What is the BEST handgun round?!?
The debate over the 9mm and .45 ACP is one of the most heated conversations in the firearms community.
Enough to spawn sayings you’ll see all over forums (and t-shirts).
Both handguns have a huge following thanks to their popularity and success in the field.
But which one is better?
Well, let’s take a look at these two rounds and pit them against one another.
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Comparing 9mm vs .45 ACP
Let me start out by saying that the biggest mistake that most people make is taking a black-and-white stance on the .45 ACP and 9mm.
You’ll hear a lot of people say that the .45 is better because it shoots a bigger caliber bullet, or that the 9mm is better because of its capacity.
Click here to learn about more handgun calibers.
Both are valid points and very understandable reasons to prefer one over the other.
Even if you think size > capacity or more bullets is better, you have to admit having bigger bullets and having more bullets on tap are both worthy considerations when choosing one gun over the other.
The truth is…neither gun has a total advantage over the other one, and your personal preferences will play a lot in determining which handgun is for you.
Let’s take a look at the selling points of each one to help you decide.
About the 9mm
Praised for being compact and easier to handle than its .45 ACP counterpart, the 9mm has become one of the most popular rounds in the world.
For more than 30 years, the Beretta M9 and M9A1 have been the standard sidearm of the US Army, Navy, Air Force, and Marine Corps.
Prices accurate at time of writing
Prices accurate at time of writing
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The U.S. Army eventually switched over to the Sig Sauer P320…but they’re still sticking to the 9mm caliber design.
Prices accurate at time of writing
Prices accurate at time of writing
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Even the FBI has phased out the .40 S&W in favor of the 9mm.
This begs the question: is bigger necessarily better?
The 9mm features some interesting advantages. It offers a larger magazine capacity given a similarly sized firearm.
And its compact nature makes it easier to carry, especially when concealed carrying.
Not to mention, the 9mm brings less recoil, making it an easier gun to handle – especially for beginners.
The 9mm also has a higher muzzle velocity than the .45 ACP because of its lighter bullets.
This has caused a debate in the firearms community over what’s better, a fast and light cartridge or a heavy and slow one.
While the answer to this is up to debate, one thing is certain: higher muzzle velocity does equal slower bullet drop.
Something to remember when shopping around for a 9mm…
One of the most attractive things about choosing a 9mm for home defense is having more rounds at your disposal.
Unfortunately, this benefit may be completely canceled out in some states due to magazine restrictions.
So, it’s a good idea to review any state and local laws before buying a 9mm if you’re set on having a 15+1 capacity gun.
Regarding the .45 ACP
From the days of the trusty Colt M1911 to modern .45 caliber handguns like the Glock G21 and the Sig Sauer P220, the .45 ACP has always been one of the most reliable calibers on the market.
After all, it was the official sidearm of the United States Armed Forces during two World Wars and remained so up until the 1980s.
Many handgun enthusiasts believe that bigger is, in fact, better and love everything that the .45 ACP has to offer.
Over 100 years of testing and iteration have produced some very powerful .45 ACP ammo.
While slightly impractical, the .45 ACP also has the coolness factor on its side. It’s a battle-tested gun that’s played an important role in American history during the 20th century.
Measuring the Two
There’s no doubt that the .45 ACP is a classic handgun that’s not going anywhere in the foreseeable future, but it doesn’t outperform the 9mm.
Oomph
Advancements in modern defensive ammo technology have helped the 9mm improve by leaps and bounds over the past 30 years. It’s not the same underpowered round that the FBI abandoned in the early 1980s.
With a good modern defensive ammo, the 9mm is just as powerful as any .45, just in different ways.
When comparing the two handguns, it’s important to note that the 9mm hasn’t become better than the .45 ACP…it just caught up to it.
A picture is worth a 1,000 words…new self-defense 9mm ammo opens up to create some nasty possible wound channels to stop attackers in their tracks.
That is some nice expansion from the 9mm ammo.
But let’s not forget the same developments in HP ammo that allowed 9mm to catch up to .45 ACP FMJ also allowed .45 ACP HP to beef up.
While the 147-grain Federal HST expanded from 9mm (roughly .35cal) to on average 15mm or .61-inches, the .45 ACP expanded from (again, roughly) 11.5mm to 25mm (.45-inches to 1-inches).
Both (roughly!) doubled in size…and since .45 ACP is bigger to start with, it became massive in the end.
Affordability
Furthermore, the 9mm is more practical for the average shooter looking to spend time on the range.
With boxes of 9mm Luger rounds being 30% to 40% cheaper than .45 ACP ammo, increased range time isn’t going to break your budget. And after all, spending more time on the range does make you a better marksman.
(Here are more tips on how to shoot more accurately.)
However, there’s always something to be said for having really big bullets ready to deal with really big problems.
Velocity & Penetration
Also, for you suppressor enthusiasts out there, the .45 ACP is an inherently subsonic bullet.
It fires slow and heavy bullets at a lower muzzle velocity than the 9mm and can be suppressed to near-whisper levels.
One of the main reasons why military personnel and many LEOs made the switch to 9mm handguns was because of the deeper bullet penetration.
Unlike in a home defense situation, the ability to shoot through a wall, automobile, or various other objects in a combat scenario can be the difference between life and death.
According to a report released in 2014 by the FBI, the 9mm was preferred in the field because of the following:
- LEOs have a 20% to 30% accuracy rate in shootouts, so magazine capacity is important.
- 9mm Luger rounds have a deeper penetration than other handgun calibers.
- FBI agents have been observed to shoot faster and more accurately with the 9mm.
- The wound tracks of a 9mm and a .45 ACP are almost the same.
- The 9mm’s lighter recoil makes it easier for follow-up shots.
Considering all of the advantages of the 9mm, why would anyone want to use the .45 ACP?
Well, when it comes to home defense, the .45 ACP does hold one distinct advantage over the 9mm…shallower penetration.
Don’t worry, that’s a good thing here.
As mentioned in the video, over-penetration isn’t as big of a problem with the .45 ACP because of its slower, bulky bullets.
This means you have less of a chance of hitting an innocent bystander through a wall in a self-defense scenario.
Choosing the Best Caliber for You
You might be wondering which caliber is better for you…the 9mm or the .45 ACP.
Well, this might rock your world but I recommend getting one of each — assuming you have the money.
Many shooters like to have the .45 ACP for home defense. It’s the perfect grab-and-go handgun to fire in close quarters where recoil won’t pose such a problem.
Alternatively, the 9mm’s stopping power and smaller design make it ideal for open and concealed carry.
While there are a number of compact .45 ACP models on the market, as a general rule, you can expect the 9mm to be more comfortable to carry in a holster.
Also, in a self-defense situation outside of your home, there’s a higher likelihood that you won’t be firing your gun at close range like in a home-defense scenario.
In this situation, you’ll be glad that you have the faster follow-up speed and a larger magazine capacity of 9mm.
Conclusion
Ultimately, both calibers are great to have. Some people prefer the heavier feel of a .45 ACP, while others find the light and slim nature of the 9mm easier to use.
Either way, the 9mm and the .45 ACP both pack a strong enough punch to neutralize any threat.
Where do you stand on the 9mm vs .45ACP debate? Let me know in the comments below! No matter what caliber you pick, we got you covered in our Best 9mm Ammo and Best .45 ACP Ammo reviews.
And speaking of comments…we had some fun with this video…
414 Leave a Reply
Thanks for the article. I've been rocking the 9mm for years now, but have recently purchased the G21. I noticed when shooting the .45 that my groupings were actually better- maybe due to the fact that I knew it was a bigger caliber so I was really focusing on my grip. I've become a huge fan of the .45 and might make it my EDC. But again, 9mm is a hell of a round.
The only two cartridges I shoot better than 9mm are .32 ACP and .22 LR. My deep carry/BUG is a Seecamp 32. I'm quite comfortable carrying a .38 Special +P J frame, backed up by the little Seecamp, or else I'll tote a Sig P365. Extended shootouts in self defense scenarios seem to be pretty rare. Most bad guys make peepee poopoo while running the other way once lead starts flying. If 5 in the J frame + 7 in the Seecamp aren't enough, I'll quote Confusious.
Who cares which one you choose. More importantly find the handgun that fits your hand the best without worrying about caliber. What ever you decide on practice and train until the handgun becomes an extension of your hand. Me personally a 10 mm leave’s either one in the dust.
Article literally says cops miss 75%-80% of the shots they take so it’s better they use 9mm bc there’s better penetration. IDK about U, but being that terrible of a shot, I really don’t even want them firing at all, let alone firing rounds with *BETTER* penetration! They can’t hit what they’re trying to hit, so why wld U want them to penetrate further into what they’re accidentally hitting‽‽ I wonder if cops around the globe are as pathetic as American cops? Ours are pathological liars, overly violent & abusive, obviously terrible shots, they know fuck-all about the constitution they’ve sworn to uphold, they murder innocent (& sometimes not so innocent) people with impunity, they don’t know the law & more often than not, they just make it up as they go along and oddly enough, their new laws always seem to align with whatever they want in whichever situation they find themselves in. They’re poorly trained, only solve ~2% of crimes, they spend most of their time harassing people who’re financially disadvantaged; what it comes down to is, they’re an incredibly well-funded & state-sanctioned criminal organization. I say “criminal organization” bc they literally do more damage & steal more goods than the actual criminals they’re meant to be stopping. And to think, I used to want to be one!!
There are several aspects of this comment that are interesting. Not sure of your profession but it's likely you've never had to engage in self defense or offense while being shot at. Accuracy in all shootings are abysmal during a shootout. Furthermore, you mentioned that the police do not "know "fuck-all" about the constitution"... How many of the military members (such as myself) do you believe have taken the time memorize all of it... Hell some of the guys I worked with, I wonder if they can even read, especially when some of them are joining the military just to become a US citizen and can barely speak English (this is pertaining to the US military).
That said, i see the concern with the 9mm and the extra penetration concerns with the missed rounds nearing 80%. The other aspects of targeting innocent, happens with every profession... Also, many that are disadvantaged financially maintain such of their own choice.
Take care.
Bootlicker
You forgot to mention capricious and arbitrary.
No, the article says that cops miss 80% of shots so they need MORE ROUNDS IN THE MAGAZINE. Better penetration is another issue. And remember: police shootings often involve people who are moving, using cover, and shooting back. It’s not, as you seem to think, like the range…
There are an estimated 60 million police interactions per year in the US with about 2% resulting in use of force. About .1% of that is use of deadly force and of that, about .01% is non-justified use of force, (i.e. the police were being attacked in almost every case) Would you consider that the media only really talks about the negative interactions and thats all you ever hear about? If you're open minded I'd check the Bureau of Justice statistics. “It's easier to fool people than to convince them that they have been fooled.” ― Mark Twain
American police aren't the best and have small bits of corruption like making up that they smell weed for an illegal search, occasional excessive force, etc. but if compare them to the rest of the world american police look like saints. All of south america police are directly involved in drug trafficking/kidnapping even running the convoys themselves and you can do illegal things like drunk driving and get out of trouble for $10-100 usd and get out of murders for a few thousand payoffs I lived there for a year so seen it first hand. Large parts of europe police are tied to the mob and in general way way way more useless then american police.
Isn't the 9X19 basically a "warm-to-hot .38 Special" in semi-auto form, somewhat in the same neighborhood as a .38 Super? It's obviously not a.357 Magnum. But, it makes sense as a military, police and security round. With the trainloads of money spent for cartridge development, it is what the government and private agencies want and can use, as well as civilians. The .45 has also been improved.
My first experience with the 9mm was in my early 20s. I was underwhelmed with the twin pops and barely noticeable recoil, the very thing that makes the 9 so likeable to many. I grew up with the .357 and .45, and admit I didn't have confidence in the 9mm. I suppose for me confidence was the deciding factor. If things go to hell, and I absolutely have to stop a lethal threat, that first magazine of .45s is reassuring. A few sources have said having a magazine with 15 to 18 (or more) rounds often results in the shooter blasting away. Nothing wrong with having high cap mags these days, what with gangs and disparity of force. But, if one believes a Gatling gun's worth of rounds on his belt equals survival, what makes one think he or she will survive long enough to make use of all that firepower? Extra rounds are comforting. I agree. Solid hits are the goal.
Other than rifles or shotguns, I still believe the .45 can hit hard, deep and quickly enough to do the job.
Look at the velocity numbers: you’re talking bs.
If you're the only one carrying a gun use a gazillion round 9mm because you'll likely be able to get a lot of rounds off before anyone can stop you. But, the reality is, in a gunfight, unless your opponent is really really careless, you will likely only get one or two shots off before you're rendered dead (if you're really really careless). Carry the biggest most impactful bullet in your gun and shoot as straight as you can as if your life depends on it. Or, just practice, practice, practice; excellent aim with your first shot will pay off much more than bullets being spewed all over the place.
Best advice here. We get so hung up over caliber its now merely clickbait in 2023. Ill echo you coz I agree. Biggest bore you can shoot well and comfortably carry in a gun you actually train with. Cheers
I agree with TZH. Training is the key to hitting the target. Training will also increase confidence and help manage the "amp" effect. I have both 9mm and .45 and they are similar in size (compact) and weight. I can take head shots with all rounds in a target range with all my guns. With a military background and my own hand loads, this is no problem. The best advice is to train as often as possible. I carry differently depending on where I am going and what I am wearing also but the 45 is just as easy to carry as the 9mm, just fewer rounds.
I also load the heaviest round in all calibers with no care about velocity because the handgun will never reach a high enough velocity to match even the slowest rifle round. The effect on the target is improved with weight of the round when it comes to pistols. Also the heavier the bullet, the slower the velocity in all calibers. If Kimber made a 357 sig, I would carry that one.
Everyone always mentions more capacity for the 9mm, but forgets to mention that you'll need those extra shots. And aggressive Prosecutors love to bring up how many times you shot them. If you feel the need for extra bullets buy the FNX tactical.
I found, for my wife, the 380A worked best for her. For me the recoil is like a BB gun but has a similar down range damage as a 9mm. The Ruger LC380 is her favorite. She is a small gal so it is perfect for her.
I am purest. I like the beef of an ACP. My favorite round is a sig 200 g. I tested this as a defense round and appreciated the impact force on soft targets. If you want to make a mess, the sig elite 230 g is a show stopper. The recoil may be heavy for some but since I am a big guy, recoil has never been an issue for me.
I carried the 45 in VN and I had to use it. One shot one kill at 30’. I love it. The energy at impact is greater than the 9mm.
9 mm gets females through the firing range. It is a problem at every agency. 9mm is the solution.
I may be considered old and not moving with the new trends, and all is good and dandy with the 9 mm and all the new ammo development, but if the .45 was good to stop the japs and the Krouts, in my book it is good to stop anyone who wants to harm me or my family.
I own both, a Colt Combat Commander .45 and a Glock compact 9mm. The Glock is my every day carry. When comparing the Glock compacts in both calibers, the 9MM was less bulky, lighter, and easier to conceal. If the gun isn't comfortable to carry, chances are you won't every day carry, defeating the purpose. Less recoil making for more accurate follow up shots is a huge advantage in a high stress defense situation.
My 9mm compact carry S&W Shield semi-auto has a ported barrel. It squirts so much flame out the sides it would scare an attacker half to death.
My military friends please chime in. ANYONE who says in a civilian encounter they can end it in 3 shots is either a psychopath, a liar, or never been in a SHTF scenario.
I know the adrenaline dump and freeze response. I (thankfully) also have an overactive amygdala response that kicks in to take over. If you THINK you can shoot accurately while adrenalized, then you're whistling past the graveyard. You WILL freeze and lose hearing and fine motor control. Veteran SWAT freeze. I freeze. You'll freeze. It's human nature.
The big question is how to navigate the freeze and put rounds on target. Unless you've been through Massad Ayoob's classes you'll be useless once you're adrenalized. Clumsy, deaf and with tunnel vision. You think that you can outshoot a cop? You think you can outshoot SWAT? BS!
What ends an engagement is being able to walk away from it. My preference is 9mm because I know how I'll respond. I know pros miss under stress. I'm not deluded in thinking I'll be able to point and shoot more accuately than them. I want more rounds with less recoil.
Hi Catherine. I had been on the fence regarding 9mm and .45ACP for a long time, then I purchased a couple of different 9mm semi-autos for carry over a few years, with .380 for backup. Mostly for the reasons that 9mm is easier to carry, has more rounds without reloading and is probably much lighter. I read a lot before I chose 9mm over .45 and was swayed a lot by the later technology of 9mm catching up with the power of .45. My sub-compacts with their polymer frames are smaller and easier and lighter to carry than a steel 1911. I found all-steel frames a tad heavy for pocket carry, even in compact size. I have several carry modes active: inside the waist band, outside the waist band, pocket (jeans, shorts and jacket), and .380 on the ankle. And yet after reading all the evidence about the benefits of 9mm and I'm already heavily invested in 9mm (and happy with it) I'm still looking at buying a 1911 because it seems everyone feels like they have to own at least one. I'm still on the fence about it though. Would I even ever use it? I like the reasoning that .45 might be less likely to over-penetrate walls at home, but does it really make that much difference between a 9mm and .45 at home? Where's the test evidence? For home defence, I have .38 special, 9mm and a 12 GA shotgun squirreled away. I don't own a .45 (except a Pietta .45 Long Colt cowboy gun I target shoot in the mountains- I love it), but my son does have a couple of 1911's which he loves and says they are fairly easy to conceal carry. He also has his .40, but he finally switched to 9mm for carry after squealing about the price of ammo (I chuckled!). He now carries a S&W M&P Shield like mine, or a Glock 19. I carry a Shield or a Glock 26. All are in 9mm, we both love our Glocks and Shields. My .380 S&W Bodyguard is so small, I'm considering getting a slightly larger .380 with a longer barrel for backup. This is to ensure good expansion of defensive hollow point ammo, plus the Bodyguard gets lost in my pocket and slips sideways on me. A different pocket holster might cure this, but so would a larger gun, with the benefit of longer barrel. I like the Bodyguard, unlike a lot of people. I've modified the trigger pull on mine a bit. This really is a science, but I benefited from reading a lot before I purchased, whereas my son didn't. He bought 'bigger is better' and eventually converted down to 9mm. I hope this is of interest, Colin
Sorry I mis-spelled your name Katherine. I also agree about freezing and adrenaline and tunnel vision and all that. I've never been in a gunfight, but I have been terrified out of my mind once, many years ago, when someone did come after me with a gun, and I couldn't get my knees to stop shaking. Obviously, he didn't get me. But, had I not experienced it, I never would have believed it would happen to me. People don't.
@Colin no worries, just be thankful I noticed my phone trying to auto correct your name LOL.
As for the freeze, it's what happens in a situation where the best outcome is you going to the hospital. It's a hardwired biological response. When it happens your mind will likely go blank. In my case it was no weapons displayed (but you must always assume present) against a $h!t brick linebacker. Thankfully my training kicked in. Whatever you repeat thousands of times will be available for use.
As for the rounds, force x mass = power. Hollowpoint should (mostly) negate over penetration. As for the tool, I like the Springfield XD series. If someone wants to open carry (which I personally discourage) the XDM appears rather intimidating and is quite accurate in calm hands. For CC, I like the XD-M elite 3.8 inch. It's compact yet carries 14+1 stock mag or 19+1 extended mag. Grip safety with chambered indicator. So you can literally grab, draw, aim and shoot. Then rinse and repeat as needed.
For resources, I recommend Rory Miller as he dedicated a chapter in his book to the freeze, Marc MacYoung how to avoid violence in general, and Tim Larkin. What if you can't deploy your firearm in time? Well then YOU become the weapon. Physics works whether you're a criminal or a descent citizen.
Thanks for the tips Katherine. We are now restricted to 10 or less rounds in the mag here in Washington State. Happily, I have some larger mags which were purchased prior to the 10+ ban, so are 'grandfathered' in! Have a great weekend, Colin.
You paint with a really broad brush, and worse yet you are wrong. Adrenaline is not the deterrent you make it out to be, some people actually perform better when "amped up" so maybe YOU would be would be useless but you have no right to comment on anyone but yourself.
A range instructor at Camp Pendleton once told me most would-be handgun shooters would be more effective hurling the pistol itself at the enemy rather than trying to aim and fire it. He was being serious.
If you can place three (3) rounds in the black, the debate of 9mm vs 45 acp does not really mater.
If you can place three (3) rounds in the black, the debate of 9mm vs 45 acp does not really mater.
Totally agree the new technology of the self defense bullets put’s the 9mm in the same class as the 45acp, but I still prefer the 45acp but I carry the 9. Great review
.45 is, by nature, subsonic. No need to buy specialized rounds. It's quiet and very handy if you plan on silencing a weapon. 9mm is just as quiet if you opt for subsonic which takes away the muzzle velocity and negates the price difference.
My non-silenced weapons are 9mm my silenced weapons are .45. I agree with the writer, owning both and applying to individual situations and preferences is right on the mark.
The hydrostatic shock of a .45 is inherently greater than a 9mm. Remember the reason for the .45. Before 1911, soldiers hit with a 9mm were still advancing even if mortally wounded. They had a narcotic in them to keep them advancing until they expired. The .45 stopped them in their tracks. Badda Bing, Badda Boom. I witnessed a man in the early 80s with over a dozen 9mm rounds in him. During the EVAC of Vietnam, one VC was dusted with a .45 to the shoulder and he was DRT...dead right there.
There really isn’t any hydrostatic shock to speak of at handgun cartridge velocities to begin with.
With handguns, it’s all about holes: how many, how big, how deep, and where hit.
Hi Wyatt, not trying to be a jerk here. It's possible the man in the 80's wasn't being hit by modern 9mm expanding hollow-point defensive ammo. I'm not an expert on injuries caused by ammo though. But if the FBI and cops have reverted to 9mm, I'm comfortable with it too.
I have both 9mm & 45. I have the M&P Shied, Glock 30. I also have rhe Rock Island 9mm/22 TCM & an AR 45 pistol. CZ Evo carbine & pistol, a Mac 1911 45 & many others. I like both & depending on where I'm going, how long I'll be gone, what I'm wearing & how I'm feeling makes my choice for me. I really enjoy carrying the 22 TCM, 19 rnd, 40 gr hollow points. 1 thing that cannot be argued is that the 22 cal is very effective. I grew up listening to stories & seeing the animals taken with the caliber. The 22 TCM is just a steroid version. President Reagan was hit with a 22 lr & we almost lost him. I guess a lot has to do with comfort of us & quality of placement. If your g it someone in the head, point blank, w/a blank round it should kill them. Go with comfort, shot group & confidence.
I have both and like them both. The 9mm is easier to conceal but when I am on my Harley I like to carry the 45. When I was taking a defensive hand gun course( took it multiple times ) at Front site all of my instructors had 45's. When I asked them why ....they said it was because they didn't make a 46. I was using a Glock 17 and really liked it. My 45 is a Sig dual tone full size 1911 and I like the weight and the 5 inch barrel. My 9mm is a FN509C with a red dot and I really like using it as my CCW.I use Federal HST in both for defensive carry. Interesting discussion thanks for the insight and thoughts.
The big difference in the 9mm and .45 which is often overlooked is if you have less than a 4” barrel on your 9mm even the the best (law enforcement grade) ammo under preformed; which negates the smaller 9mm’s
I like and own both and both have a mission. In church it's a full size 45 at home full size 45 SHTF one for travel one for house to house. CCW 9mm
See you miss a HUGE point. Capacity is not as huge a deal as you make it out to be, you tried to compare police shootings to civilian ones. They are extremely different. The average civilian self defense shooting ends on average between 2-4 rounds. The average is 3. To think you need 15+1 is just imo foolish. Hi capacity is unnecessary 5 and above is just fine. And sure it won’t hurt to have extra rounds it’s just not necessary, people try to bring up the “multiple attackers” argument and the same applies, 2-4 rounds to end an attack.
Also avg distance of civilian gunfights are extremely close quarters. Another fact if you shoot an assailant far enough away that bullet drop comes into play, you will have problems in court. Pew Pew needs to talk to professionals rather than giving amateur opinions.
Absolutely agreed.
I would prefer to ha ve more rounds just in case. You never know what you can walk into or what might bust in your door
Hi Chris. To add to this argument, I read a recommendation which said that in a civilian SHTF encounter, that five rounds would be enough to cover such an encounter based on research which had been carried out on previous encounters where the number of shots fired had been recorded, so purchasing a firearm with seven rounds would be considered more than enough (IN MOST CASES). So therefore if you had a smallish gun with 6+1, you'd be covered. If you had a slightly larger gun with a magazine holding seven or eight rounds (or more, such as a double-stack with 10, like a Glock 26) was available, you'd essentially have plenty (in most cases). That would be 1,2,3,4,5 to stop the threat and the rest to miss with. The common reasoning that having more than enough, or having the most you can carry because you can 'never have enough', well, it's not something anyone can make others believe one way or the other. Everyone has their own confidence level and may believe what people tell them or what they read. My own belief is that in most cases, people will over-carry for the sake of extra confidence. However, 'over-carry' isn't really the correct term, because one never knows what one will walk into, or how one will react to an encounter. Besides, small guns can be easily supplemented with an extra mag or two or a back-up gun. As Katherine pointed out as well, there are so many variables which can come into play like freezing, forgetting to take the safety off, tunnel vision, forgetting everything you've ever been taught, gun jam, on and on. In the end, I would rather have a 6+1 which I knew how to shoot well than a 15 rounder I couldn't.
I went with the 45 Smith&Weston MP2 Shield that is good foe me. I was one of those folks in the military in the 80's that had a 45 side-arm, so used to it. I also have a bull-pub style shotgun for home defense as well. Yes you are right there is a lot more 9mm ammo than there are 45 ACP. Their is also a cost difference thou if you look around you can get ammo cheaper.