I know there are a few .45 PCC that are being sold to the public. Other than .45 being slightly larger bullet than 9mm and shooting heavier bullets, what are some other advantages?

- 9mm and .45 both have subsonic ammo that isn't hard to find
- Both have good self defense loads
- 9mm magazines carry more rounds than the .45 ACP ones  
- Looking at ammo costs, 9mm is about $0.10/round cheaper than .45 for the comparable new factory FMJ plinking ammo. For fancier stuff, 9mm is cheaper per round as well. Looking at .45 FMJ ammo, you start getting to the price point of what 5.56 FMJ would be if you don't mind running a AR15 pistol/SBR instead of a PCC

I could see the benefit of a .45 PCC for someone that shoots .45 as their primary handgun ammo. Is there something else that I am missing for why I might need a .45 PCC in my life?

If you want one just buy one.  Everything you mentioned is true.  I would add if your going to buy a silencer then 45 ammo of different varieties does silence better than 9mm and 556.  You made two comments as to why your hesitating.  One being cost of 45 ammo compared to 223/556 this all comes down to your intended use.  The 2nd was about someone already having a 45 pistol that could potentially share ammo with the new 45 subgun.  That depends on whether you already have a 45 as to whether it applies to you.  I took the statement as in real time the subgun could share ammo with your pistol.  While that potentially could be a benefit it just doesn’t happen in the real world defensive scenario.  Actually I haven’t seen any defensive training companies teach that but maybe its assumed as tactic in a extreme scenario.  Rifle ammo is always better in a defense scenario even out of a short barrel as long as you have shot that short barrel a bunch to verify it’s works constantly.  I guess the same could be said about .45 subgun with regards to verifying it’s works constantly.  The 45 will silence better than the 223/556 but loses at terminal ballistics on body armor because most level 3 Kevlar stops 45 acp.  To me the reason I jump into a 9mm subgun pistol configuration was the ability to conceal carry in my state and not wanting a 4th ar15 in a pistol configuration.  I fully understand though that a 10.5 inch ar15 pistol with a folding buffer tube and pistol brace is a better defensive option.  I did go down the road your on and felt like 9mm was the better choice between 9mm and 45 acp.

If you reload then there is not much difference. If you only run factory both calibers can be cheap to shoot if you watch sales. What are you planning to use it for? HD, sport or just fun? I would look at what type of action you want to use. In AR you have blowback, radial blowback and gas operation, in blowback you will need to run heavy buffer springs and buffers for both calibers, radial blowback (CMMG) still needs heavier springs/buffers from what I have seen, I currently don't have a radial blowback gun? Gas/DI will use standard carbine springs and modded bolts, in 9mm if you reload it is an option but running factory ammo the gun gets dirty fast usually in 75-100 rounds depending on ammo then it needs a quick cleaning to keep running reliable. If you reload there are several powders that burn clean and I have gone 500-600 rounds before having issues, then a quick clean and all is back running. The 45 has the same issues but are less pronounced,  I have gone 400 rounds of wwe white box before needing to clean it. The main issue with DI pcc is the gas port being so close to the chamber it pulls crud into the carrier.
The recoil with DI is alot less then with blowback in my opinion and is down right amazingly soft in FA.

Steel targets clang way louder

PCCs are a want not a need.  I still want a couple myself, but practicality goes to a 5.56 or 300 Blackout SBR.

I have PCC's in both 45ACP and 9MM.  The 9mm is an SBR'd UZI Pro and the 45ACP is an SBR'd Kriss Vector.  Love them both and shoot them both as often.  I don't use them for HD but seeing as how I have suppressors for both, if I were to use one for HD it would be the Kriss Vector.  The 45ACP is a little quieter suppressed with all ammo and launches a heavier pill.

Is a 45 really quieter than a 9? In pistols it’s the other way around

Yes, my 45 PCC is quieter suppressed then my 9mm suppressed PCC, and so are my pistols.  In my 3 host 45ACP pistols standard 230 grain ball ammo is much quieter then my 2 host 9mm’s shooting 147 grain sub-sonics,  maybe because the 147 sub-sonics still move faster than 230 grain ball ammo.

.45acp makes bigger holes.

I have more 9mm PCCs than .45, so sticking to 9mm will give you more choices in a PCC, and 9mm is cheaper and has less drop at distance. Higher capacity too.

I still like .45 more. I typically shoot PCCs at 50 yards or less, I have plenty of rifle cartridges to go farther than that.

I think the smart choice would be to pick the caliber you stockpile the most, to me that's the biggest plus for a PCC, shooting a round you already keep a stash of.

Or pick the carbine first, then decide on what caliber is available. I have a SUB2k in 9mm, since its not rated for +P I wouldn't be interested in that in a .40s&w version.

But both my Hi Point carbines are .45 & 10mm, they're much heavier so I think they're better suited for heavier calibers.

If we're talking a blowback AR then no question 9mm, just way more options out there.

What's the softest shooting .45 PCC? It has to be something other than blowback.
KRISS has great recoil mitigation, but it weighs 7 lbs
https://kriss-usa.com/15-rifles/short-barrel-rifles/176-kriss-vector-gen-ii-sbr-black-kv45-sbl20

This new AlphaWolf PCC has short stroke, gas piston operation
https://www.lonewolfdist.com/Detail.aspx?PROD=922220
and is supposed to have mild recoil, weighing in a 6.4 lbs

Then there is the 5 lb. CMMG Guard series with "Radial Delayed Blowback"
https://www.cmmginc.com/product/rifle-mkg-45-pdw-45-acp-nfa/
which is an SBR, but they have a pistol brace version (4.7 lbs)
https://www.cmmginc.com/product/pistol-mkg-45-psb-45-acp/

If you don't have a 45 pistol there really isn't that good of a reason.  The 9mm picks up 200 to 300 fps in a carbine where as the 45 might get 100 fps, so the performance gap is pretty much nil in a carbine.

I already have a CZ Scorpion that I really like. I have just been seeing more .45 ACP PCC come out when for awhile it seemed like everything was just 9mm. Was just curious the reasoning for that. Seems it is like stated above; punches bigger hole, slightly quieter when suppressed but comes at the cost of more expensive ammo, less capacity and more recoil than the 9mm.

.096 mm

In both my pistols and PCC's my suppressed 4's are quieter then my suppressed 9's.  I use standard 230 grain ball ammo in the 45 and 147 grain sub-sonic in the 9mm.  The 45ACP travels at 800fps and I believe the 9's are moving at about 925fps.  The slower of the two sub-sonics seem to suppress better.

NO. Not at all. There are at least two people in this thread that are very mistaken. 45acp is always subsonic, that’s true, but even subsonic 45acp is typically louder at muzzle and at shooters ear when suppressed than supersonic 9mm when in comparable guns.

45acp cans have 1.5x larger hole than a 9mm can (pi*r^2). It makes the baffles much less effective. I DARE anyone to find comparative meter testing that shows 45acp to be quieter. I read a lot of dumb mistaken stuff on here, but man, this is really something I wouldn’t have expected it’s just so easy to look up as incorrect.

My understanding is that the best .45ACP dry suppressors have noise levels typically in the 140-ish decibel range.

The best 9mm suppressors typically test in the 130-ish decibel range.

The .45ACP is quite a bit larger in diameter that the 9mm.
Suppressor efficiency is mostly a function of internal volume of the suppressor as well as the ratio of baffle face area relative to bullet caliber/diameter.
With 9mm and .45ACP suppressors being relatively similar in size, the 9mm will be easier and more efficient to suppress...given the use of subsonic cartridges in either caliber, of course.
Moreover, I think it is important to consider how either caliber sounds when suppressed.

My .45ACP can sounds more like a muffled gunshot, while my 9mm can sounds more like a pellet gun.
I can also carry twice as many cartridges per any given weight with the 9mm.

So personally I like the 9mm more.

I have built several New Frontier Armory AR-based 9mm pistols that use the same magazines as my Glock 17's and 19's so I have magazine commonality with guns that I already own and I also have a lot of 9mm stockpiled.

YMMV...

The only thing you got wrong was YMMV. It doesn't because There are no 45acp setups that are quieter than their 9mm counterparts ;)

Well now that solves what caliber for my next PCC.

9mm > 45ACP suppressed.
9mm > 45ACP verses armor

I wanted to try out the Extar in 45 but what's the point, I like quieter mode better. Might as well add another 9.

For some reason I thought 45 would be as quiet as 9. Interesting considering 45 and lower velocity.

45 ACP is a dumb cartridge and you don't need it.

I just saved you a lot of wasted time, money, and effort. You are welcome.

I have shot an AR 45 (blowback) with ball 230 grain suppressed. At least 400 rounds over a 3 or 4 occasions, and it was very quiet and really pleasant. I have also shot way more 9mm 115 gr. ball and subsonic ammo . To me, the benefit of the 45 was mostly due to the fact that 230 gr. ball ammo is common and fairly inexpensive for a subsonic round.

Compared to 9mm, there's no need to hunt down special subsonic rounds for the 45. Any 230 grain will do. With the heavy buffer in the 45, you felt a healthier "bump" from the recoil, but nothing particularly offensive or harsh. I personally liked the kick of the 45, and it knocked around the steel targets with much more authority than the 9mm. I also feel that the 45 ball ammo is a proven defensive round, so even without a high performance hollow point, 45 ball ammo is effective stuff. With a 10" barrel and suppressor, it was easy to shoot and very quiet.

It's an academic issue anyway. The 9mm subsonic is around 160 grains, vs. 230 for the 45, so the 45 always packs a bigger punch. But if you are shooting the carbine for high volume, fun blasting, then get the 9mm for less expensive ammo. You really cannot find any center-fire ammo less expensive than 9mm pistol ammo, and you can buy the occasional box of subsonic ammo.

Even though I really like my 45 AR, and think it a better defensive caliber than 9mm, for economy and volume shooting, I vote 9mm.

Here's a post of some of my rifles and the process I went through tuning the 45. (don't be thrown off by the pictures, I've had uppers with both 9mm and 45, each in 10.5 and 16" barrels, and I've tested them all, but didn't get a picture of every combination).
https://www.ar15.com/forums/ar-15/put-together-a-New-Frontier-45-auto-AR-this-weekend-update-with-16-barrel-and-pics/15-727285/