Looking at doing another Blowback PC AR build. Just completed a 9mm. Am kindve on a budget. If stuck with building a straight blowback gun, what other pistol calibers run the best?

Searching around on here, it sounds like 10mm is a bit too much of a round to run in a blowback gun without delayed blowback or direct impingement. Some people have done it but these seem to have a lot more issues than the 9mm ones.

Not the biggest fan of .40 S&W. Almost would rather have a 10mm over .40. Although 10mm is pretty expensive and .40 S&W is cheap. So I would consider the .40 S&W.  Although a big knock on the .40 S&W is that the round has a weak case head and in the case of a blowback gun that maybe an issue.

But, im really curious how everybodys .45 ACP builds are making out? .45 ACP has a lot less pressure than even a 9mm despite being a bigger caliber. But one big problem with the .45 ACP is if I were to do a glock mag build in .45 ACP, the largest factory mag is 13 rounds. Theres Korean made mags that are bigger and the Kriss Vector extensions but Id rather not get into those. .40 S&W they make higher capacity magazines.

Probably would do a 16" barrel rifle version as well which the .40 S&W might make more sense. Dont know if a .45 ACP will gain anything from a 16" barrel? I know the 9mm does. I can tell the 16" barrel in 9mm def hits the targets harder over a handgun barrel.

But im still leaning towards the .45 ACP and sticking with the old school calibers.

are you only wanting to stick with factory mags?

I'm running an 8" .40SW CMMG RDB setup in full auto and using ETS 30 rounders.  

ETS also makes 30 rounders for .45.  I haven't tried them in .45 but my 40SW 30 rounders have been working fine in full auto.

I'm in the process of doing a .45ACP CMMG RDB but will be using an MGI GreaseGun magwell.

I've done all my CMMG build using their BBL/BCG combos which retail around $350.  I've never bought their complete uppers.

ETA, while the CMMG RDB setups have had ejector spring problems for some people (including myself).  I have yet to hear of these failures in the .45's.  I presume lower pressure.

Does anybody make a 45 ACP dedicated lower that isn't Glock mag?  Maybe grease gun mags or whatever, but not using a mag block in a standard lower?

Bazooka Bro's Hybrid lower, takes GG mags and still takes AR15 mags

The grease gun mag ones look a little goofy to me for some reason. Straight .45 mags in a 30 round capacity or so stick out pretty far. I know many think glock mags look goofy over colt mags but I would probably consider the Glock route for me.

And as far as Glock mags. My mileage with Glock Magazines have been factory mags and Magpul are best. Ive dealt with South Korean mags that dont even fit into the gun. ETS ive had issues with and dont care for the clear magazines. Magpul only makes 9mm Glock mags though. And  factory mags in .45 ACP...the largest they come is 13 rounds. Too bad they dont make a 21 or 23 round .45 ACP mag or whatever would be comprable in size to the 33 round Glock mags or the 27 Round Magpul 9mm Mags.

Anybody have or built both a .40 S&W and .45 ACP to compare? Im kindve thinking 9mm and .45 ACP are the best options for a straight blowback AR system. 9mm is snappy but pretty low recoil and .45 is a gentle shove when shot in handguns. Ive shot .40 and its much more snappy and abrupt recoil than the .45 and 10mm is similar but just a bit more snappyness.

Yes rudy at macon armory.

I have one and it works fine. Drops into a standard lower.

He also makes a di upper for 10mm and 45. Theyre great.

I've had an Olympic Arms .45ACP AR upper for a dozen years.  They also sell 10 mm, and .40 S&W, as well as 9 mm uppers.  The upper is complemented by a standard magwell sized polymer mag body magazine with an appropriately sized insert.

I have not had any issues with the OA system which has a proprietary conical barrel extension and bolt face, and an ejector mounted in the upper receiver.  The OA mags are probably incompatible with other systems.  The high capacity version .45 ACP mag has a 17 round capacity; 10 mm mags hold 18, .40 S&W hold 20, and 9 mm hold 25.

You don't see much said about Olympic Arms stuff here.

Best of luck with whatever you decide to pursue.

MHO, YMMV, etc., be well.

I maybe wrong but they mightve gone out of buisness. Seems like the Glock mag options are the latest and greatest.  Just not the most cosmetically appealing to many.

Wonder if they make a lower which uses H&K UMP mags?

As far I know they did go out of business..for awhile.... and then opened back up but didnt tell anyone I guess.

http://www.olyarms.com/shop/pistol-caliber.html

RMWxtreme (dot com) sells a stripped grease gun lower that uses standard AR lower parts kit.

Can't vouche for it personally, (yet).

My 45-ACP simple blowback runs nice and smooth (not yet with grease gun mag lower) and actually has less felt recoil then my 9mm blowback.  It may be a bigger bullet and a bigger bore but the recoil is "less sharp" then the 9mm if that makes any sense.

Probably due to the .45 being lower pressure than the 9mm. Debating between .45 and the .40.

They didn't go out of business; they were phone orders only for a while.  I ordered a few things from them in that time period.

Another thing that I failed to mention about the OA mags is that they have a LRBHO trip built in so no extra mechanism is needed.  My experience has been that their trip isn't 100% but pretty close to it.

I've found the OA system to be pretty tame for a direct blow back .45 ACP carbine.  Probably due to the conical extension/head having a gradual opening instead of an abrupt opening as does traditional direct blow back systems.

MHO, YMMV, etc.  Be well.

I had an Oly 45 upper for like one week.....
I bought it used to get the MGI GG lower it came with.  I test fired it and was surprised how smooth it was.  


However, the bolt was cut so that it wouldn't work with my RDIAS in full auto and the spring was super strong which I'm sure would result in a really high cyclic rate so I sold it.  
Both of those points would be non-issues for someone that wanted a semi.

Kriss USA has been making mag extensions for G21 mags for some time. Turns them to 30rds of 45
Kriss mag ext

Interesting how people percieve recoil differently. Many say 9mm blowbacks have recoil especially with the heavy buffer. Other say 9mm carbines have no recoil and heavy buffers tame the recoil. Im running a pretty heavy buffer and spring and my 9mm dosent have much more recoil into the shoulder than a .22 LR IMO. However, the gun does jump a little bit more than a .22 as far as staying on the sights.

Ive only shot a 9mm in a PCC though. Havent tried .45 or .40. I have the experience of shooting friends handguns in 9mm, .40, .45, and 10mm to go on when comparing the rounds. I liked 9mm or .45 the best in the few handguns ive shot. Cant say which I liked better but id own both cause theyre different. .40 and 10mm were just a 9mm with a bigger kick and .45 to me was kindve its own animal.

I wish 10mm was closer to the price of .40 because I would rather have the full size 10mm chambering over a .40. Then you pay all that money for factory 10mm ammo and its not loaded much hotter than the cheaper .40 S&W ammo . If you reload, its a different story. Both .40 and 10mm use flat point bullets more akin to rimmed revolver rounds. I feel that would present more of an issue with feeding in a Glock AR over the round nose bullets of 9mm and .45.

.45 ACP also seems to have less variation in the factory loads. 230 grain ball ammo is usually around the same power level and velocity. While with other calibers, some brands are hotter than others in the same loading.

I had to phone call special order the full auto carrier (with weight installed) and send OA a check for about $80 plus a copy of the M16 Form 4, then swap the bolt from the semi carrier (a quick, easy process).  I've always shot it with a standard 9mm, two-piece "mechanical" buffer and the standard carbine spring.  I don't have a timer, but the ROF doesn't seem excessive.

MHO, YMMV, etc.  Be well.

https://thisguysguns.wordpress.com/2018/11/06/building-a-cmmg-guard-45-aka-an-ar-45-that-doesnt-suck/

How does the accuracy compare between the calibers (9mm, .40, .45)? 9mm usually has the highest velocity and the flattest shooting. .45 might drop off before the others.

As with anything firearm related, good barrels and good ammo, will produce good accuracy. All pistol cartridges are going to "fall off". Each loading is different, and barrel length also comes into play.

Tldr version: buy quality barrel in either flavor and you'll be fine. Just don't expect 300yard shots without some holdover.

Regarding 45 mags that are Glock pattern:  I’ve had great luck with the 26 round SGM Tactical ones.  I’ve got four.  One needed a little break in but after 50 or so rounds it was fine.

Been using them in my CMMG Banshee for a year or so. Many hundreds of rounds through them of all type.

I have a 40 S&W AR pistol that I love. I run it with 155 grn JHP's and it runs like a dream. Just don't use aluminum cases in a blowback. Mine also runs steel fine. I use a TACCOM 3g delayed recoil system.

My 40 AR pistol will stack bullets @ 25 yards.

I built a 45AR on a billet NFA upper and lower.  Rudy provided bolt, barrel, and buffer.  It is dedicated to Glock Mags.  It is as reliable as any other AR.

I run Kriss Extensions.  All run smooth and Kriss is a great system.
Accuracy:  Mine is a 8".  If I put a bipod on it, and get prone, I can put everything in 2 inches at 100y with a red dot and a target that has a 3' spot on it.
Frankly, this is my favorite play toy.  It's now suppressed with a Octane 45K.
Build with confidence.  Rudy has the stuff that works.

I have built a couple of 45 AR's.  A 16" rifle and the 10.5" pistol below:
Attached File

I ended up going with a glock lower simply because mags were easier to find and cheaper.  That is a mag from a Glock 36SF which makes it easier to store in the gun safe.  I pretty much shoot cast bullets primarily and so far no real issues.   I wish it had a LRBHO, but that would involve buying a proprietary upper, and I just wanted to build them as cheaply as possible. Standard AR upper and Wilson stainless 10.5" barrel. I use the Kaw Valley 10 oz buffer.  Other lighter buffers caused problems in both the rifle and the pistol.