My initial foray into building pistol caliber carbines (PCC) was a 9mm AR using Colt style magazines and another using a modified Colt lower to accept Sterling SMG magazines.
That was 3-4 years ago. Back then, PCC’s chambered in .45ACP were in the early stages of development and still had a ways to go. The two big hang-ups were the blowback designs in use required a very heavy bolt/bolt carrier to work and availability of .45 magazines that could be adapted to the AR lower.
The heavy bolt/bolt carrier magnified the recoil. And like the 9mm AR’s, there was a lot of experimenting with different buffer weights and spring rates to get the gun to function properly. Changing ammo types or adding a suppressor may require a change in buffer or recoil spring.
Magazines were another problem. Most designs used a magwell adapter that fit in a standard AR lower and use existing, readily available .45 magazines. Military Grease Gun magazines were plentiful and at one time inexpensive. Designs using Uzi .45 magazines were also available. Olympic Arms built a dedicated .45 upper with a unique magazine that basically had a magazine adapter block attached to Uzi magazines.
Fast forward to today. There
are now direct impingement designs using lighter bolt/bolt carrier designs and
dedicated lowers designed to use Glock,
H&K USC and other readily available .45 magazines. A lot more time,
research and experimentation has gone into refining the .45 AR.
What got me re-interested in a .45acp AR was an article in an old issue of Firearm News. It talked about a new PCC being built by CMMG that was neither blowback nor direct impingement. The lugs on the bolt and barrel extension were cut at an angle to cause a delay in the bolt opening until pressure was at a safe level. No heavy bolt/bolt carrier and gas is not directed into the receiver as in direct impingement designs.
The cover says CMMG’s Guard 9mm Carbine, but they actually tested the .45 Guard Carbine. Could not find this article online on the Firearms News website, but did find this article about firing the .45 Guard carbine with .450 SMC ammunition.
CMMG’s GUARD Withstands The Mighty 450 SMC
The down side (for me) was this PCC used Glock .45 magazines which slant to the rear of the gun and I just don’t like that look. Buying or building guns based on “looks” may be a stupid reason but that’s the way I am. Looks were one of the reasons I built a 9mm AR that used Sterling SMG magazines – it looked right. The other reason, Sterling 9mm magazines are the best 9mm magazines in the world; very easy to load. Anyway, further research showed that CMMG was selling their new design bolt, carrier and barrel to build your own PCC.
This was enough information to get me started. I went to the parts cabinet to see what I had on hand, then to the internet to order what I didn’t have. The first thing you will find out is the CMMG bolt/carrier/barrel combo is expensive – $374.95 and no free shipping! Shortly after receiving the order confirmation email, I received another saying it was on backorder, 2-4 weeks. Got it in three weeks. Plenty of time to round up all the other parts.
Most .45 builds use a magwell adaptor. I have always preferred dedicated lowers for each upper. Didn’t like the idea of a piece/part temporarily attached to the receiver that could fail at the worst possible time. It costs more, but that’s what hobbies are for, to drain your bank account.
There are no dedicated .45 lowers at this time other than for Glock mag’s and that’s a non-starter for me. I was able to modify a Colt style 9mm lower to take Sterling SMG magazines, so surely it would be possible to modify a 9mm lower to accept .45acp magazines as long as the magazine width wasn’t excessive.
My first though was .45 Thompson SMG magazines; readily available, not expensive (my opinion) and for their capacity, not excessively long. With an internet search, I found the dimensions for Thompson mags and the width stated was in my opinion not too wide to fit in an AR lower. I ordered two magazines, a 20-round and a 30-round. AND THE INTERNET LIED TO ME! The dimensions listed were wrong! The width in my opinion WAS too wide! Set these aside. Lesson learned.
Additional research did show some REAL gunsmiths/machinists have made lowers that will accept Thompson mags. This requires machining of the bolt carrier and upper receiver to work. Which is beyond the capability of a guy with a set of files as his machine shop.
Next up was the .45 Uzi magazine, used by several manufacturers of magwell adaptors. Ordered one. This magazine was a 22-round magazine. It was 11-¼ inches long! It stuck out of the magwell 9 inches. It looked ridiculous. In comparison, the Thompson 20-round mag is 6-1/4 inches long and the 30-round magazine is 8-5/8 inches long.
Then I found this article:
This Guy’s Guns website has an excellent article on building a .45 AR using CMMG components that in his words results in a gun that, “Doesn’t suck.”
His plan was to build a .45 upper and use it with a magazine adapter block in an existing AR-15 lower receiver. He already had one with an approved ATF Form 1 to SBR an AR-15, so he could use the .45 acp upper with the 8” barrel on the ATF approved lower without paying another $200 for the tax stamp and waiting nine months for approval.
Building his upper was simply assembling the usual parts. And was easier than a normal AR upper build since there was no gas tube to line up with the receiver through the barrel nut.
His lower was already finished. He just had to find a magwell adapter that used available magazines. There were several to choose from. He tried two different ones and chose what he considered the best.
Another advantage of the CMMG bolt design is it can handle higher chamber pressure ammunition and is rated to safely use .45 Super or .450 SMC (Short Magnum Cartridge I think). I have never heard of these two rounds.
In the website’s authors words, “Just think of it: A pistol caliber carbine launching a 200gr projectile at 1450 feet per second! That’s 933 ft-lbs of energy. That’s more than .357 Magnum energies! Most importantly, you can still shoot .45 ACP, so you aren’t tied down to one cartridge.”
A review and video of author firing his CMMG equipped AR using both .45 acp and .45 Super:
The key element I got from his article was his choice of magwell adapter and magazine. He went with the Macon Armory magwell adapter which uses a proprietary magazine. The problem with this is you can’t buy the magazines just anywhere, only from Macon Armory. And if the company goes under, you’re out of luck unless you really stock up. But the magazines are made of steel with a steel follower, so they should hold up for a long time. The Thompson SMG mags I bought were probably made during World War II. So are some of my Colt .45 1911 magazines. All are still fully functional.
The author mentions the Macon Armory magazines are “slightly” larger than the Colt pattern 9mm magazines. If I can make the slightly larger Sterling SMG magazines fit and work in a standard Colt 9mm lower, surely I can make the slightly larger Macon Armory .45 acp magazines work in a standard Colt 9mm lower. Hopefully. This could be an expensive assumption.
His plan to use an existing lower makes sense and saves money. Lots of money if you plan on building SBR’s: $200 for each gun plus cost of lower receivers and parts. You could conceivably build uppers in many different rifle calibers and .22LR, 9mm and .45acp to use on the same lower. You would have the same stock, cheek weld, trigger pull, and controls regardless of caliber. Makes sense.
But I’ve always preferred building complete weapons so I went ahead and ordered a dedicated 9mm lower receiver from Black Creek Precision in Jacksonville, FL. Same lower I used on my PCC that used Sterling 9mm magazines.
Part of researching .45 AR’s was reading everything available about them on AR15.com. All 39 pages! I noticed that the owner of Macon Armory, Rudy or the Mad Machinist, had a lot of input and shared a lot of information.
I ordered two 20-round magazines from Macon Armory. The very next day, Rudy called me and left a message. He wanted to insure I knew his magazines could only be used with his magwell adapter which was currently out of stock. He had no record of me previously buy a magwell adapter. I emailed him next day saying I was stocking up on magazines for when his magwell adaptor is available, which is my plan B. Should my 9mm lower modification fail, I will buy his adapter. I eventually order two more 20-round magazines two times for a total of six 20-round magazines.
When the magazines arrived, I started measuring. The width of the Macon Armory (hereafter called MA) magazines are slightly smaller at the back than Colt 9mm magazines. They are the same in the front. Front to rear, the Macon Armory magazines are only 0.005 inches bigger that the Colt mags.
So, it seems that I would only have to remove a very small amount of material for the MA magazines to fit. But that wasn’t true. The Colt magazines have a ridge on the back. The metal tab that trips the bolt hold open lever rides in this ridge. This ridge fits into the groove in the back of the magwell. The MA magazine doesn’t have this ridge. Measuring again showed I would just about have to completely remove the groove from the back of the magwell.
Macon Armory Magazine on left, Colt Magazine on Right
Modifying the Upper and Lower Receiver
Before starting in on opening up the magwell, I had to remove the 9mm bullet guide and the built-in 9mm ejector from the lower receiver. These are held in by very small set screws. The set screw for the bullet guide is on the front of the magwell. The same set screw that holds the ejector holds the trip for the bolt hold open. Remove this too. More about last round bolt hold open later.
The built-in ejector won’t be needed as the CMMG bolt has an ejector like 5.56mm bolts. And I’m hoping the bullet guide won’t be needed either. The CMMG produced carbine doesn’t use one.
To make the MA magazine fit, I used a flat file to file down the back of the magwell until the groove was nearly gone. This could have been easily and accurately done on a vertical mill which I don’t have (yet). I regularly checked progress by inserting the magazine into the magwell till it fit. Took about an hour and a half.
A major advantage to the MA mags, they are designed to use the standard AR magazine catch. They don’t have to be modified to use the AR mag catch like Grease Gun or Uzi mags. Just file the magwell till the magazine slides in and locks in place. Once it will, you’ll notice there is there is more play (slop) side-to-side than the Colt magazine. I guess that 0.005 difference in magazine width really does affect fit.
Turned out to be a non-problem. When the gun is assembled and the bolt is forward, the movement is reduced to what I considered acceptable.
I assembled the lower receiver to include the buffer tube, recoil spring and buffer. On the bolt hold open lever, the BCP lower doesn’t use a roll pin to hold it in place. They use a small set screw with a shaft for the lever to rotate on. Makes it easy to remove it as I had to do multiple times. On final assembly, this setscrew will get some blue Loctite
A note about the upper receiver. The ejection port on a normal AR-15 upper is not big enough to allow a .45 case to eject. Options are to use an upper for a .458 SOCOM, .450 Bushmaster, etc. or to enlarge the ejection port yourself. I have an upper with a large ejection port, but it has the forward assist and shell deflector which I didn’t want. So, I ordered a slick side upper from AIM Surplus, LLC and used the SOCOM upper as a guide to enlarge the port. About 45 minutes with a Dremel and done. I didn’t enlarge the whole ejection port, just enough at the front of the port to allow a .45 case to clear the receiver.
Fine Tuning
I assembled the barrel and upper receiver, just hand tightening the barrel nut. I installed a compensator I got from KAK Industry onto the barrel to protect the threads. Note: the threads on the barrel are .578 x 28 or 37/64 x 28, apparently normal for .45 barrels. Inserted the charging handle and bolt/bolt carrier. Assemble upper receiver to lower receiver.
The author of This Guy’s Guns article noted that there were several points of contact among the moving parts that benefit from rounding and smoothing some of the sharp edges. You will notice this the first time you manually cycle the bolt with a loaded magazine in place. Refer to his article. He has a lot of good photos.
He rounded and smoothed the lower edges on the front of the bolt carrier. The carrier I received from CMMG already had this done.
During rearward movement of the bolt, the two lugs on the bottom of the bolt were catching on the rim of the cartridge in the magazine. Minor rounding and smoothing solved this problem.
With those two areas fine-tuned, I loaded a couple rounds in a magazine, locked bolt to the rear, inserted a magazine, released the bolt. My first failure to feed. The first of many.
The issue preventing reliable feeding is the feed ramp. There’s a flat area below the feed ramp that has a sharp edge. The bullet slams into this sharp edge and stops dead in its tracks. This has to be rounded and smoothed to insure proper feeding. I would file and smooth a little, then work the bolt by hand until I could feed a few rounds without issue. Took several hours.
The bolt release in BCP’s original 9mm lower receiver design used a standard Colt 9mm bolt release. In their updated design, they use a standard AR15 bolt release. Works well with the Colt magazines. To make this work, they added a piece between the bolt release and the magazine to kinda pass on the required movement.
But to make this system work with the MA magazines, you need to either widen the opening in the back of the magazine or grind down the additional piece. Or a third choice, use a Colt 9mm bolt catch. That’s what I did. Got one from Rock River Armory for $8.50. I had to trim a very small bit (1/64 maybe) because the bolt catch was barely catching on the rim of the rounds in the magazine.
Fine Tuning the Magazines
Concerning the magazines, they are the hardest to load magazine I have ever seen. Rudy mentioned in his phone message that I didn’t buy their magazine loader and the mags were hard to load without it. He wasn’t kidding. Two or three rounds was the most you could get in without the mag loader and even with the loader, it wasn’t easy. Seemed like the mag follower was hanging up in the mag body.
I could load all 30 rounds into the Thompson mag by hand, no magazine loader, without much effort. I thought maybe the MA mag spring was overly stiff. I swapped the Thompson mag spring with the MA mag spring. No difference.
Maybe the Thompson mags were just broken in. I took one of the MA mag’s apart and rounded and smoothed the edges of the follower that ride against the mag body. I reassembled and noticed a slight difference.
I’d read where other brands of magazines were initially hard to load, but with use, got easier. I took a strip of wood that would fit in the magazine and worked the follower up and down the full length of the mag. A lot. The mag was easier to load. Not great, but better. Could get a few more rounds in before needing the mag loader and was easier to load when using the mag loader. Went ahead and did this to the number two magazine.
Final Assembly
That’s pretty much it on the build. I torqued down the barrel nut and compensator. I liked the look of the gun without a forearm, kinda like a modernized Grease Gun; but went ahead and added a seven-inch free-float forearm. Put a pistol brace on the buffer tube to keep it legal.
Photo-shopped a stock onto the finished gun (pistol) to see what it would look like as an SBR.
To actually put the stock on the gun without an approved ATF Form 1 would be a violation of the National Firearms Act of 1938 that could result in a $10,000 fine and ten years in jail. As stupid as that seems, that’s our legal system; until we can get common sense back into government.
Can you see the difference?
Off to the Range
I lubed the gun up, added an inexpensive red dot sight and headed to the range. The first trip would mainly be to check functionality. Didn’t have much choice because I left my target stand leaning against the garage.
I loaded two rounds in each of my four magazines. The first two magazines loaded the first round when I pushed the bolt release. When fired, the second round loaded with no issue. After firing the second round, the empty magazine locked the bolt back. These were the two magazines I took apart, rounded and smoothed the edges of the followers then broke the magazine in by repeatedly jamming the piece of wood in the magazine.
The two unaltered magazines didn’t do as well. The number three magazine would not load the first round into the chamber, FTF every time. If I could get the first round into the chamber and fire it, the second round would also FTF. The magazine would not lock the bolt back when empty.
The number four magazine did a little better. Some FTF’s on first round, but if I could get the round in the chamber, the second would load without issue. The empty magazine would not consistently lock bolt back.
In a later email conversation with Rudy, I mentioned that some of the magazines seemed easier to load than others. He said that they had a problem with coating on some of the magazines and lightly sanding the inside helped with the difficult loading issue. If normal usage doesn’t improve ease of loading I’ll consider sanding the insides.
Took gun home, removed barrel and worked on feed ramp some more. Also took magazines three and four apart, smoothed and rounded the follower edges, reassembled and “exercised” the magazine springs.
Second Range Trip
Reassembled and re-torqued barrel. Loaded everything into the Jeep including the target stand.
Magazines one and two worked flawlessly, feeding without issue and locking back the bolt after the last round. But not much improvement with magazines three and four. First round was jamming against the bottom of the feed ramp when I hit the bolt release. Found out if I drop a round in the chamber, let the bolt go forward, then insert the magazine, the second round would load from the magazine when I fired the first.
Main purpose was improving functionality but I was also trying to get the iron sights somewhat zeroed. It was nearly impossible to take up a steady shooting position with the long magazine. The best I could do was right arm and magazine resting on the shooting table, left hand on the magwell. Not that steady. I need to get the shorter ten round magazine Macon Armory offers.
As expected, the “pistol brace” makes a lousy stock. Fired about 75 rounds and my shoulder was feeling it at the end. No next-day bruising. It just got to where shooting wasn’t fun. I see a Form 1 application going forward eventually.
More to follow.
Useful Links
Black Creek Precision Dedicated 9mm Lower Receiver
AR15.com – Thoughts on the CMMG Guard
AR15.com – .45 acp Conversion Guide
Review: CMMG 9mm MKGS Guard DRB2