380 AR

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Archibald Stanton

Archibald Stanton

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It’s not a typo, I was not talking about 308.

I can’t imagine the people in here dying from laughter at the title. Once it is out of your system, hear me out. There may be a thread on this already but I could not find it using the search forum. I am sure every single person thinks this is an idiotic idea but I have my own reasons for wanting to do it. That being said, I am welcome to any and all ideas on the best way to go about building an AR in 380 acp.

yes, I know that 99.99999999% of responses will say just do it in a 9 mm. I already have plenty of guns in 9 mm and plenty of ARs in other calibers. yes, I am fully aware that I could just reload 9 mm down to 380 levels but that is not what this is about. This is just a project for my own satisfaction.

it could be that not a single person on this site even wants to attempt seriously answering this question about what would be involved in actually building it but if anyone is, I would love to hear your thoughts. I am assuming most of it will be built on an AR 9 platform.

i would assume the bcg would need to be different and the barrel would need a different twist. Maybe 1:10?

thanks in advance for anyone wanting to look at this with real ideas
 
At .380ACP pressure, I'd guess you could use a 9mm bolt without issue, and the extractor would probably work just fine. You would need a new (or set back) barrel with an appropriate chamber, and you might need to block the magazine at the rear for feeding.

Hey, it's not the craziest idea of the day!
 
edwardware said:
At .380ACP pressure, I'd guess you could use a 9mm bolt without issue, and the extractor would probably work just fine. You would need a new (or set back) barrel with an appropriate chamber, and you might need to block the magazine at the rear for feeding.

Hey, it's not the craziest idea of the day!
Click to expand...
Do you think a 380 would generate enough pressure to use the 9mm bolt? I would think there might not be enough but I am not sure. What about the buffer and spring size?
 
Archibald Stanton said:
Do you think a 380 would generate enough pressure to use the 9mm bolt?
Click to expand...
Without question, the mass and spring rate will need to be retuned for .380ACP. . . but the spring rate is an easier place to start, so start there. Likely you'll end up modifying both.

I'm assuming this will be in a blow back (unlocked) 9mm system. If you plan to start with a locking/gas operated system. . . well, best of luck. Pipsqueak cartridges don't shine in gas operated actions.
 
I think you could reduce AR PCC bolt mass and spring load to get a .380 to eject; after all, there are untold numbers of .380 blowback pistols that work just fine.

The first thing I don't know but you should find out is whether the bolt face and extractor reach of a 9mm P bolt will hang on to the smaller .380 rim, .374" vs .394".

What I think the big obstacle will be is a magazine. Will a 9mm P magazine work? Can you put a spacer in a 9mm P magazine to make it work?

Just for fun and lacking a PCC, I got out a couple of 9mm P pistols and gymnasticated them with .380.
They hand fed from a Glock magazine which is pretty much the default for PCCs.
The 9mm Glock extractor pulled them out, too.
They also slurped right through a 2011 even though that is a .38 Super length magazine.

So it looks like you are ok on the magazine and probably on the bolt face.
 
Jim Watson said:
I think you could reduce AR PCC bolt mass and spring load to get a .380 to eject; after all, there are untold numbers of .380 blowback pistols that work just fine.

The first thing I don't know but you should find out is whether the bolt face and extractor reach of a 9mm P bolt will hang on to the smaller .380 rim, .374" vs .394".

What I think the big obstacle will be is a magazine. Will a 9mm P magazine work? Can you put a spacer in a 9mm P magazine to make it work?

Just for fun and lacking a PCC, I got out a couple of 9mm P pistols and gymnasticated them with .380.
They hand fed from a Glock magazine which is pretty much the default for PCCs.
The 9mm Glock extractor pulled them out, too.
They also slurped right through a 2011 even though that is a .38 Super length magazine.

So it looks like you are ok on the magazine and probably on the bolt face.
Click to expand...

I was more worried about the bolt. I know of several 9 magazines that will work for it so it’s good to know the bolt might work as well
 
I played with 380 caliber for a special purpose application.

I wanted an automatic to allow really low cost & high volume “just for fun” target shooting:
  • A barrel at least 3.2” long for a decent sight radius and less muzzle blast
  • Able to use easy to acquire range pickup brass. This comes down to 9mm, 380, 40 & 45.
  • The ability to function leading free with tumble lube bullets no heavier than the Lee 102 RN (mine drop at 0.359” & 108 gr). This rules out 40 & 45.
I felt the 380 was much more likely to give me leading free shooting with tumble lube. I also liked the idea of the reduced muzzle blast. Again, this is just for making holes in paper.

I bought a HiPoint 380 but was not entirely satisfied. It took quite a bit of work to get it to cycle ok (still not great) and I just could not shoot decent with the trigger pull. While researching my feeding issues, I found that the HiPoint 380 is really just their 9mm gun with a different chamber and the black coating machined off the rib on each side of the slide. Same magazine, frame, slide & springs.

I knew I could make the trigger “better” but really did not want to do the required modifications to this relatively “new” gun. I felt that the less than great feeding was at least partly due to my “short” 380 ammo in a gun that was really set up for the length of 9mm ammo.

I decided to double down. Based on my research, I purchased a fleabay HiPoint 380 parts kit (everything except frame and mag) for around $80 plus $15 shipping and had my son 3D print a “LoPoint” frame. I bought a couple of extra mags from the factory. I made a chamber reamer and converted the barrel to 9mm. I assembled the gun and had lots of problems (the “LoPoint” has lots of room for improvement). Although it had many problems, I noted real quick that the trigger felt much better. I slowly modified the frame as needed to work through all of the problems. Now I am rather pleased. It feeds good (much more reliable than the 380), it cycles with a Promo charge of 2.65 gr (right at 380 power level) that give zero leading with the BLL coated Lee 102 RN, the trigger has a much better feel, and I shoot it better than any of my son's 9mms that I had been shooting before I got this gun.

My cost for the ammo is right at $0.025 each with the light bullet (assuming $1.10 per pound even though I am using range pickup), low charge of cheap powder ($16 per pound from a 8# jug bought ~4 years ago) and reloaded primers ($0.002 each).

And if I want, it also shoots good using my full power 9mm ammo with powder coated Lee 120s.

From my experience, a blowback 9mm loaded down to 380 power levels left me pretty happy. I think a 9mm blowback AR can probably be converted to 380, but here are some things to consider:
  • Get and/or assemble a 9mm AR.
  • Try loading some 9mm to 380 power before making any mods. If needed play around with buffer weights and/or springs before making any other changes. You should be able to figure out what it takes to get good function pretty quick and easy.
  • Make up some dummy 380s. See how well they feed from the magazine up into the chamber. Play around with this for a while to confirm feeding reliability is good enough.
  • If the 380s do not feed as good as the 9s, think long and hard about about going the next step. Just sticking with low power 9mms might just work better.
  • A 9mm AR-15 barrel can probably be modified on a lathe to remove just enough (the length of a 9 case minus the length of a 380 case) from the rear to leave you with a 380 chamber. Next the rear of the barrel could be turned down to remove the existing "ring" (material that is clamped by the barrel nut), this area threaded, new material screwed on, and then machine for a new "ring" that is is forward of the original ring by the same amount that was removed from the rear of the barrel. The chamber will be a "sloppy fit" for 380s, but it should work. Reloading brass fired from this chamber might not last very long.
  • Alternately, just buy a barrel blank and have it machined to the external dimensions of a 9 mm barrel, but chamber it for 380. This will give you a real 380 chamber.
 
Last edited:
P Flados said:
I played with 380 caliber for a special purpose application.

I wanted an automatic to allow really low cost & high volume “just for fun” target shooting:
  • A barrel at least 3.2” long for a decent sight radius and less muzzle blast
  • Able to use easy to acquire range pickup brass. This comes down to 9mm, 380, 40 & 45.
  • The ability to function leading free with tumble lube bullets no heavier than the Lee 102 RN (mine drop at 0.359” & 108 gr). This rules out 40 & 45.
I felt the 380 was much more likely to give me leading free shooting with tumble lube. I also liked the idea of the reduced muzzle blast. Again, this is just for making holes in paper.

I bought a HiPoint 380 but was not entirely satisfied. It took quite a bit of work to get it to cycle ok (still not great) and I just could not shoot decent with the trigger pull. While researching my feeding issues, I found that the HiPoint 380 is really just their 9mm gun with a different chamber and the black coating machined off the rib on each side of the slide. Same magazine, frame, slide & springs.

I knew I could make the trigger “better” but really did not want to do the required modifications to this relatively “new” gun. I felt that the less than great feeding was at least partly due to my “short” 380 ammo in a gun that was really set up for the length of 9mm ammo.

I decided to double down. Based on my research, I purchased a fleabay HiPoint 380 parts kit (everything except frame and mag) for around $80 plus $15 shipping and had my son 3D print a “LoPoint” frame. I bought a couple of extra mags from the factory. I made a chamber reamer and converted the barrel to 9mm. I assembled the gun and had lots of problems (the “LoPoint” has lots of room for improvement). Although it had many problems, I noted real quick that the trigger felt much better. I slowly modified the frame as needed to work through all of the problems. Now I am rather pleased. It feeds good (much more reliable than the 380), it cycles with a Promo charge of 2.65 gr (right at 380 power level) that give zero leading with the BLL coated Lee 102 RN, the trigger has a much better feel, and I shoot it better than any of my son's 9mms that I had been shooting before I got this gun.

My cost for the ammo is right at $0.025 each with the light bullet (assuming $1.10 per pound even though I am using range pickup), low charge of cheap powder ($16 per pound from a 8# jug bought ~4 years ago) and reloaded primers ($0.002 each).

And if I want, it also shoots good using my full power 9mm ammo with powder coated Lee 120s.

From my experience, a blowback 9mm loaded down to 380 power levels left me pretty happy. I think a 9mm blowback AR can probably be converted to 380, but here are some things to consider:
  • Get and/or assemble a 9mm AR.
  • Try loading some 9mm to 380 power before making any mods. If needed play around with buffer weights and/or springs before making any other changes. You should be able to figure out what it takes to get good function pretty quick and easy.
  • Make up some dummy 380s. See how well they feed from the magazine up into the chamber. Play around with this for a while to confirm feeding reliability is good enough.
  • If the 380s do not feed as good as the 9s, think long and hard about about going the next step. Just sticking with low power 9mms might just work better.
  • A 9mm AR-15 barrel can probably be modified on a lathe to remove just enough (the length of a 9 case minus the length of a 380 case) from the rear to leave you with a 380 chamber. Next the rear of the barrel could be turned down to remove the existing "ring" (material that is clamped by the barrel nut), this area threaded, new material screwed on, and then machine for a new "ring" that is is forward of the original ring by the same amount that was removed from the rear of the barrel.
  • Alternately, just buy a barrel blank and have it machined to the external dimensions of a 9 mm barrel, but chamber it for 380.
Click to expand...
A lot of perfect ideas, thank you
 
Contact any custom barrel maker and order a short chambered 9mm. Specify what you plan to do with it.
9mm AR bolts are AFAIK push feed which means you NEED a .380 chamber. I can say for certain, .380 will not headspace on the extractor and fire.

I would expect a 9mm bolt/buffer to work fine with .380 blowback.

A .223 Rem bold modified to be permanently in the lock position would be a simple way to get a .380 comparable breach face, unless you were comfortable welding/pressing a 9mm bold down.

Pressing a ring in would be easy enough but getting the extractor to work would be more difficult. A .223 bold already fits a .380 case.

9mm barrels are already slightly long. Maybe a .223 Rem bolt pinned permanently extended mated to a milled down 9mm barrel after reaming could work.

.380 magazines that feed is another issue. .380 is a straight wall case, so most 9mm mags will be too wide.
 
The original Colt 9mm SMG buffer is 5.5 ounces and I know the CMMG buffers are also 5.5 ounces. This is the minimum buffer weight for a blow back 9mm AR setup. A lot of guys run a buffer weighing between 7.5 and 8.5 ounces in their 9mm AR's.

A 380 AR does sound neat. I would first try a 5.5 ounce 9mm buffer first.

A H3 carbine buffer will weigh between 5.4 and 5.5 ounces (same as a Colt or CMMG 9mm buffer), the H2 weighs 4.6 to 4.7 ounces and the H (or H1) buffer weighs 3.8 ounces, and a standard buffer weighs 2.9 ounces. Now you will see + 0.1 ounce difference when weighing multiple buffers from different manufacturers (on average).
 
12Bravo20 said:
The original Colt 9mm SMG buffer is 5.5 ounces and I know the CMMG buffers are also 5.5 ounces. This is the minimum buffer weight for a blow back 9mm AR setup. A lot of guys run a buffer weighing between 7.5 and 8.5 ounces in their 9mm AR's.

A 380 AR does sound neat. I would first try a 5.5 ounce 9mm buffer first.

A H3 carbine buffer will weigh between 5.4 and 5.5 ounces (same as a Colt or CMMG 9mm buffer), the H2 weighs 4.6 to 4.7 ounces and the H (or H1) buffer weighs 3.8 ounces, and a standard buffer weighs 2.9 ounces. Now you will see + 0.1 ounce difference when weighing multiple buffers from different manufacturers (on average).
Click to expand...
I have a few different buffer weights I can try. The only extra springs I have are fairly heavy tension so I would need to go lighter I think
 
Archibald Stanton said:
Do you think a 380 would generate enough pressure to use the 9mm bolt?
Click to expand...

I do, might have to remove some weight but I would try it without first.

The 380 is slightly smaller OD than 9mm but they appear to be just fine in the 9mm double stack double feed mags. Might have to load them a little long if you run into feeding issues.
 
jmorris said:
I do, might have to remove some weight but I would try it without first.

The 380 is slightly smaller OD than 9mm but they appear to be just fine in the 9mm double stack double feed mags. Might have to load them a little long if you run into feeding issues.
Click to expand...
I'm gonna try the 9mm bolt as is first and see how it goes
 
Archibald Stanton said:
I'm gonna try the 9mm bolt as is first and see how it goes
Click to expand...

Let us know how that works out. The 9x19 rim diameter is 0.392" while the 380 rim diameter is 0.374". I know that to a lot of people 0.20" doesn't sound like much of a difference but it is quite a bit of difference and one might have issues with the 9x19 extractor grabbing onto and holding 380 cases.
 
berettaprofessor said:
Or...just go here: https://www.armorydynamics.com/Armory-Dynamics-380-ACP-Glock-AR-Bolt-Carrier-Group-BCG_p_681.html

Bolt carrier $158.99, 10 inch barrel $136.99. They've even got a glock-style AR lower for $299...says it works with any standard AR bupper.


Or buy it complete; only $13K: https://enterbras.com/index.php/en/carbine-caliber-380-ar-15-33-shots-detail
Click to expand...
Unfortunately I have been on that waiting list for a while and have zero hope anything will come up. I even tried to send them an email directly and I got a reply back saying their inbox was full. It might be different if I went off site to send it but when I tried to send it through their site, that was the message I kept getting. So there must be thousands and thousands of unanswered emails sitting in their inbox. That alone would make me nervous about using them, even if they did reply
 
Last edited:
12Bravo20 said:
Let us know how that works out. The 9x19 rim diameter is 0.392" while the 380 rim diameter is 0.374". I know that to a lot of people 0.20" doesn't sound like much of a difference but it is quite a bit of difference and one might have issues with the 9x19 extractor grabbing onto and holding 380 cases.
Click to expand...
That is what I am most worried about but I am still going to try it
 
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