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Posted: 3/1/2017 12:13:18 PM EDT
Hey everyone, I’m looking for input and advice from anyone who’s built a .40 AR-15.

What parts did you use?
What was necessary to get the gun to run reliably?
Would you do it again?

I have a palmetto state forged lower with LRBHO, but no other parts yet.  Quarter Circle 10 is the brand I’ve been looking at to complete my build, but it seems their .40 BCG is out of stock everywhere.

Thank you for your help.
Link Posted: 3/1/2017 12:42:24 PM EDT
[#1]
What style of magazines are you using? I was not successful in finding something the width of an Uzi or M16/9mm that would work with the .40 cartridge. The cartridge stack ends up half way between a single-column and an optimized double-column.

I tried IMI .41 AE mags, ProMag and IMI .45 mags, and an early Rock River .40 AR15 mag.

You can only stuff so many rounds into the mag. Stuff too many, and the rounds push outward rather than downward. This makes for a pregnant mag body which won't fit in some magwells, and will jam as the rounds feed.

Here's a picture showing how the .40s stack in an Uzi .45 mag vs a Glock mag.

Link Posted: 3/1/2017 12:55:18 PM EDT
[#2]
I plan to use Glock mags.  Here is a link to the PSA lower I picked up. It will support all small frame Glock mags.
Link Posted: 3/1/2017 1:08:07 PM EDT
[#3]
@goloud
I believe he had and sold a .40 SBR
Link Posted: 3/19/2017 10:58:56 AM EDT
[#4]
QC10 lower, barrel, and bolt. ETS mags ended up not working for me, but it runs like a champ with standard Glock mags.
Link Posted: 3/21/2017 12:19:36 PM EDT
[#5]
Awesome looking .40 Build there.
What weight buffer are you using?
Link Posted: 3/21/2017 2:55:36 PM EDT
[#6]
Thanks, I using a 9mm buffer, cant remember which one. I am getting slightly bulged cases from the bolt opening a little to quick, but I am not reloading so I don't really care. Another guy has a similar setup but with a 10.5" barrel and he is getting the same bulged cases using a 9mm buffer. So you may want to try a .45 buffer. DannerTrax
Link Posted: 4/10/2017 2:26:26 PM EDT
[#7]
I would like to build 40 S&W upper for the Palmetto lower I have. Will the Quarter Circle 10 bolt work with a Palmetto glock lower?
Link Posted: 4/10/2017 3:44:45 PM EDT
[#8]
I'm hoping so as I picked up a QC .40 bolt and barrel. I'm waiting for their upper to come in stock so I can see how it all fits together.
The QC40 bolt is cut to be used with Glock Mags and .40 Mags are small frame like 9mm Mags so in my mind it should work.
Link Posted: 4/22/2017 6:11:42 PM EDT
[#9]
Those lowers are on sale this weekend. What is the cheapest I could build an upper?
Link Posted: 5/27/2017 10:51:54 PM EDT
[#10]
joe bob outfitters,Quarter circle 10, new frontier armory ,lone wolf distributors, palmetto all show 40s&w ar stuff. I've been looking building an ar40 aswell but have questions about parts compatibility. For obvious reasons all the manufactures list their stuff as only compatible with their stuff.

Not to highjack the thread but I'm sure it would help the O.P. also. What is compatible with what and what works?
Link Posted: 6/2/2017 9:16:47 AM EDT
[#11]
Link Posted: 6/14/2017 4:02:36 PM EDT
[#12]
Update:
I have completed my build, but it is not problem free.
I ended up using all QC10 parts for the upper.
For the lower I used a Maxim defense CQB pistol brace with a JC Silent Capture 9mm buffer.
This buffer required me to remove the extra weight from the QC .40 bolt, which might be a large part of my problem.

When I manually cycle the gun using the LRBHO or by quickly racking the charging handle rounds are nose diving into the barrel or are pushed to the left of the barrel.
If I slowly move the charging handle forward in a controlled fashion everything chambers just fine.
This has lead to my very uneducated guess that the non-weighted bolt and JP buffer are moving too fast for the Glock magazine to correctly present and feed the .40 round.

My current plan is to replace the JP Silent Capture buffer with a standard buffer from Maxim defense and reinstall the weight into the .40 bolt.
I am using Glock factory 22 round .40 magazines, so another work around might be some extra strength springs for them that would feed the rounds in a quicker fashion.
Link Posted: 6/16/2017 9:15:51 PM EDT
[#13]
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