I just finished putting together a 9mm AR SBR.  It's built on a standard AR lower that I had SBR'd.  My build uses a New Frontier Armory 9mm modular BCG, Spikes T3 buffe, PSA magwell block, 4.5" KAK arms barrel and 9" YHM unltralite quad rail.  I'm using C Products 9mm Colt style mags.  I'm having a few issues and not sure if the are magblock, magazine or BCG related so looking for some input on how to address them.

Here are the issues I'm having:
-About 50% of the time the last round from the magazine doesn't feed properly.  It doesn't seem to matter if I load 1 or 32 rounds in the mag it's only the last round that doesn't feed correctly.  Here is a picute of the malfunction.


-I can't get the mag to fully seat witht he bolt closed weather it is loaded or empty.  I've tried pushing pretty had and can't get the mag to seat unless the boldt is locked back.  It seems like the mag is hitting the BCG.  Just not sure what part is the root of the problem.  Here are some pictures of the mag seated with the upper off.



-Recoil and weight:  The felt recoil was a lot more than it should be for a 9mm SBR.  It is noticeably more than my 300 BLK SBR shooting subs and I have to shoot it slower to stay on target.  I know this is problably due to the BCG and buffer going back too hard and slamming into the back of the buffer tube.  The BCG I'm using is a little over 16 oz and the Spikes T3 is 5.4 oz.  I know the buffer is lighter than most 9mm specific buffers.  Trying to figure out if I should go for a heavier 9mm specific buffer, increased power buffer spring or both?  The gun is already heavier than I would like so I'd preffer to avoid adding more weigt if possible.(For reference: It currently weighs 6 lbs 4 oz unsupressed or 6 lbs 14.5 oz with my Ti-Rant 9 an 3-lug.  For comparison my 300 BLK SBR that has a barrel that's 5.5" longer weighs 5 lbs 14 oz  unsupressed, or 6 lbs 11 oz with my Omega direct thread set up.  Do most 9mm ARs weigh as much as mine?  Is there somewhere I should be looking to cut weight?

Here is a picture of my set up.  Currently it just has a cheap red dot I pulled off a .22.  I'll put a better red dot on if I get it running well.

Do you have a spacer in the back of your buffer tube? A standard buffer will move too far to the rear and cause issues like broken bolt catches if not corrected. This could also cause feeding issues as the bolt is moving forward with more velocity. It wouldn't hurt to try a heavier 9mm dedicated buffer, there is one from New Frontier armory that is 8 oz and only like 40 bucks. I run the Spikes 9X one.

Your mag catch looks like it could be screwed in a few more turns, this might help hold the mag in when trying to seat on a closed bolt. If that does not help try another mag catch or get a 9mm dedicated one. Your C products mags look like they don't need the front of the follower to be shaved down like mine did. But check to make sure they are not hanging up on anything as that could hinder the final round or two from feeding properly. Play with the height of your mag block. It looks as though it is not 100% flush with the lower. It being higher will put the magazine that much closer to hitting the bolt.

Yes 9mm AR's are heavy and yes the recoil is more than my 300BLK also. Nature of the beast being a blowback design.

Good luck 9mm AR's can take some tweaking to get them running good when throwing parts together but are fun once they run. **Edit to answer a missed point/question.

CP mags were always hit or miss.  I guess they served a purpose for some when the original Colt (Metalform) mags were $50+.  Now that you can get Metalform mags for as low as $20 though, there's little reason not to buy them.

Metalform mags (direct, Colt, Brownells, PSA) are the original 9mm mag and much less troublesome, so you may want to try and pick up a single metalform mag and try it out in your build to see if it works without issue before you do anything else.

Or, you can search for old posts from people that went through various steps to get the CP mags to work.  I think some people ground on the followers, others tweaked/ground the feed lips, some modified their magblock or lower, and so on.

As far as recoil and weight, if you want a lighter build, you can get a lighter weight stock and/or handguard, or have the barrel profile altered, but at the end of the day, it's a blow back 9mm AR and a lot fo the mass is in the bolt and buffer.  You could also try one of the hydraulic or other specialty buffers - some say they totally change the feel of the gun.  The more you shoot with it though, the faster you should get as you get used to it.

Joe Bob's has 7.5 oz. 9mm specific buffers for $34.95.  I have picked them up for less in the past, but I $35 is the best price I've seen recently without too much searching.  I have a few, but most of the time I just use a 5.5 oz buffer and standard spring in my 9mm AR's.  I don't really notice a significant difference in feel between the 5.5 oz and 7.5 oz buffers in semi auto use.

Good luck.

1) Correct buffer weight, spring and spacer
2) Metal Form mags and don't look back
3) +10%-15% mag spring replacements on mine solved a lot of issues even with Colt stamped mags. Wolff Springs sells them in UZI 10packs and you just cut for 20s and 25s.
4) Good factory ammo. Hand loads are hit or miss around minor/subsonic load velocities.

Thanks everyone for the replies.  I'll pick up a metalform mag to give a try.  If it's a mag issue I'd rather just buy mags tat work than to have to tinker to get the C Products mags to work.  I'll also pick up a 9mm specific buffer from Joe Bobs.  Hopefully it will function well after that.  Do all direct blow back 9mm need this much mass in the bolt carrier and buffer system?  The CZ scorpions are direct blow back, how much weight do they have in their bolt carrier assembly?

Regarding the mag block bot sitting quite flush with the upper:  It's a top load mag block, it slides in from the top with the bolt catch off with two little ears to keep it from going past flush with the lower.  There isn't anything other than the mag catch to keep it from sliding up a little that I can see.  Is there a way to keep it from sliding up a little?  It doesn't come up past what's shown in the picture.

Where can you get metalform mags for $20ish?  The cheapest I can find them in stock after doing a quick search is $39.  

Lower looks out of spec to me.....Mag Adapter riding a bit high which may be cause of the issue with seating even on empty mag.  

Definitely get  9mm specific buffer and preferably one of the 7-9oz extended versions.  I prefer 9oz

Ditch the YHM Handguard to save some weight. it weighs twice what many of the lighter weight options on the market weigh.

Definitely try some new mags

The 9mm colt Ar mags at PSA are made by Metalform.  They run them on sale for 19.99 on occasion.

Currently OOS

As of right now they are in stock with the daily deal of 19.99. Get them while you can!

A blowback 9mm AR will definitely recoil harder than a gas AR in 556 or 300blk, so expect that.

Thanks, when I looked before they were out of stock.  I saw your post and just got one on order.

I have an update after making a few changes.  As several of you suspected a big part of my problem was magazine related.  I got a metal form mag from PSA and that fixed two of the issues I was having.  I can seat it fully with the bolt closed with no problem and it doesn't have a feed issue on the last round.  Looking at it compared to the two C Products mags I have it has a different follower and the feed lips at the top of the mag are a little longer and farther apart.  Here is a picture of the two side by side.



It looks like the feed lips on the C Products mag are too close together and probably hitting the BCG when I try to seat the mag with the bolt forward. I'm going to try prying them apart and see if that fixes that issue.  I tried swapping the mag follower between the mags and the feeding issue on the last round followed the orange follower from the C Products mag.  Does anyone know if I can buy replacement mag followers like the ones that are in the PSA mag?

I also ordered a I also ordered a Sprinco 308 Carbine Extra Power buffer spring(orange).  I put this in with a few quarters behind it to act as a spacer to keep the buffer from going to far.  This with my Spikes T3 buffer seemed to be a good combo of buffer and spring weight.  The recoil was noticeably softer than before and it seemed quieter.  I'm no longer unhappy with the amount of felt recoil or how it sounds suppressed.

Thanks everyone for the suggestions and input.