9mm Build Likes It Hot

[John Hearne]

I have a RRA 9mm upper that sits on a brand new Stag Arms lower with a Hahn bottom load magazine block and 9mm buffer. The bolt was ramped by a local black gun gunsmith. Prior to this incarnation, the upper was on a FA FN lower from work.

I've noticed that the upper, regardless of which lower its on, only runs 100% with 9mm NATO spec loads. It seems that the hotter the round is, the more reliable the weapon is. Worst luck has been with Winchester 147gr JHP. These induce classic short stroking symptoms. 115gr FMJs only fail occasionally but when they do, it looks like short stroking. I'm trying to get some +P+ JHPs and see if they run to verify my theory.

I was wondering if there's any way to make the gun more reliable with lower pressure rounds. I'd love to be able to run on it on Winchester White Box. I was wondering if I just need to break the gun in more or possibly ditch the 9mm buffer and try something a little less heavy.

[ST911]

I saw this problem in a batch of Colt SMGs. Gas port and buffer tweaks were required for their load(s).

[Robb Jensen]

I have a RRA 9mm upper that sits on a brand new Stag Arms lower with a Hahn bottom load magazine block and 9mm buffer. The bolt was ramped by a local black gun gunsmith. Prior to this incarnation, the upper was on a FA FN lower from work.

I've noticed that the upper, regardless of which lower its on, only runs 100% with 9mm NATO spec loads. It seems that the hotter the round is, the more reliable the weapon is. Worst luck has been with Winchester 147gr JHP. These induce classic short stroking symptoms. 115gr FMJs only fail occasionally but when they do, it looks like short stroking. I'm trying to get some +P+ JHPs and see if they run to verify my theory.

I was wondering if there's any way to make the gun more reliable with lower pressure rounds. I'd love to be able to run on it on Winchester White Box. I was wondering if I just need to break the gun in more or possibly ditch the 9mm buffer and try something a little less heavy.

Are you using a buffer spring that's stronger than a normal CAR buffer spring (like a David Tubb/ISMI or Wolff)? Try an almost worn out CAR buffer spring that's on the edge of spec (no shorter than 10 1/16") Also Randall of www.ar15barrels.com and Ken Elmore of www.specializedarmament sell a plastic insert for 9mm buffer springs which help 9mm ARs to run better (it goes on the tail end of the spring opposite of the buffer).

Winchester White box is terrible as of late (it was never really good). Try MagTech if you can find it or American Eagle.




I saw this problem in a batch of Colt SMGs. Gas port and buffer tweaks were required for their load(s).

Colt SMGs don't have gas ports............they're blowback operated.

[John Hearne]

The recoil spring is the "9mm Carbine Recoil Spring" as offered by Brownells. I don't know if it's a special spring or not. I have a conventional carbine buffer spring that I can try and see if it matters.

[Robb Jensen]

The recoil spring is the "9mm Carbine Recoil Spring" as offered by Brownells. I don't know if it's a special spring or not. I have a conventional carbine buffer spring that I can try and see if it matters.

Since they're marketing it toward 9mm it sounds like it's an extra power one. I would say try the standard power CAR buffer spring that you have.

[AR-15A3]

I agree try a standard or almost worn carbine recoil spring first and see if it will improve.

I've got an all RRA 9mm A4 carbine with top load Hahn mag block and it runs fine with different brands of FMJ rounds, but my reliability on hollow point bullets are not predictable and depends on the shape and how large the hole of the hollowpoint.

Mine also runs well with 9mm NATO surplus ammo.

[ST911]

Colt SMGs don't have gas ports............they're blowback operated.

Doggone it. And the ones I was thinking of weren't SMGs, but 5.56 SBRs. Fingers faster than brain today. Disregard last.

[John Hearne]

Interesting update. Took my 9mm AR back to the range. Shot a mix of 147 gr JHP, 100gr frangible, and 115 gr Winchester FMJ. Before the range session, I generaously lubed the bolt with Slip 2000. The setup ran perfectly for the session. Apparently, the gun was a too dry at the first session, CLP was the lubricant then.

[The Archangel]

I just got back from the range and put 250 rounds of UMC 115gr. FMJ and had a double feed rate of 1 in every 12-15.

My set up is:

RRA 9mm Upper Receiver
5" TROS 3-Lug Barrel
AAC Phantom Suppressor
Modifed 25 Round Mags
DD 7" Rail

I put a decnt amount of CLP on the bolt and carrier the night before. The DF / FTF starting to happen after the first magazine.

Maybe it was the ammo, but I'm open to any input on getting this thing running 100%.

[John Hearne]

What are you using for a magazine block?

[The Archangel]

What are you using for a magazine block?

RRA.

Anyway, someone suggested that I try Winchester White Box (I know some are against this brand because of QC, but I had absolutely no malfunctions with it so far.

The culprit turned out to be the UMC ammo which seems to be underpowered.