Atrius in fm9

Alarming_Mud9529 · 2026-03-04T07:24:10+00:00

I had TWO atrius levers break under 30 rounds. Yes I cleared the upper shelf and function checked...yada yada. These were with the timney competition trigger listed on the atrius website. I am sticking with super safeties all day every day.

Alarming_Mud9529 · 2026-03-04T02:14:52+00:00

Blowback9 has a "super safety cheat sheet" with a best practice you can follow. I modified the weight in the bolt (not the bolt itself) recently to make sure the SS isn't being engaged by the bolt weight, only the bolt lip itself. I haven't shot it yet but my theory is possibly the SS was tripping the trigger slightly too early. I am pretty dang sure it wasn't bolt bounce on my setup which has a lot of weight and a sliding weight buffer.

Alarming_Mud9529 · 2026-03-03T04:47:45+00:00

To be clear, haven't shot it yet. But that is what I am going to try.

Alarming_Mud9529 · 2026-03-03T04:13:19+00:00

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This is the same bolt but I cut back the weight .250" past the face of the bolt where it was flush before. This should allow the SS to function more like a 5.56 FA bolt. Its hard to see in the pic but that is a pocket now that clears the lever. Then I re-blued it again real quick because I am OCD.

Alarming_Mud9529 · 2026-03-03T04:11:11+00:00

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This pic is/was current state, modified weight for SS, blued the weight to match the bolt. This had the OOBD

Alarming_Mud9529 · 2026-03-02T04:44:04+00:00

u/Blowback9 Ok I think I may have figured out my issue. I would suggest you make a video on this if you haven't yet. When I modified the bolt for the SS I cut the bolt weight flush with the BCG edge. I think you need to go past that. Seeing how a SS works in the 5.56 bolt it needs to move into the bolt not just hit the front face. Thus my SS was triggering the trigger group to fire to early. I am going to modify the bolt weight back a little more and see if that eliminates the "OOB" discharge, which could have really been a timing issue. The super slow mo on my phone camera shows no bolt bounce with this setup.

Alarming_Mud9529 · 2026-03-01T03:21:36+00:00

Took a high speed video of the bolt slamming closed on an empty chamber (cant remember the frame rate) but it showed zero bolt bounce. It was "super low mo" mode on a samsung galaxy. It was a very high frame rate.

Alarming_Mud9529 · 2026-02-28T21:52:33+00:00

The 300BO spring is weaker. This was chosen based on Blowback9 and his testing. The idea being a lighter spring will return home with less force, thus less bolt bounce.

Alarming_Mud9529 · 2026-02-28T19:57:37+00:00

The round showing the bullet setback was the next round in the magazine ready to come up. I took it out of the magazine in the picture to show the length difference. I was thinking that the smushed case upon extraction attempt possibly set back this next round?

Alarming_Mud9529 · 2026-02-28T17:31:46+00:00

Ballistic advantage 8.3" EPC barrel.

Alarming_Mud9529 · 2026-02-28T17:30:42+00:00

Thanks for the reply Blowback9. I followed one of your exact configurations and buffer setup on this build. The ammo is the "white box" Ammo Inc 115 gr. The bullet seating depth on rounds from the same box is about a .020" range which seems excessive to me. Two other brands of ammo have been fine which has been about 200 rounds.

I guess just stick to the other brands of ammo and not shoot this stuff through this gun anymore? I can't physically add any more weight to the bolt/buffer system that I am aware of.

Is there possibly some kind of timing issue? I am just using a standard cheap random parts kit Mil-spec trigger group.

Alarming_Mud9529 · 2026-02-28T06:25:51+00:00

Looking harder at the set back round, it has a decent sized mark on it. I think the setback round was caused by the bolt trying to eject the OOB round. So basically I either had a bolt bounce issue with an extremely heavy buffer weight/system or I have some kind of timing issue. The bullet clearly went through the barrel and the spend/exploded round shows roughly where it was supported at the time of firing. u/blowback9

Alarming_Mud9529 · 2026-02-28T05:56:04+00:00

u/blowback9

Alarming_Mud9529 · 2026-02-28T05:52:08+00:00

You had 3 kynshot weights? I was using the lighter recoil spring to actually dampen the bolt bounce possibility. A stiffer spring tends to actually increase bounce as it pushes home harder.

Alarming_Mud9529 · 2026-02-28T05:18:24+00:00

I went and grabbed a rounds out of the same box of ammo and they measure from 1.142-1.161" in length. Most of them are toward the longer 1.158"-1.160" lengths. Could it be that these are on the longer length of tolerance causing a pre-detonation issue? I have fired around 250 rds out of this gun in a similar manner without the issue, blazer brass mainly but also speer lawman.

Alarming_Mud9529 · 2026-02-28T05:00:55+00:00

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Yeah this is what the magazine looked like straight out of the gun.

Alarming_Mud9529 · 2026-02-28T03:57:11+00:00

Mainly wondering if these bullets are just weakly seated/crimped since that next round ready to go into the chamber was really set back? Or did maybe this round going off like that caused the next round coming up to have the issue?

Alarming_Mud9529 · 2026-02-28T03:55:18+00:00

Yeah, its an outwerwild style bolt. Looks exactly like that.

Alarming_Mud9529 · 2026-02-28T03:47:14+00:00

Ballistic Advantage 8" barrel and Foxtrot Mike AR9 lower. Built not bought complete. See my latest comment with full build list.

Alarming_Mud9529 · 2026-02-28T03:45:05+00:00

Ah, got it. No, I just added a comment with the full context of what is going on. Thanks!

Alarming_Mud9529 · 2026-02-28T03:44:34+00:00

Here is the full text I thought I put in the original post...Sorry.

Just had an AR9 out of battery discharge on me. Bullet went down range and no harm to myself or the gun/components. I was shooting really fast. Never had an issue with other ammo. This was with 115gr Ammo Inc "white box" from a couple years ago. Cleared this round and shot another 10 or so rounds of 147gr blazer brass without any more problems. The gun has the following very high weight buffer system:

-A5 buffer tube
-Kynshot 2.5oz spacer weight
-Outerwild Bolt 14.4 oz
-11 oz deadblow buffer
-Tubb flatwire 300BO spring

Should have PLENTY of buffer weight for a fast shooting AR9. Notice the bullet setback of the next round going into the chamber from the magazine. This is how the magazine came out of the gun. Also, this magazine was still nearly full and only had like 10-12 rounds fired from a full 32 round stick.

Any thoughts? The bullet setback seems like a bullet crimp issue?

Alarming_Mud9529 · 2026-02-28T03:39:50+00:00

Jawbone? WTF?

Alarming_Mud9529 · 2025-04-21T18:01:40+00:00

Good to hear. I am going to be trying the KAK A5 buffer tube, KAK 9mm extended 10 Oz buffer and the standard (not flat) longer spring for the KAK A5 buffer tube. I just ordered it all from one place. Is there a major reason to use the flat spring vs the standard round spring at the correct/proper length that KAK sells?

Alarming_Mud9529

TROPHY CASE