AR-9 Strange 'Push' feel to recoil

MindTheGAP · Aug 15, 2019

Hello all,

I have a low round-count AR-9 I built a couple years ago, probably around 500-1,000 rounds between each full teardown / cleaning.

I've gone slightly longer on a FULL teardown this time, about 2,000 rounds or so, but still do the FCG and barrel / BCG.

What I've started to notice is that some (~50%) of my shots, not correlated to weapon warmed / cold, feel as though there's almost a delay to the BCG moving back after firing..the best way I can describe it is it feels less like a snappy recoil and almost like it's a push-back, with a noticeably slower delay between bang / BCG blowing back.

I will be tearing it down to components and cleaning the inside of the buffer tube this week, but my concern is there's something wrong with the lock time in some fashion. If it matters, it does this whether suppressed or not, though I wish I'd paid more attention to notice if it did it less due to more back pressure when running a can.

Any ideas from the collective?

GuitarmanNick · Aug 15, 2019

I am fairly new to the 9mm AR but there is no lock up on the bolt. Sounds like something is dirty or needs lubrication. If you have used any steel cased ammo, check for varnish build up in the chamber. Inspect it for any unusual marks.

MindTheGAP · Aug 15, 2019

You are correct, there is no lock up. I was wondering if I'm getting undue friction somewhere causing it to 'delay' slightly. I also literally have never fired steel case ammo through it (don't like what it does to the chamber over time) so I'm not sure. My next step is, once it's fully broken down, to throw a bore brush in a drill and hit the chamber.

cstone · Aug 15, 2019

Any new shiny areas on the bolt or carrier?

Blow back weapons are simple but in the AR they can be picky with ammunition, buffer weight, and buffer springs. As you noted, not cleaning them over time will cause the bolt movement to slow down a bit.

I normally strip the BCG down and throw it into the ultrasonic every so often. Hit the chamber with some Hoppe's and let it sit for ten minutes and then scrub with a nylon toothbrush. When I am shooting my PCC ARs I will lube the BCG at the range if it seems like it is getting dirty. Once I got the buffer/spring sorted out, mine have run like a sewing machine.

Be safe.

MindTheGAP · Aug 16, 2019

I'll be hitting the chamber tonight and let it soak a bit. Also the inside of the buffer tube was pretty dirty, likely due to using the can for about 500 consecutive rounds. Thanks for the tips.

danb · Aug 16, 2019

500 consecutive rounds is a very short trip to the range.

MindTheGAP · Aug 27, 2019

danb said:
500 consecutive rounds is a very short trip to the range.

Your helpful input has been noted.

alucard0822 · Aug 27, 2019

Extremely common for 9mm bolts to bind in the buffer tube, especially if there is a tension adjustment screw in the lower, or fouling/excess grease, I would check that first, along with anything that might have come loose, put back wrong, or got inadvertently bent, like the ejector. The bolt should move freely when putting upward or downward pressure on the barrel to show any misalignment in the receiver/tube/bolt. If everything is lined up, and the FCG or ejector isn't binding then timing is largely a function of bolt/buffer mass and spring rates, the expanded case in the chamber under pressure is what basically keeps the bolt closed, it won't release until the pressure drops sufficiently, usually as the bullet leaves the barrel. There can be issues with jamming and insufficient bolt mass or spring weights, or issues with out-of-spec chambers, but that shouldn't affect a gun that was fine before taking it apart to clean it.

MindTheGAP · Sep 4, 2019

alucard0822 said:
Extremely common for 9mm bolts to bind in the buffer tube, especially if there is a tension adjustment screw in the lower, or fouling/excess grease, I would check that first, along with anything that might have come loose, put back wrong, or got inadvertently bent, like the ejector. The bolt should move freely when putting upward or downward pressure on the barrel to show any misalignment in the receiver/tube/bolt. If everything is lined up, and the FCG or ejector isn't binding then timing is largely a function of bolt/buffer mass and spring rates, the expanded case in the chamber under pressure is what basically keeps the bolt closed, it won't release until the pressure drops sufficiently, usually as the bullet leaves the barrel. There can be issues with jamming and insufficient bolt mass or spring weights, or issues with out-of-spec chambers, but that shouldn't affect a gun that was fine before taking it apart to clean it.

Great advice, I'll swap in a few different buffers / springs I have. IIRC, I'm using a Wolff AR-10 high power spring with a tungsten 8oz buffer, and I do in fact have a tension adjustment screw in there so I'll swap that out too.

Thank you so much for the input, will try returning everything back to as basic as possible.

alucard0822 · Sep 4, 2019

MindTheGAP said:
Great advice, I'll swap in a few different buffers / springs I have. IIRC, I'm using a Wolff AR-10 high power spring with a tungsten 8oz buffer, and I do in fact have a tension adjustment screw in there so I'll swap that out too.

Thank you so much for the input, will try returning everything back to as basic as possible.

No problem, have built and worked on enough AR9s, there really isn't a "mil-spec" standardization with 9mm dedicated receiver sets. Bolts tend to be oversized, and sometimes buffer tubes are just slightly out of alignmennt, especially under the stress of recoiling. Have one I built where the QC10 lower, slick side upper and KAK bolt bind up whenever the tension screw is tightened, probably a slightly out-of-spec takedown pin hole, or possible buffer tube bore/threads in the lower, swapping to about 4 or 5 other brands of 9mm bolt have ) issues, and that bolt has 0 issues in another AR9. Otherwise the binding has almost the exact feel you are describing.

Magnumite · Sep 5, 2019

What is the tension screw being mentioned?

alucard0822 · Sep 5, 2019

Magnumite said:
What is the tension screw being mentioned?

some lowers have a nylon tipped screw to "tighten up" the gap with the upper, it is adjusted to put a little upward pressure on the rear takedown pin lug, problem is, if anything fits a little loose, it puts the bolt path in the upper out of alignment with the buffer tube in the lower, and being 9mm bolts tend to be larger in diameter in the rear compared to 5.56 bolts, that slight misalignment can cause it to bind in the buffer tube where it wouldn't with a 5.56 bolt, and probably wouldn't be as bad if there was a little slop between the receivers.

MindTheGAP · Sep 5, 2019

Okay so - update.

I broke the lower down entirely, checked the bolt over for any wear spots indicating some kind of repeated wear / bind, and removed the tension screw. I also (though I struggle to envision how it would impact this issue) removed the trigger takeup screw and swapped it for a regular grip screw / bolt just to be safe. Last step was doing a full cleaning of the barrel face / chamber and bolt + checking the height of the ejector (since the AR-9 lower in question is the kind with the two screws pinning it into the lower rather than integrated into it.)

Upon reassembly / oiling (also had wiped the inside of the buffer tube clean) it seems to have completely eliminated the problem through 250 rounds of testing, so now will come the laborious task of single-factor checks. First one I'll be checking is the tension adjustment screw, as based on your advice, I'm willing to bet that's what was causing the issue. I had another couple ar-9 bolts I could test but since I wasn't seeing any undue wear on the bolt itself, I figured I'd keep the variances as low as possible. Bigger thing this is reiterating to me is the value of high-end, matched receiver sets (for lack of slop / not needing the rubber plug / tension screw).

Magnumite · Sep 5, 2019

Thank you for the response and excellent explanation and pic.

AR-9 Strange 'Push' feel to recoil

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