AR10 bolt slamming home when the butt stock is bumped

yoshmyster

I was fiddling around with my MATEN's Adjustable butt plate to my shoulder. After the adjustment I put the AR on to the floor butt first and the bolt slid home without the magazine installed. I figure about 4 inches from the floor? It won't do this with an empty magazine in the rifle but it will load with a dummy in the magazine. Not sure if it will bump fire but I don't wanna find out.

Not sure is the bolt hold open lever is not full engaging. Or the sheer weight of barrel is pushing the BCG back enough when the butt stock is bumped and making the magazine hold open lever disengage or?

I think I'll start by cleaning the bolt hold open lever (pocket, detent/spring and the lever itself) to make sure that it's free moving. Any guesses?

Horse Plains Drifter

Too light of recoil spring maybe? Rifle stock bumps the floor. Rifle stops, but bolt continues to move. It only takes a silly millimetre of movement to release tension on the bolt stop causing it to release. I haven't tried that with my M1A, but if you are changing mags, you can seat a loaded mag and then give the butt plate a hearty slap which will release the bolt.

chiefr

Check your retainer pin to make sure it is not bent or worn and the spring pushes it out firmly.  

Grunt2

chiefr said:

Check your retainer pin to make sure it is not bent or worn and the spring pushes it out firmly. 
Right on! ...FIRMLY!

bpost

That is normal with the empty mag not installed.

B17-P51

Is this a DPMS by any chance?

bustedknee

Is this a recent build by any chance?

Mobuck

Make sure the bolt catch is engaging the bolt vs snagging the front of the bolt carrier.  I had a guy return an AR for a very similar issue.  Slightly off kilter gas block wasn't gassing the bolt hard enough to get full bolt travel.

yoshmyster

Horse Plains Drifter - That was my second check. I have one of those JP Enterprises silent captured spring thing. I changed it to the next heavier spring red/blue since I don't have the heaviest red/yellow (maybe the yellow flaked off?). Oh and the BCG is the low mass but on a AR10 what is low mass? I may have to order the red/yellow from JP. Here I thought I was done ordering from them. Had I did this couple weeks back I would've ordered the silly spring.

I close the dust cover and drop the butt and the bolt moves because the dust cover opens.

chiefr - Retaining spring under the bolt catch? That's new and pristine. Do I need one that's heavier? And now the whole area there is clean. The bolt catch and the ambi magazine release might be touching slightly, though. I found this while looking for cause.

bpost - Normal without a magazine but is it normal with a loaded magazine installed?

B17-P51 - DPMS based, yes. So just be mindful not to drop the butt with a loaded magazine installed?

bustedknee - Recent build, yes.

Mobuck - How do I check that? Dekem the bolt and look for wear?

bpost

It should not close with an empty magazine installed.  Me thinks the issue is the bolt hold open system.  An empty mag pushes the hold open up with magazine spring pressure so the jar of dropping it should not release it.  The AR9 has the same issue, bounce it on the butt plate and it will release the bolt, but the bolt is very heavy and works differently than the AR10.  The AR9 does not stay open on the last shot.

Mobuck

The bolt should retract rearward well past the bolt catch. If by some combination of buffer/buffer spring/foreign material in the tube behind the buffer, the BCG doesn't fully retract and it's possible for the bolt catch to catch the front of the bolt carrier rather than the front of the bolt itself. If this is the case, very little movement/bump is required to cause the angled front of the bolt carrier to slip past the bolt catch.
I've watched guys manually retract the bolt and push the bolt catch to lock the bolt open only to have it slip and send the bolt into battery. They simply didn't pull the bolt back far enough. Also an AR that's not set up correctly will cycle and appear to operate correctly but is not achieving full bolt travel with similar results.
Manually retract the bolt as far as possible and activate the bolt catch. Look inside and see if the catch has risen in front of the bolt. Take the rifle out and load ONE round in the mag. Leave the empty mag in place and fire. Do this 3-5 times. If the bolt catch works properly every time, the system is working as it should. No need for other testing.

B17-P51

My son had a DPMS LR 308 Hunter Lite of mine and informed me that when fired with the last round of any magazine (tried several) one of these three things would happen:
1) Bolt would not lock back
2) bolt would lock back but would slam shut upon removal of the magazine
3) Bolt would lock back and stay, but slam shut upon the insertion of a new magazine.
He told me this after I had already sold the rifle, so I was not able to experiment with it.

Mobuck

Classic symptoms of the bolt not fully retracting resulting in the bolt catch hooking the front of the bolt carrier rather than the front of the bolt.

yoshmyster

bpost - Bolt does not drop with an empty magazine installed. Bolt does slam closed with a dummy round in magazine and no magazine installed.

Mobuck - I was noticing the bolt catch and the magazine release kind a rubbing when I put pressure on the bolt release (to keep it engaged) and manipulate the magazine release. Maybe a little filing on the bolt catch tab if this is the problem. I'll have to remove the magazine release and see if the bolt catch engages more? But I think the chunk of chunk of a BCG will just move enough to disengage the bolt catch. I'll try what you suggested.

B17-P51 - I'm not quite ready for the range. I don't want to be that guy at the range.

Mobuck - Me or B17-P51?

bustedknee

Bolt stop/release would be my first suspect. Check the pin and spring.

Does it move freely but not too freely.

I have had only a couple problems but they turned out to be rather simple.

I really over-thought them at first.

yoshmyster

bustedknee - The thumb side checked and cleaned. Secondary by the mag release button trigger finger side I have not.

The bolt catch free enough.

Checked how the bolt catch and BCG jived. The bolt catch is fully engaged. It's definetly the BCG being heavy and the buffer spring not being stout. The bump moves the BCG and inertia takes over. According to few of the M1A forum guys say it's how it's supposed to be. One guy told me not to bump the butt.

So any one else try what I've been experiencing with your own AR10?

Mobuck

"Mobuck - Me or B17-P51?"

I was referring to the post by B17-P51.

yoshmyster

Another reason why I never had this experience. All my ARs other than this has a Juggernaut Tactical "Hellfighter" installed (no open bolt button) so the BCG is always "home". California.