Bolt cam tension

[CHA-LEE]

I think I have a bolt cam timing issue on my AR. The failure happens during the chambering of a round. The bold carrier will not go all the way forward acting like the bolt is caming over and trying to lock too soon. When I take the bolt carrier out and look into the upper I noticed that the leading edge of the notch in the upper that is suppose to clear the bolt cam pin has some wear, like the bolt cam pin is coming over too soon before its in the notch. When I take the bolt carrier out and manually move the bolt in and out there is very little resistance. I think that the three gas rings are worn out and not creating enough friction on the bolt carrier allowing the bolt to start to cam over to the locked position before the bolt snaps up against the breach face and engages the locking lugs.

So to make a long story short, how much tension should the three gas rings create resisting the in/out movement of the bolt within the carrier? As it currently is I can push and pull the bolt in and out of the carrier with very little effort. I am assuming that there needs to be more friction resisting front to back bolt movement to keep the bolt from moving back until the locking lugs are engaged. But I am an AR novice and don't know how it should really feel. What do you guys think?

[20X11]

Has the rifle previously run? Have you replaced any parts? What buffer/gas system combination are you using?

[dwalker460]

If you can pull the bolt out til it stops, turn the BCG upside down and place the bolt face on your table and the bolt does not slide in, your gas rings are likely OK. If the bolt slides into the BCG on its own or at the slightest touch, then you need new gas rings.

How many rounds has the BCG seen?
If you hand cycle the rounds does the bolt close properly?
is this a new build?
Can you post pics of the wear you have observed?

[ray1970]

I've experienced worn gas rings and they didn't cause the issue you are describing. They simply started creating short cycling problems on my rifle.

[275RLTW]

Throw another bolt in it & see if this resolves the issue. There shouldn't be an issue with the cam pin or the bolt as they should go hundreds of thousands of rounds if decent quality. If you have excessive wear on the upper lugs of your receiver then that is an issue I've never seen before and would probally require a call to the mfg. I'm not up to speed on gas ring tension but gas rings are cheap. If you doubt them, go buy 2 sets (1 for a spare) and see if the problem is resolved with new ones.

[BPTactical]

DWalker is spot on as far as testing the ring tension. Cam pins can wear, does yours have a ridge from where it has been riding?
If the bolt carrier cam slot is worn(unlikely)it will be quite obvious, it will appear peened.
Make sure the lugs in the barrel extension are in good shape and clean, I have seen chunks of a primer get in behind the lugs and prevent it from rotating properly, preventing lock up. A burr built up on them will do the same thing.
Make sure the gas tube is in good shape, not burred or bent and the gas key is properly tightened as well.

[CHA-LEE]

The AR associated with this issue is a ZM Weapons Piston AR. It does not have a "Normal" bolt carrier as the bolt carrier does not go back into the stock. Since it is a piston driven bolt there is no gas that gets all the way back to the bolt carrier like a normal AR.

There is virtually no bolt tension within the bolt carrier. I did a ghetto rework on the three gas rings to expand them a little bit. This created more bolt to bolt carrier tension and it takes more effort to move the bolt back and forth. I am going to test fire it tomorrow to see if it makes any difference. If it does make a difference then I will order up some new gas rings and see if that is a long term solution.

[dwalker460]

Well there is the issue, AR's are not meant to be piston weapons.

[CHA-LEE]

Well there is the issue, AR's are not meant to be piston weapons.

I will keep your comment in mind in the event you need some assistance with something some day......

[CHA-LEE]

I tested it out today with the readjusted gas rings (more friction) and it worked flawlessly. Looks like I need to order up some new gas rings to solve this issue for good.