View Full Version : Bolt cam tension
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?
Has the rifle previously run? Have you replaced any parts? What buffer/gas system combination are you using?
dwalker460
11-23-2012, 18:53
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?
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.
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
11-23-2012, 20:14
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.
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
11-23-2012, 22:41
Well there is the issue, AR's are not meant to be piston weapons.
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......
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.
Now I'm mildly confused. The gas rings don't serve their normal purpose since this is a piston rifle, right?
Great-Kazoo
11-24-2012, 21:13
You have a ZM, very nice.
Now I'm mildly confused. The gas rings don't serve their normal purpose since this is a piston rifle, right?
They don't but evidently you still need the tension from the gas rings to keep the bolt from camming over too soon.
They don't but evidently you still need the tension from the gas rings to keep the bolt from camming over too soon.
My PWS runs without any gas rings. Interesting.
Kaiser.Shooter
11-24-2012, 23:20
No gas rings needed in a piston AR, most likely it's a carrier drag if you see the upper wear. If that's an issue POF has a solution for it POF-USA .223 Roller Cam Pin:
http://www.emergingtacticalsolutions.com/images/128911272872269344802.jpeg
No gas rings needed in a piston AR, most likely it's a carrier drag if you see the upper wear. If that's an issue POF has a solution for it POF-USA .223 Roller Cam Pin:
http://www.emergingtacticalsolutions.com/images/128911272872269344802.jpeg
That is a great idea. I will order one of these bad boys.
Kaiser.Shooter
11-26-2012, 21:43
If you would like to give one a try you are welcome to borrow it. I'll go someday this week to BLGC so you can join me or test it wherever you can
If you would like to give one a try you are welcome to borrow it. I'll go someday this week to BLGC so you can join me or test it wherever you can
Thanks for the offer. But I just ordered one and should get it some time next week.
Kaiser.Shooter
11-29-2012, 16:09
Right on! I use them in all my piston AR's.
Another good add to prevent carrier drag would be a bolt spring sold by adams arms or buy one at ace hardware (compression spring #6) cut it shorter, remove the gas rings and install the spring.
While I haven't tried it, I believe my Sig716 user manual states the gas rings aren't necessary.
Sounds like some piston guns need them for proper timing, some don't.
Right on! I use them in all my piston AR's.
Another good add to prevent carrier drag would be a bolt spring sold by adams arms or buy one at ace hardware (compression spring #6) cut it shorter, remove the gas rings and install the spring.
I really like the spring idea. I will get one at ACE and give it a try.
Kaiser.Shooter
12-07-2012, 14:14
Please post an update how your rifle will run after new bolt cam pin and spring installed
I didn't get a chance to test this bad boy out this weekend. I was too busy freezing my ass off. Maybe next weekend will work out better.
I didn't get a chance to test this bad boy out this weekend. I was too busy freezing my ass off. Maybe next weekend will work out better.
I heard there was enough hot brass out there on the comp pistol berms Saturday to keep everyone warm. [AR15]
The ZM guns are a modified DI gas system not a piston gun . May be worth a call to them before monkeying with the gas rings . Pretty sure they are still there by design . Al Zitta is a 3gun shooter and I would expect would be quite helpful .
I FINALLY got this issue figured out. It ended up being the alignment of the barrel in the upper receiver. When the barrel was installed and barrel nut tightened the alignment pin was mashed into the left side of the alignment notch of the upper receiver. The barrel was basically twisted counter clockwise slightly in the receiver. I removed and reinstalled the barrel in the proper alignment with the pin at 12 o-clock and it has been running like a champ ever since. I am glad that I got it figured out, but it sucks to have wasted so much time and $$ on replacing stuff that would have never fixed an improperly installed barrel.
I also wanted to add that I did work with Al Zitta (Mr ZM Weapons) on troubleshooting this and he was super helpful.
Kaiser.Shooter
04-02-2013, 14:47
I'm glad you made your carbine to work properly! So it's not a piston system, right?
I'd buy the roll cam pin if you don't need it, LMK
You are correct and much to my surprise its not a piston system. I am going to leave the bearing roller cam pin in there because its "Cooler" than the stock one :)
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