Long/short:

BCG got stuck half unlocked while at the range the other day.

New 1-piece gas ring found to be .006 smaller in diameter than BC chamber.
Caused blowback without release of lockup, jamming rear of bolt lugs into front of chamber lugs.

Anyone else have this (or otherwise bad) experience with these rings?

H

McFarland has put out undersized batches before. In 2010, some delivered by Brownells measured .499" but they're supposed to be around .503"
http://www.m4carbine.net/archive/index.php/t-55376.html


I have personally observed problems caused by an undersized McFarland from such a batch.


I've just stuck with conventional 3-piece gas rings. They haven't given me any problems yet. I have replaced some near end-of-life, but they're consumable and inexpensive.

Yeah.
Seems every time I try something new and fancy it fails.

I'm back to basics now and refuse to change.

I bought a BCG three or four years back that was sold with the McFarland one piece gas ring as an "upgrade" feature.  I only recently, as in last summer, put it in a build.  I was shooting at the range and noticed that little by little, after a few additional shots, the bolt was not going fully into battery.  I was beginning to get light strikes and had to tap the forward assist for a proper battery.  After a few more shots, even the forward assist would not help.

I took the BCG apart, expecting to see some galling of the rings, but they looked perfect.  I couple of drops of lube and everything worked fine again.  I asked the vendor if I might have received some bad rings, and he said they were known for needing ongoing lube for break-in.

I had the same experience. Went back to the tried-and-true 3 pc gas rings and was once again trouble-free.

A solution in search of a problem.

I just thought about something and your problem could have been out of spec ammo.

If you were comparing the diameter of the rings, to the diameter of the opening in the carrier, that is not a good indication of the dimensions.  The carrier chamber has a step on the inside, that is where the rings seal.  If the rings are .006 smaller at the carrier opening, it doesn't mean anything.

Nerp.

They're too small.

They are the EXACTLY same diameter as the tail end of the bolt.

No seal at all.

McFarland "gas rings" are nothing more than Spiraloc locking ring retainers.

They are commonly used for piston pin retainers on aftermarker piston kits. First one's I saw came with a Wiseco kit for a CR250.


Last time I checked, a .500" ID Spiralock was $1.59 each, vs $3.50+ for a "gas ring".