Any and all help is appreciated guys...
I have a CMT M16/Full Auto BCG purchased with and ran in a CMT upper (Gasbuster CH, CMT lower parts) for about 750 rounds.  After months of admitted neglect I attempted to field strip the BCG the other night, and after removing the retaining cotter pin found that my firing pin WILL NOT drop free.  I've hit it with a ton of Break-Free Powder Blast and it will not give way even under force.  Also when the retaining cotter pin is re-installed the bolt will not travel all the way to the rear (cam pin gets about 2/3 of the way home).  Can it possibly be that gummed up or is something more sinister at play?  I have had it apart and cleaned once before after around 500 rounds so I haven't been a complete tool regarding maintenance.  Can't say I have ever heard about this issue and was hoping for some solid advice.  Is letting it soak overnight and keeping my fingers crossed the only option?  I've had constant problems with this build that all center around a first-round feeding issue.  When loading the weapon (either dropping the bolt or cycling the CH) the round being fed will intermittently and randomly hang up.  Most of the time it requires just a tap of the forward assist and sometimes not even that.  This was all the source of another troubleshooting thread at least 6 months back that continued for six pages or so and never was resolved despite my best efforts and some excellent input from folks way more knowledgeable than I in these matters.

I thought the source of my problems was the barrel and/or chamber, since the build originally started with a Giffmann Group Buy barrel.  I had that swapped out (by Giffmann) for a Sabre barrel of the same 1/7 twist rate with M4 feed ramps.  Multiple lowers were tested and the only thing left to speculation without being swapped out at the time was the BCG and the upper receiver.  The problem frustrated and disheartened me enough to put the project on the back burner, not being able to get my first AR working properly was pretty discouraging.  The upper sat around awhile, admittedly dirty and neglected, until I got another complete rifle together.  This second build is a factory complete Sabre upper on another lower I built with CMT internals.  I include all these relevant details to stress that this has been a problematic build from the get-go, one with a randomly intermittent feeding issue of the first round that clearly does not involve any of the usual suspects.  The gun feeds and fires every round under gas impulse.  Even though I may be a learning novice to AR's, I am not riding the charging handle or fudging my builds.  The second build works FLAWLESSLY and everything in the first build has been looked over and triple-checked to the best of my modest ability.

I decided before ordering an extra BCG for the sole purpose of testing to try swapping the CMT BCG originally in the rifle for the Sabre from the factory complete upper.  The problem still occurs with random frequency doing dry runs in my basement, as I'm sure it will while testing at the range tomorrow.  Only after the first round is chambered and firing commences does the rifle feed flawlessly.  Most importantly, and my reason for posting tonight after being bummed out that swapping the BCG seemed to have no effect, is that before now I had never been able to notice any unusual scratching, wearing, or other damage that the experts were saying to keep an eye out for.  While doing dry runs I slapped a mag to ensure it was fully seated, and to my suprise the first round of the mag came flying out of the ejection port when I slapped it.  That's when I noticed that my repeated feeding runs had mangled the right feed lip of that particular mag, especially on the rear corner.  As soon as I saw that I thought "This one bad mag has been the source of my hangups!"  Then I realized I had made a point of checking and testing 13 different mags during the original troubleshooting thread, and that the one in particular that was just mangled had previously been New In Wrapper.  Of the other 13, only one showed any similar damage and that was not nearly as bad.  So now, if you are still with me and I haven't rambled incoherently too much, the story ties together into something quite multifaceted for any troubleshooting gurus out there.  Are these all signs of a bent upper or a FUBAR BCG?!?!  Allow me to summarize:

-Problematic build from the start.  Sticky first round feeding issue.  Operates flawlessly under gas impulse.  Ruled out riding the charging handle.  Swapped barrels, lowers, and magazines, tried different ammo types, performed visual inspections throughout process.  Posted a previous thread, hell even featured video of the problem!

-Got bummed and let it sit after about 750 rounds of testing with several cleanings of the internals but only one field strip and cleaning of the BCG around 500 rounds.  Let it sit for a few months.  Now when the firing pin retaining pin is removed the firing pin still feels rock solid, as if it were fused to the bolt.  Bolt will fully seat to rear with full travel of the cam pin with the firing pin retaining pin removed (obviously) but not once it is re-installed.  Can a few hundred rounds of gunk and a few months of neglect do this or is it possibly the source of earlier problems?

-Noticed unusual damage for the first time tonight after swapping in a different BCG.  During repeated cycling of the action to test for feeding issues, the right feed lip of a brand new mag was mangled bad enough to allow it to send the 28th round flying out the ejection port when doing the standard smack test.  Is this evidence of a culprit to this mysterious issue?  Is the mag body trashed now?  I imagine having the dust cover closed when that happened could get annoying.

Thanks for reading and for any input.

-

Well, let's see if I understand this correctly. When charging the rifle with either the charging handle or bolt catch, it hangs up on the first round. You've swapped:

1. Bolt carrier assemblies.

2. Complete lower receiver assemblies.

3. Magazines.

and the problem still remains. You say that a little tap of the forward assist usually closes the bolt home. How far back does the bolt hang up? If it is just barely back from fully locked, then I'd say to check the gas tube/gas key alignment. If it is back a lot farther than that, it might be an issue with the upper receiver itself. Does the bolt carrier bind any in the upper? Are you keeping the bolt carrier lubed enough?

Since you've stated that you've tried different lower receiver assemblies, we should be able to rule out a weak buffer spring and the bolt carrier hanging up on the hammer. I am stumped, with no other ideas than the ones I mentioned in the paragraph above.

As for the stuck firing pin, you may have to soak the bolt in Kroil overnight. I ALWAYS clean my rifles the same day that I shoot them.

Quoted: Well, let's see if I understand this correctly. When charging the rifle with either the charging handle or bolt catch, it hangs up on the first round. You've swapped: 1. Bolt carrier assemblies. 2. Complete lower receiver assemblies. 3. Magazines. and the problem still remains. You say that a little tap of the forward assist usually closes the bolt home. How far back does the bolt hang up? If it is just barely back from fully locked, then I'd say to check the gas tube/gas key alignment. If it is back a lot farther than that, it might be an issue with the upper receiver itself. Does the bolt carrier bind any in the upper? Are you keeping the bolt carrier lubed enough? Since you've stated that you've tried different lower receiver assemblies, we should be able to rule out a weak buffer spring and the bolt carrier hanging up on the hammer. I am stumped, with no other ideas than the ones I mentioned in the paragraph above. As for the stuck firing pin, you may have to soak the bolt in Kroil overnight. I ALWAYS clean my rifles the same day that I shoot them.

Thanks for the feedback M4Madness.   You are correct in review and the bolt hangs at what seems to be first encountering the bullet in the magazine or the magazine itself.  I did a bind test with the BCG and upper in the previous thread, noticed no hangups and had free travel.  I have experimented with lubrication...bone dry, sloppy wet, and lubed to specification all seem to have no bearing on the randomness of the matter.

We can definitely rule out a weak buffer spring.  As for the carrier hanging up on the hammer,  I do keep consistently  getting wear marks on my hammer after a couple of hundred rounds but can also duplicate the problem on an unfired factory built CavArms MkII with an unnotched hammer so I have chalked those up as normal with some reservation.

A bent upper (how rare is this?) or simply a matter of lubrication?  I cannot believe the latter with all the time, attention, and energy I have devoted to troubleshooting and providing info on this matter.  I've run it so dry some of you would blame that alone, so sloppy wet that at the end of shooting I would blame it myself.  My love of shooting is too firmly entrenched to be besieged, but I'm ready to chuck this build out the window.

Look at the front of the gas tube where it enters the gas key. If it is dinged or shows wear, that's the culprit. Usually easy to remedy with a slight pressure on the tube in the direction needed for alignment.

Hope it's that simple.

Tack

derrrrr..brain fart. Sorry

Quoted: Quoted: -Noticed unusual damage for the first time tonight after swapping in a different BCG.  During repeated cycling of the action to test for feeding issues, the right feed lip of a brand new mag was mangled bad enough to allow it to send the 28th round flying out the ejection port when doing the standard smack test.  Is this evidence of a culprit to this mysterious issue?  Is the mag body trashed now?  I imagine having the dust cover closed when that happened could get annoying. This one has me a little concerned, Is the mag in the mag well so loose that the bottom of the mag can be cam’d back towards the butt stock enough to actual make contact with the bottom of the carrier?  If so, then the first mag feed could be as simple as a low feed out of the mag, and the rifle self-loading with the assistances of the recoil cam’g the front of the mag upwards for a clean feed. Take a look at the upper receiver for the tell tail signs of bullet tip strikes under the feed ramps for signs of low mag feeds.

Dano523, what's got me stumped is that he says he has tried multiple lowers with the same poor results.

Bent firing pin, smashed gas rings, out of spec carrier and or bolt?  Remove extractor-is there an o-ring on the extractor spring causing too much extractor pressure, try removing it if so.  Keep looking, you can figure it out.