Popped Primers and a Stuck BCG

[theJanitor]

So i went to the range to verify the zero on one of my rifles and after 75 rounds or so, had a stuck BCG, It would positively not move forward or backward. So after using a brass punch and a hammer to move the carrier back, to eject a spent case, and forward again to separate the upper/lower, and back again to get the carrier out of the upper; i find a primer stuck in the cam pin cavity. it prevented the pin from rotating, essentially freezing the carrier in the upper.

Then, after checking to make sure nothing else was damaged in the upper, BCG, barrel, i reassemble and shoot some more rounds. 50 or so rounds later, the carrier won't go into battery. i take everything apart again, and another primer is floating around in the FCG (i assume, restricting the hammer and preventing movement of the carrier).

Details:

ammo: XM193 stamped LCO8
lower: LMT Defender2000, mil-spec trigger
upper: LMT with Noveske 13.7 barrel
bcg: Young MFG

[sinister]

This topic has gone round-and-round for years.

Barrel makers have been putting out rifles and carbines for decades with either a short-throated .223 Remington chamber as opposed to a military 5.56mm chamber (actually the throat, but some also have chamber diameter variances).

Most commercial .223 Remington throat leades are rather short, anywhere from 2.440 to 2.450 long. Colt 5.56mm military throats are fairly long at 2.550 inches. Military match throats are a long 2.500 inches to handle single-feed 80-grain Sierra bullets.

Use a short commercial throat with military spec ammo (M193 or M855) and your chamber pressure goes up when the bullet starts right on or jammed into the lands. With a slightly longer throat the bullet gets a "Jump" or "Head start" as it leaves the case mouth before the bullet starts engraving into lands at the throat leade (pressure doesn't start to build until the bullet ogive starts engraving at the bore's stricture at the lands).

Too-high pressures as carbine bolt carrier group extraction begins while chamber pressures are peaked cause the case to fail at the weakest point -- the primer fitted into its seat. As the bolt unlocks and extraction begins (while case walls are still obturated against the chamber walls), pressure forces primers out of the case head even past military primer staking as soon as the case head leaves the support of the carbine's bolt face -- and the primers go flying into a carbine's upper or lower receiver (most are not fortunate enough to have the things eject overboard and out the ejection port).

This makes for some interesting stoppages with blown primers anywhere within the voids of the upper or lower receiver/trigger mechanism.

Simplest fix is ream the chamber to longer specs (2.475 to 2.550 inches). Ned Christiansen markets a tool, and others like Sully will also do the work in their shop. It's literally a five-to-ten minute fix.

[Molon]

This topic has gone round-and-round for years.

Barrel makers have been putting out rifles and carbines for decades with either a short-throated .223 Remington chamber as opposed to a military 5.56mm chamber (actually the throat, but some also have chamber diameter variances).

Most commercial .223 Remington throat leades are rather short, anywhere from 2.440 to 2.450 long. Colt 5.56mm military throats are fairly long at 2.550 inches. Military match throats are a long 2.500 inches to handle single-feed 80-grain Sierra bullets.

Use a short commercial throat with military spec ammo (M193 or M855) and your chamber pressure goes up when the bullet starts right on or jammed into the lands. With a slightly longer throat the bullet gets a "Jump" or "Head start" as it leaves the case mouth before the bullet starts engraving into lands at the throat leade (pressure doesn't start to build until the bullet ogive starts engraving at the bore's stricture at the lands).

Too-high pressures as carbine bolt carrier group extraction begins while chamber pressures are peaked cause the case to fail at the weakest point -- the primer fitted into its seat. As the bolt unlocks and extraction begins (while case walls are still obturated against the chamber walls), pressure forces primers out of the case head even past military primer staking as soon as the case head leaves the support of the carbine's bolt face -- and the primers go flying into a carbine's upper or lower receiver (most are not fortunate enough to have the things eject overboard and out the ejection port).

This makes for some interesting stoppages with blown primers anywhere within the voids of the upper or lower receiver/trigger mechanism.

Simplest fix is ream the chamber to longer specs (2.475 to 2.550 inches). Ned Christiansen markets a tool, and others like Sully will also do the work in their shop. It's literally a five-to-ten minute fix.

Excellent information as allows sinister, unfortunately, it doesn't apply in this case. Noveske barrels use either a 5.56mm NATO chamber or the Noveske NMm0 chamber which is designed to handle the heavy OTM NATO pressure loads, even on full-auto in hot environments The more likely source of the problem is the rejected, out of spec XM193 ammunition.

[theJanitor]

yeah, i'm aware of how that happens, but i'm not about to ream out a noveske barrel yet. just wanted to share my experience from this weekend. this barrel has gone through a thousand rounds of 5.56 or so without a problem.

[Molon]

yeah, i'm aware of how that happens, but i'm not about to ream out a noveske barrel yet. just wanted to share my experience from this weekend. this barrel has gone through a thousand rounds of 5.56 or so without a problem.

Shoot some MK262 through your barrel and see if you have any blown primers. I'm betting you won't.

[theJanitor]

If I had Mk262.:( all i have left that's 5.56 is green tips

[sinister]

The only way you're going to know for sure (without sectioning the barrel) is to pull a bullet and use a Stoney Point guide to check bullet depth and verify if the bullets jam against rifling leades.

I found even with good, high quality match bullets (like Hornady 75s) there's enough case lot-to-lot difference that you may go 500 to thousands of rounds with the same load, then find the ogive has changed. This happens with different bullet making machines when they change bullet dies.

[Ned Christiansen]

Couldn't help but notice the title of the thread. If it says "popped primer" I'm interested.

As to chambers and my reamer-- there is something about my reamer that may cause a little confusion, and I'm trying to be pro-active in letting it be known-- my reamer gives a chamber that is 5.56 NATO "plus" if you will.... the freebore is longer than a NATO chamber, the throat angle is the same. I mention this because you can drop my reamer into a gen-u-ine 5.56 chamber and get some chips, a few. But it doesn't mean it was a .223 chamber.

I have been working on another solution for some time and next week or so it will finally be available-- not a solution really but a means of determining WTF chamber you have without having to make a chamber cast or buying a bore scope.

It looks like a headspace gage but it's not. It is made to gage only that which is forward of the shoulder-- if you have a 5.56 NATO chamber, it will drop in and "clink" off on the chamber's shoulder, and drop back out freely. If you have a .223 or other smaller-than-5.56 NATO chamber, it will try to wedge itself into the shorter throat. It won't get locked in there or damage your short chamber-- if you don't do something like try and hammer it in.

It's tapped 8-32 on the back end. Not sure how I'm going to do this part yet, but I'm thinking it'll either come with an 8-32 set screw locked in there so you can use a pistol cleaning rod, or it'll just come with its own 8-32 threaded rod.... or maybe an option for one or the other. If somebody wants to have one tapped 8-36 to use with a military cleaning rod-- I think there's an adaptor out there somewhere.

I've wanted for a long time to offer guys a "figure out which" option for under $50 as opposed to the "buy the reamer for $250 and just do it " approach.

[topraider]

Can u post the lot # for the XM193 that u fired.

[theJanitor]

sorry, can't say for sure, i emptied the individual boxes into a can and was shooting out of that. all of the boxes were discarded a couple of weeks ago.