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10/30/2020 2:42:12 PM
Posted: 12/28/2003 9:08:29 AM EST
Hello All,

I apololize for the long post but feel it's best to be specific when discussing a KB. If nothing else, maybe it's interesting reading???


5 years ago my Dad had a KB with his Colt SP1 caused by a barrel obstruction.  The upper receiver broke at the square lug where the rear take-down pin inserts to hold the upper and lower reveiver together.  The Bolt Carrier Catch broke.  The bolt itself needed to be hammered out of the upper receiver which showed the extractor had been broken in half and the extractor spring had been mashed flat. Further, the 20 round magazine had been swelled to the point of breaking the welds. All in all everything other than the lower receiver and small parts were destroyed.

Current Situation:

My Dad had purchased 5+ twenty round boxes of LC tracers at a gun show this past spring. After firing a couple, he noted excessive pressure signs such as severaly flattened primer and gas leakage around the primer itself.  He decided to pull down the remaining LC tracers and re-load the bullets into his own commercial cases using a standard loading.  Unfortunately, he missed a 30 round mag which had been loaded alternating the LC Tracers and FMJs-1/5 alternate. It was this magazine which he used during our last shooting session.

My Dad currently uses a Bushmaster 16" carbine and I stopped by to do some shooting with him the other day.  As I watched his first tracer fly across the coulee I heard the remaining rounds in his magazine fall to the ground.  

As far as a KB goes, this was rather "weak". The casing showed the obvious "belting" and the primer had been blown out of the primer pocket. IIRC, the case had ruptured to the point of tearing a hole in the brass at the base of the shell.

After cleaning and inspection of the rifle for obvious cracks to the upper or lower receiver, no cracks have been found.

The magazine swelled minimally and the floor plate will go back in, however only about 50% of the retaing ears contact the floor plate so it will need to be squeezed back to spec.

The bolt, which has seen around 15,000 rounds, does not show any obvious signs of damage.  All locking lugs are present and uniform.  The extractor did not break.  Extractor spring and rubber insert are present and show no sign of flattening or stress. The ejector functions smoothly with strong spring pressure.  The bolt carrier appears to be serviceable without obvious signs of damage.  The firing pin retaining pin has a nasty mark and is bent which made re-insertion difficult so it will need to be replaced.

The lower did not crack, nor have any signs of swelling appeared such as the mag well itself. All small parts in the lower receiver appear free from cracks or breakage. Also the upper receiver is free of obvious cracks...

Barrel shows no sign of an obstruction being the culprit.  No dark spots or swelling.

My question, before my Dad takes it out and begins shooting it again, is...What am I missing???

TIA for your advise.

Link Posted: 12/28/2003 12:23:38 PM EST
Pull the barrel and check the upper barrel receiver socket for cracks in the threads/socket to upper receiver.
Link Posted: 12/29/2003 10:38:26 AM EST

I'm not ignoring your advise, it's just that my Dad has the rifle and where he's at right now I can't reach him by phone.  Hopefully I'll get a chance to look at it again before he shoots it.

I don't really have any dedicated AR tools. I'm assuming I'll need a barrel wrench, which I have for some reason. (It's one my Dad made years ago when he was young and serious about ARs) And a punch for the gas tube pin.  Also a pair of expansion pliers for the snap ring.  My only other question is, do I NEED a torque wrench for re-assembly?  What's your recommendations for a guy who hasn't "been there, done that"?

I've never dinked around with the upper receiver before.  As far as I'm concerned that's where combustion takes place, which involves fire, and my family has seemed to be more worried about mastering alcohol through the generations than fire.[beer] I've been burned and KBd myself before (Not an AR) so I try to stay away from the more critical components. But then again, I could probably save my Dad a lot of cash in repair bills if this is going to be his life legacy..."Mr. KB" has a nice ring to it.

Lastly, I would imagine checking head space after re-assembly would be necessary.  If I were to buy a "Go" or "No GO" gauge, which is more handy and or, is having both necessary?

Thanks again,

Link Posted: 12/29/2003 12:41:11 PM EST
[Last Edit: 12/29/2003 12:47:06 PM EST by MickeyMouse]
Link Posted: 12/30/2003 12:00:35 PM EST
[Last Edit: 12/30/2003 12:02:15 PM EST by I-M-A-WMD]

Thanks for the additional input. I'll be sure to check the additional points out next time I inspect his rifle.

His bolt face has what I considered "normal" pitting.  He used to shoot his brass long past the "expiration" date so to speak and I think this led to some premature pitting.  The firing pin hole was round. Also, the cam pin may need replacing.  It certainly shows apparent wear although once again, not sure if it was from the KB or useage.

I'll go to the FAQs and check around as to what tools are a necessity for getting into repair and builds. I would probably enjoy the hell out of dinking with them. I've thrown a couple lowers together but always figured best to wait until I was good and serious before messing with the top ends.

Thanks again.

Link Posted: 12/30/2003 4:29:17 PM EST
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