Author Topic: Different than expected issue got me a different than expected outcome..oh yeah  (Read 374 times)

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Offline M1A4ME

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Well, every now and then I get a failure to chamber in a couple of my AR15s.  None of the others ones has had this happen.  Just a couple of the 18" builds.  Well, technically I did have a failure to chamber issue about 15 years ago on a carbine build, but a set of RCBS X dies and a bolt change cured that rifle.

Anyway, in between range sessions and other work my mind wondered if it was the new Lee press.  Turret with aluminum die plates.  I got to wondering if something was moving, giving, stretching during the resizing step.  No issues on pistol cases - but those are straight walled, or slightly tapered (9MM).

I decided to do a couple things.  One was hold back/save every cartridge that had an issue on one of those rifles till I had a few.  The other was to dig out the old Wilson case gauge I bought for that carbine issue 15 years ago. 

Did one other thing, just for the heck of it - ordered a 6X47 resizing die.  Figured it the issue turned out to be something with an occasional case not being fully resized I could use the 6X47 resizing die to resize the loaded round since the same case length/diameter/shoulder as the .223 die would resize the case while the 6X47 die would not contact the neck or disturb the bullet diameter/shape.

So far I've only collect a couple non-chambering rounds.  Dropped them into the case gage and got a nice "clink" sound but them did not fully drop into the case gage leaving the base and rim sticking up above the top of the case gage.  Thought it was a incomplete resize issue and ran them through the 6X47 resizing die.  Wiped off the lube, grinned and tried them in the case gage.  Same clink, same failure to drip down flush with the top of the case ga. = smile gone.  Darn, so what is the issue with the case gauge?

Grabbed the calipers and finally got tired of trying to find a meaningful difference from cartridges that would fit and the two that would not fit into the case ga.

Ran the two offenders through the 6X47 resizing die again, wiped the lube off and they still would not fit into the case ga.

Finally grabbed the round that would fit and (who know why) dropped into into the case ga. base first and it fell into the gauge.  Then I grabbed the two that wouldn't fit into the ga. and neither one would drop in base first.  It was the base/rim that was off, not the case walls/shoulder/neck.

So, how does the rim being off keep a round from chambering?  Only way I can see is if the bolt face on those two rifle are tighter than usual.  Have not yet grabbed three or four ARs out of the safe and removed the bolts to measure the diameter across the bolt face inside.

Oh, chucked the two offenders into a drill, grabbed a piece of 220 grit sandpaper and spun the rims on the sandpaper till they would also drop down into the case gauge base first.

Assumed/guessed.  Spent some money on a die I might not have needed (unless I bulld a 6X47 AR15.....)  Spent some time trying stuff till I found something different by accident (dropping the cases in the case gauge rim first.)

Brass was a mix of different headstamps (commercial and surplus from here and other countries) loaded for plinking.

Now, might grab the two 18" barreled AR15s that were showing the problem and see if those rounds will now chamber (quicker than measuring bolt faces but I still want to do that, too.)  Never heard of that being an issue on AR15s but who knows.
I stopped carrying the SIG 556R.  SIG changed models and couple/three times and stopped supporting it with parts.  So, I stopped supporting SIG.  Back to the tried/true AR15 Carbine.

Offline Wobbly

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So, how does the rim being off keep a round from chambering?  Only way I can see is if the bolt face on those two rifle are tighter than usual.  Have not yet grabbed three or four ARs out of the safe and removed the bolts to measure the diameter across the bolt face inside.

Quite a lot of the Mil Spec surplus brass has been fired from machine guns which sometimes call for a higher loading. When any round is fired at, or above, SAAMI pressures, the head and rim of the case expand. Or, it could be that one of the chambers is simply out of spec and the brass expanded same as above. Or, both reasons.
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