Does a bolt properly engage the barrel extension when the upper is fully assembled?
Not to take this thread too far off track, but I've heard that the anodizing on 1911 aluminum feed ramps helps protect the softer metal underneath. Wouldn't this also be the case if one took a dremel to the upper in question? If the upper's ramps are what's off, light grinding looks to be the easy fix, but would it cause more problems in the long run? Just curious.
He followed up in post 7:
https://www.m4carbine.net/showthread...89#post1881189
Granted, he could have embedded them rather than linking the way he did.
The upper in the OPs pictures is indeed factory assembled. All the forge codes and codes on the bolt are of a matching early 2000s upper.
"Not every thing on Earth requires an aftermarket upgrade." demigod/markm
From my understanding, a factory built Colt upper will have a small proof mark on the upper next to delta ring on the ejection port side. Here's a pict on a A2 upper, in the mark closer to the ejection port door. On the M4 uppers, it should be in a similar location. Check both sides of your upper.
coltproof.jpg rbfdc4.jpg
(Images from Urban_Ops from a post over at ARF)
I've built over 100 uppers using individually sold Colt M4 uppers receivers (meaning not disassembled from complete uppers) and none of them had the VP ("Verified Proof" from a test fire) proof mark. All the factory built Colt uppers that I have (several) and upper receivers that I've built from "pull downs" all had VP proof marks.
Last edited by recce556; 03-23-14 at 16:18.
Thanks for the help so far everyone.
To answer some questions:
Does the bolt engage the extension?
I don't have a ton of experience on the AR15 platform, but based on a Colt AR15A4 I had for a bit (not any longer), the bolt seems to engage like it should. The bolt lugs go through the barrel extension just fine and the BCG shoves forward into place as it should.
Canted FSB?
Near as I can tell, the FSB is straight and true.
Marking @ chamber swell:
If I get a chance I'll look at it, though again I am inexperienced so it's a bit difficult for me.
If your bolt is engaging then it's just poorly cut feed ramps on the receiver. At least you know now your fix will be cheap, good luck.
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