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usgi barrel?

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bumppo View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bumppo Quote  Post ReplyReply Direct Link To This Post Posted: Aug 11 2020 at 8:08pm
sorry, blackfish, I wasn't paying attention earlier.
so an Underwood barrel was installed by NPM on their own receiver, (normal business) and this product was the "original configuration", not the gun as I have it. this Underwood barrel was later salvaged from that NPM receiver and re-installed on a Standard receiver by a re-build armory, not by Standard. and the proof mark on the bottom of the handgrip - a P inside a circle - is what marks proofing of the rebuild as a whole unit, which should have included an inspection of the re-installation of the barrel, but maybe that part of the re-build's inspection was sub-par or just skipped altogether? 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bumppo Quote  Post ReplyReply Direct Link To This Post Posted: Aug 11 2020 at 8:44pm
jackp,
I can't recognize which part is being detailed in that diagram! 
unless I'm assembling some parts wrong, there must be excessive wear someplace, causing excessive looseness in the action, making it bind (and sometimes pop apart), causing a misfeed every few rounds. this much seems clear.
tomorrow I'll be following blackfish's suggestion - double-checking the barrel/receiver alignment in case re-installation of the barrel at the rebuild armory was sloppy and this sloppiness went undetected during the re-build's final proofing. barrel misalignment might be distorting the operating rod's path, causing it to dismount from the receiver's outer slide rail, allowing the bolt to over-rotate and drive its left lug into an exposed edge of the left inner rail where there's a shallow cut out probably designed to allow removal of the bolt for maintenance.  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Smokpole Quote  Post ReplyReply Direct Link To This Post Posted: Aug 11 2020 at 8:55pm
Proof rounds are fed singly and would not necessarily expose a misalignment of the barrel. They are too long to be fed from the magazine. A misaligned barrel can cause the slide to disengage and can cause misfeeds. Don't bother with the witness marks. Take 2 straight edges and place them on the bottom of the reciever and on the flat of the barrel while the reciever is held upside down in a vise. The two straight edges should be parallel to one another. Witness marks are occasionally out of line. It isn't the bolt that causes the slide to jump. It is pressure from the recoil spring. Properly aligned, it pushes the slide to the rear in a straight line. Misaligned, it lifts the handle end of the slide and causes the slide to leave the groove.
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painter777 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote painter777 Quote  Post ReplyReply Direct Link To This Post Posted: Aug 11 2020 at 8:57pm
We don't know who replaced the barrel.
Yes it started its life attached to a NPM receiver.
But has found its way to your Stan Pro.
Installed by 'Who' we don't know.
If it had been done during the Post WWII rebuild programs it would be marked which facility did it on the left side of the stock.

But we don't know if this is the original or a replaced stock. As stated earlier the Crossed Cannons stamp is a fraud.

Also Stan Pro did not put a Circled P Proof stamp on the bottom of the pistol grip.
A Circled (Sans Serif) P Proof stamp (Circle .50" ) was used on early Inlands, WRA, UEF, SG and S'G' IIRC.

Back to the glitches,
Your Op slide spring is a bit on the short side. It should measure 10 1/4".
As far as how your internals work, I think you should watch this M1 Carbine Cut Away link:

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blackfish View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote blackfish Quote  Post ReplyReply Direct Link To This Post Posted: Aug 11 2020 at 11:36pm
Here's another site with some good "videos" of the carbine's inner workings too

http://www.uscarbinecal30.com/operation.html

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painter777 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote painter777 Quote  Post ReplyReply Direct Link To This Post Posted: Aug 11 2020 at 11:53pm
Thx Blackfish,
I forgot about that one.
Much nicer than the ole grainy one I sent him.

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