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TSMG Machining question


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Inside my M1 receiver just behind the magazine slot there is an oval shaped depression machined into the interior top surface. It does not appear to have any function. What is it for?

Edited by StrangeRanger
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It was machined there so when the tooling used to machine the radius on the front of the receiver where the bolt enters would clear the top of the receiver.

 

That's pretty much what I thought it might be. Thanks for the confirmation

 

I also recall seeing a narrow channel running the length of the receiver as well on some M1 receivers. The AOC M1's seem to have a more pronounced oval at the front location when compared to Savage plant M1's.

Mine is a Savage and Got Uzi's M1A1 is a Bridgeport but the cuts are very similar. Without actually measuring, his appears to be very slightly deeper

 

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7) BREECH ENTRY COUNTERSINK

This is a custom made, piloted, HSS, 50 degree included angle countersink designed to be driven by an arbor through the receiver pilot hole. This cutter is used to form the cone at the front end of the bolt channel on all Receivers which guides the bolt shank into the breech.

From Doug's catalog, page 40:
It's the clearance channel for the breech entry bevel cutter which is driven through the spring-pilot hole with a 1/4 extension....Phil
Edited by anjong-ni
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The pilot hole on an M1 is at a different height from the one on a '28. The tool that works on one won't work on the other.

 

On the M1 it is in line with the bore, on a '28 IIRC (don't have one to measure) it's .10" lower. Sounds like the tool you're describing is for an M1. Its greater proximity to the inside top surface of the receiver must be what necessitated the relief cut for the tooling.

 

That brings up another question: how did they machine the feed ramps on a '28?

Edited by StrangeRanger
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  • 2 weeks later...

Can this oval shaped depression machined into the interior top surface be of any benefit for running your gun dirty?

Probably not. Most of the crud seems to accumulate in the area of the feed ramps and is blocked from getting into the main part of the receiver by the wall at the rear of the mag slot - the one through which the front of the bolt passes

 

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