interesting receiver production in 1940

[RCS]

About a dozen or so of these M1 rifle receivers have surfaced to date, all have an extra wide rib on the left side, all are revision 1 receivers. Most have had the low guide ribs repaired during rebuild and the serial numbers are from the late 30K to early 43K range.

Photos show a 42K receiver with the extra wide rib as manufactured and never having been rebuilt.Attachment 67033Attachment 67034Attachment 67035

first photo shows both a normal production receiver and a extra wide rib production receiver.

Information

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[Bob Seijas]

Yes, all part of the frantic search for the cause of the 7th round jam. Thank you and a couple of other dedicated students for narrowing down this interesting variation that most of us would not even have noticed.

[cgroc09]

39896 , rev. 1 , welded wide rib
40664 , rev. 1 , low wide rib
40796 , rev. 1 , welded rib , Navy conversion rifle
41131 , rev 1 , welded rib
42427 , rev. 1 , welded wide rib
42561 , rev. 1 , low wide rib
44393 , rev. 1 , welded extra small rib
44582 , rev. 2 , welded extra small rib, Navy conversion rifle

40782 is listed as a Navy conversion rifle but I do not have information as to what ribs this receiver has or what revision .

[Bob Seijas]

Interesting that they seem to have narrowed part of the problem down to guide ribs but they were still shearing them off.

[RCS]

The first receivers to surface with the extra wide rib were 36733 which had the correction during rebuild and 37964 (my friend in Maine owns this one) which is still original with the low extra wide rib. All the heat lots are REP with varied numbers of 5 thru 8. There are also receivers manufactured at the same time without any extra rib and with the same heat lot codes. All are revision 1.

More confusion is found with the revision 2 receivers which have either no extra rib or a extra "narrow" rib. The bolt recess started to change from square to round

[Bob Seijas]

I'm guessing the wide rib would not have helped much if it was sheared at the top. Clearly the width of the rib was not the problem, and they probably realized that after testing them. One thing that made it tough to find was that not all rifles experienced the jam, most did not. So you make some receivers with a wide rib and they don't jam... aha, we found it! Then one of them does jam. Damn!

[Bob Seijas]

I'm pretty confident I have deduced the cause of the infamous 7th round jam that plagued early rifles. Start with the fact that only a small percentage of rifles experienced it, most did not even though they had sheared ribs. The Model Shop rifles had low ribs, too, and they worked fine. The problem only surfaced well into production.
Second, Garand's Trouble Shooter Art Tuttle told me the real problem was a stack-up of tolerances in the follower. All parts had an acceptable variation from the actual drawing specification, and some machinists were not as precise as others. When a follower was at the maximum allowable slop and was installed in a receiver that was also slightly off spec, the sheared rib allowed the clip to wobble when it was almost empty, resulting in a jam.

The answer, then, is that a properly machined follower could tolerate a sheared rib and still function, that's why most did not jam. Fixing the rib allowed a sloppy follower to work reliably, and the problem was solved.

[RCS]

Attachment 67092

After Springfield adopted the revision 4 follower with later slide in mid 1940, it continued in to 1941.

Winchester started production with the un-marked revision 2, then their own variation of 45 degree front and back slide and then the short nose slide which continued into the war years. There were three different slide variations used by Winchester in 1941 as shown in this photo.