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IBM receiver with flat area

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PLN-TAP-7 View Drop Down
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    Posted: Apr 18 2020 at 2:58pm
Hi,
here is a picture of an IBM receiver with a flat area in front of the rear sight.
This carbine (in the 3 million so January to March 1944 number) must have been through the French Army and is not as out of factory (M2 stock, rear sight, bayonet lug).
Would someone know about the flat area? Has this specificity already been observed?
It may only be a machining error...


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Dan Pinto, Photo Editor

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Post Options Post Options   Thanks (0) Thanks(0)   Quote New2brass Quote  Post ReplyReply Direct Link To This Post Posted: Apr 18 2020 at 3:58pm
Good reporting!

To the flat spot I have observed them on IBM. I would have to look if it was in a specific range.

IBM only delivered for 9 months. they had only one serial number block  3,651,520 to 4,009,999.
They did not use all their numbers. So tough to put it to a date even though some book estimate ranges.

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Dave Tennent View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dave Tennent Quote  Post ReplyReply Direct Link To This Post Posted: Apr 18 2020 at 6:29pm
Here's another IBM receiver flat spot example.



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Post Options Post Options   Thanks (0) Thanks(0)   Quote carbinecanuck Quote  Post ReplyReply Direct Link To This Post Posted: Apr 18 2020 at 11:11pm
Strange timing for this topic. Just looking at an IBM right now. Showing a similar flat spot.
Serial range 366xxxx. 

I wanted to ask a question about the rear sight, but won't hijack this thread. Will start another
for that.

Here is a pic of the receiver for reference ....


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Post Options Post Options   Thanks (0) Thanks(0)   Quote PLN-TAP-7 Quote  Post ReplyReply Direct Link To This Post Posted: Apr 19 2020 at 3:23am
The first picture is IBM serial 3 821 XXX.
So we have 3 6, 3 7 and 3 8 example.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote carbinekid Quote  Post ReplyReply Direct Link To This Post Posted: Jun 09 2020 at 5:45pm
I just saw a Standard Products carbine with similar flat spots posted in the CMP forum.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote thompal Quote  Post ReplyReply Direct Link To This Post Posted: Jun 09 2020 at 11:46pm
Is there any significance to one flat area being behind the sight, and the other one in front of it?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Flat area on receive Quote  Post ReplyReply Direct Link To This Post Posted: Jun 10 2020 at 7:04am
interesting.
Of note is the fact that the flat area is behind the sight.
Would it be that IBM receivers ' got it in front and SP at the back only?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kro1970 Quote  Post ReplyReply Direct Link To This Post Posted: Jun 10 2020 at 10:47am
I have Standard Products 1985XXX and it has the flat areas in front and behind the dovetail.
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carbinekid View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote carbinekid Quote  Post ReplyReply Direct Link To This Post Posted: Jun 10 2020 at 10:55am
The STD. Product receiver I shared does have flats on front and back. Sorry that I didn’t point that out.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote New2brass Quote  Post ReplyReply Direct Link To This Post Posted: Jun 10 2020 at 11:51am
The following is just a guess!

They would "Gang" mill the receivers to the final profile. This would be an arbor with several profiled cutters would be sandwiched together to make the cut. The cutter in the middle may have left an odd profile when it got to the dovetail which was in excess of tolerances, or maybe it was done on separate passes and the arbor needed to get closer to the dovetail.

Maybe at some point there was a variation of the forgings and the cut would be to great so they reduced the profile

Again, just guesses!

Notice how the flat area is cut perpendicular to the longitudinal cuts of the receiver

@carbine kid, Pics?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote choprboy Quote  Post ReplyReply Direct Link To This Post Posted: Jun 10 2020 at 10:30pm
Putting my home-machinist hat on. The flat spots look to have been made on a horizontal mill with a straight wide milling cutter at a depth close to that of a finished profile. I would suspect that the machining steps to produce to top surface of the receiver would be to:
1) rough mill the top of the block with a straight cutter (making the top surface of the rear sight mount)
2) rough mill toward the rear site mount from either side with a straight cutter
3) finish mill toward the rear sight from either side with a concave cutter (to produce the convex top surface)

If the stop point of the #2 roughing pass was set to close to the sight mount, it would cut below the finished profile, leaving a flat spot in the final product.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sfal7418 Quote  Post ReplyReply Direct Link To This Post Posted: Jun 11 2020 at 5:55am
interesting study...my Standard 2003520 has no milled spot 
Al
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Post Options Post Options   Thanks (0) Thanks(0)   Quote foxhole49 Quote  Post ReplyReply Direct Link To This Post Posted: Jun 21 2020 at 12:48am
Just checked. No flat spot, front or rear, on IBM 37317xx, or on Underwood, 61272xx. Seems taht the flat spots are at random. Must be a reason for them.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote hunterman Quote  Post ReplyReply Direct Link To This Post Posted: Jun 21 2020 at 9:37am
For those keeping score, there is an IBM with the flat spot on the receiver on GunBroker now.  Serial # 3,654,3XX.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote tenOCEE Quote  Post ReplyReply Direct Link To This Post Posted: Jun 21 2020 at 6:56pm
The 3.71 I have has at least one flat in front of the dove tail. There's another one here I know but not sure what the serial is.
My sig: Seen an IP or S'G'? Add it to my registry. We'll check consecutives.
https://grandrapids.wufoo.com/forms/zzlnt0519k86xs/
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 03manV Quote  Post ReplyReply Direct Link To This Post Posted: Jun 21 2020 at 11:13pm
Originally posted by foxhole49 foxhole49 wrote:

Just checked. No flat spot, front or rear, on IBM 37317xx, or on Underwood, 61272xx. Seems taht the flat spots are at random. Must be a reason for them.

Yes, the reason is in the rough and finish milling of the receiver contour, as mentioned above.
The flat is cosmetic and a result of forging voids or milling variation. 
JMHO.Smile
Don
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Post Options Post Options   Thanks (0) Thanks(0)   Quote painter777 Quote  Post ReplyReply Direct Link To This Post Posted: Jun 22 2020 at 4:01pm
IBM Here 3,849,505 with Flat Spot on the front.
Can't see the backside because of Type II rear sight.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote painter777 Quote  Post ReplyReply Direct Link To This Post Posted: Jun 22 2020 at 6:02pm
Also IBM 3,857,1xx with Flat Spot on the front. (Not Mine)

Like the 3,859,505 posted above, both have Round Bolt with the X.

Flat spots on both nearly identical.

fwiw

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