7.62x39

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LyingBastard
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7.62x39

Post by LyingBastard » Sat Jul 14, 2018 12:46 pm

More as a mental exercise than suggesting it to be done, and ignoring barrel issues for now: would 7.62x39 be a good modern caliber for the 8/81? My argument:
  • Similar to 30-30 in many ways
  • Plenty around though brass casing is not found at Wal Mart
  • Bullet diameter in the 30-32cal range
  • Maybe even the rear of the cartridge is large enough for the bolt to grab
Thoughts?

And, yes, this is on the same lines as the viewtopic.php?f=10&t=5202 thread

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81police
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Re: 7.62x39

Post by 81police » Tue Jul 17, 2018 8:39 am

It would be an interesting venture. Key would be having a cartridge to operate below the maximum chamber pressures of the cartridges of the time. You wouldn't want to hot-rod these old actions!

I haven't researched them, but 7.62x39 and 6.8SPC would be cool
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LyingBastard
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Re: 7.62x39

Post by LyingBastard » Tue Jul 17, 2018 11:26 am

I was looking at some numbers for those cartridges:

Code: Select all

                        30-30 | 30Rem | 7.62x39 vs   32Rem vs 35 Rem
Pressure, CIP Kpsi                      51.5
Pressure, SAAMI Kpsi    42      38      45              36      33.5
OAL in                  ???     2.525   2.205           2.54    2.525
Base Diameter, in       0.422   0.422   0.447           0.418   0.460
Bullet diameter, in     0.308   0.308   0.312           0.321   0.358
If the 30-30win is considered close to the 30Rem as far as powder loadings are concerned, I would think that means the performance is similar. And if people claim the performance of the 7.62x39 is close to the 30-30 (some even say it is a bit below it), it might just work. Being designed for gas operated firearms the 7.62x39 should not have a peaky curve.

I would love to find a hopeless Rem 8/81 barrel so I can use to do some thinking, but if it comes to that I have no issue taking mine apart and measuring it.

LyingBastard
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Re: 7.62x39

Post by LyingBastard » Mon Jul 23, 2018 9:48 am

Interesting: I tried to put a 7.62x39 round in the bolt and it acted like it was a bit too fat for it. But this is supposed to be a 35 remington bolt, which according to my little table has larger base/rim diameter than the x39.

I feel rather confused.

LyingBastard
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Re: 7.62x39

Post by LyingBastard » Thu Nov 29, 2018 7:18 am

Slow progress
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81police
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Re: 7.62x39

Post by 81police » Thu Nov 29, 2018 8:00 am

wow, did you make that barrel extension from scratch or partially use an old one?
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LyingBastard
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Re: 7.62x39

Post by LyingBastard » Fri Nov 30, 2018 5:13 am

Er, not quite.

The blue (supposed to be purple but hey, I am not colour blind but I sure am colour dumb :lol: ) one is the one I made/printed. As you can clearly see, it is an work in progress. But, as a first attempt, it ain't bad. The metal one is the one I used as reference to make the model the blue one was printed from.

Doing that model taught me a lot about how the bolt in the Model 8 family moves, rotates, and locks.

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81police
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Re: 7.62x39

Post by 81police » Fri Nov 30, 2018 8:04 am

Loving this, please keep us updated!
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Phyrbird
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Re: 7.62x39

Post by Phyrbird » Fri Nov 30, 2018 3:14 pm

My brothers are hooked up with some investment casters in their jukebox restoration hobby. You just might use the "print" to get a good casting to be finish machined. Nice work. Which 3D printer you use? I'd really like to find a printer for small steel parts; know what I mean?
PS I'd also like to know the CAD package & method for creating the CNC file for the print. Perhaps some collaboration could help us both??
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LyingBastard
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Re: 7.62x39

Post by LyingBastard » Sat Dec 01, 2018 5:27 am

Phyrbird, AFAIK printers that can do metal parts (sintering) are still a bit pricy. Of course what is pricy to me might be nothing to others. I know they use them in aeronautical industry but I do not know if they can produce load-bearing pieces.

Good site for that is http://www.practicalmachinist.com

With that said, a lousy little printer like mine (that can only print plastic parts) probably could be used to produce the original piece to create investment cast mould.

CAD programs are many. The gold standard is the French one, solidworks. Autodesk has a free one a lot of people like because of its simple interface and ability to do organic surfaces. But it does phone home if that matters to you. On the free side, there is Freecad, openscad (feels like a programming language instead of a normal cad), and even blender (more of a 3D program for animations than for CAD). I did the part in the picture using openscad because I can be exact and can do a bit at work without anybody being the wiser.

After the object is done it needs to be converted into what is called g-code, which is the language most 3D printers and CNC mills understand. There are programs to do that, but getting it right for a given CNC machine requires understanding said machine sometimes.

FYI, my goal is to get the model properly done and then get it to a CNC mill and feed it a block of aluminum to see if everything fits. The real thing should be closer to what they use in AR so-called barrel extensions and AK/CETME/STG57?

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Phyrbird
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Re: 7.62x39

Post by Phyrbird » Sat Dec 01, 2018 3:09 pm

Good reply, it'll take a while to percolate, for now... 8-)
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