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Arisaka T-99 Mystery (Read 1268 times)
psteinmayer
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Arisaka T-99 Mystery
Mar 28th, 2017 at 4:26pm
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Ok, I have a Toyo Kogyo Series 31 that gets shot on a regular bases in CMP Vintage Matches throughout the year. Last year, I had an interesting phenomenon where I had burned discoloration of the upper portion of some of my cases. I posted this in a Jap Rifle forum last year when it happened. My load at the time was a Hornady 174 gr .3105 dia. FMJBT bullet over 40.0 grains of 4064 and a CCI-200 primer.

Going with some of the advice I received, I changed to a slightly hotter load and more accurate bullet thinking that my case necks may not have been expanding enough to seal. The new load is a Hornady 174 gr .312 dia. HPBT Match bullet over 42.0 grains of IMR 4064 and a CCI-200 primer. This proved to be considerably more accurate! However, things have gone from strange to downright weird. My son shot it on Saturday with the new load for the first time and here's what we have now:

I'm no longer getting that burned appearance on the upper portion of the case and neck. Now, I'm getting a considerable amount of carbon on the rim, head, and bolt head. I have no flattened primers, no pierced primers, no bulging cases, no split cases, no cracked case necks... in short, I have no indication what-so-ever of where this carbon is coming from. My only guess is it's leaking past the shoulder... but I have nothing on the upper portion of the cases. When my son shot it on Saturday, he gave me no indication that there were hot gases blowing out of the receiver either. After cleaning and inspecting the rifle, the bolt and chamber both look fine! Everything appears to be normal. Could it be that I'm moving the shoulder too far back when resizing, and the cases are not fire-forming to the chamber? I'm sizing the same way I have for several years, and this only started last year with the original problem. Dies are LEE Pacesetter 7.7x58 Jap. Brass is both PPU and Norma, and the results are the same for both.

I welcome all discussions
  
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AFJuvat
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Re: Arisaka T-99 Mystery
Reply #1 - Mar 28th, 2017 at 6:22pm
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My gut feeling is to start looking for a crack or a pit in the chamber.
  
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Parashooter
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Re: Arisaka T-99 Mystery
Reply #2 - Mar 28th, 2017 at 11:49pm
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Cures for "blow-by" -

Anneal case neck/shoulder.
Seat bullets deeper.
Faster powder.
More powder.
Longer/heavier bullets.
Flat-base bullets.
Trim cases as long as chamber will allow (by chamber measurement*, not "book" dimension).

Basic problem is that the bullet "uncorks" the case before neck/shoulder has expanded enough to seal chamber. We can speed up expansion or slow down bullet exit to cure it.

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*Expedient method to determine chamber length. Fired case seats buckshot into chamber, leaving impression of mouth.
  
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psteinmayer
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Re: Arisaka T-99 Mystery
Reply #3 - Mar 29th, 2017 at 2:20pm
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Thanks very much Mike and Para!

I'm going to do some of Para's suggestions.  I'm also going to size a hair longer to move the shoulder out a hair because I feel like it may be sized too deep.  I know that some Arisaka's have a generous chamber size (my Nagoya Series 7 allows neck expansion to a ridiculous size, making resizing a real challenge), and that may also lend itself to this.

Re-anneal.
Seat slightly deeper (and also crimp too).
Check chamber length and adjust trim accordingly.
Bump powder charge up a hair more (I'm already just below max for 4064, but I think I can go up another grain as Arisakas are notoriously strong).

I'm also going to take another closer look at the chamber just to be sure.

Thanks all!

  
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