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RE: 312/316 160 GN FN Ed Harris design 4 cavity aluminium all GC
I want to order/request/ pay for up-front or whatever is necessary to get a mould from the 4/28 run that has same nose diameters but will size clean and gas ck to .316 .
I believe I saw a reply that the nose area would be + .002 compared to the original Harris design. Here is how my RUS and CHI SKS as well as 3 commercial bore diameters measure with precision gage pins:
CHI .301/.302
] groove dias. on the SKS slug at .313/.3134
RUS .300/.301
Green Mountain meant for 7.62 X 39:
bbl #1 .300/.301
] grooves slug .311
bbl #3 .300/.301
Parker Hale match bbl in .308 Winchester .299/.300
As you can see, bore diameters on all these nominally .30 cal barrels are very close to .300., what varies are the groove diameters. The Ed Harris design is of 2 diameters, with the part in front of the crimp groove meant to be “bore riding” and the part behind the crimp large enough in diam . to fill the grooves . A lot has been written about the virtues of the driving bands of the cast bullet being perhaps .002 larger than groove dimension. The plausible argument to me is better sealing of the grooves prevents bore leading because hot gasses do not melt bits of lead off the bullet which by-pass the bullet and deposit on the barrel.
I believe I have proved this to myself by oversizing the driving bands only on bullets dropped out of Lee mould C312- 155-2R . I oversize the driving band area to about .319, then GC, size and lube to .316.
I see no leading , and can show 7/8 C-C 5 shot groups at 50 yds.
Conditions :
Std issue Chinese SKS ( converted from semi-auto to “Straight-Pull Repeater with a simple, removable “drop-in” kit which seals the gas block orifice).
Caldwell Lead Sled on rigid bench
2-7 power Burris hand gun scope in Scout position on forward mount of my design
5 grains of Red Dot behind the modified Lee bullet
Ave speed : 1042 fps
SD = 3.03
After all these words, I see no purpose in increasing the nose diameters just because we want fatter driving band area. Maybe I just mis-read that post which I cannot seem to find to read again.