More Ruger Bbl info from Ed Harris (formerly of Ruger)
-- Ed Harris <ke4sky@xy*z.net> wrote:
During the development of the Ruger Mini Thirty the chamber we used
was derived from prints provided by the Lapua Cartridge Factory in
Finland which made our proof ammunition. JGS has these prints and
can provide tools under the designator 7.62x39 LAPUA. This is very
similar to the CIP (equivalent of SAAMI in the EU), but held towards
the minimum tolerance. Ball seat diameter is 0.3114" with a Mauser-
style long gradual forcing cone of about 1 degree included angle,
with no cylindrical ball seat.
The SAAMI print which was standardized is derived from the CIP and
uses the gradual angle to mitigate excessive chamber pressures if European
ammunition with bullet diameters of up to .311" is used in American
barrels with .308" groove diameters. Reducing barrel groove diameter
from .310 to .308 does raise pressure slightly, by about 5,000 cup,
but remains well under 50,000 cup within the design limits of modern
rifles.
The Lee and NEI bullets which i designed for the 7.62x39 were
developed to fit the CIP-SAAMI-LAPUA type chambers, but NOT the
Chinese ones!
The SAAMI-CIP is quite different from the chambers used in Chinese AK
andSKS rifles. These have no cylindrical ball seat at all, but simply an
angle taken from the .336" neck diameter with a very long neck, like a
WWII .303 British "trench chamber." Not conducive to best accuracy.
USSR, Yugo and East German SKS rifles have chambers which more closely
resemble the CIP.
Ed Harris <ke4sky@xy*z.net>
in C-B-L Paladin wrote
I have a Ruger Model 77 in 7.62x39mm. I'm under the impression
from information published when the rifle came out that the bore has
nominal 308" grooves but a very long throat which allows the
military surplus .310" bullets to "swage down" for passage.
Would you care to comment on any aspect(s) of this, especially
as it relates to cast bullet selection(s)???
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