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From: (Bart Bobbitt)
Subject: Re: Sierra vs JLK 80 grain .223 match bullets: opinions?
Organization: Hewlett-Packard Fort Collins Site

They are expensive to shoot.  Barrel life with them is about one-third as
much compared to a .308 Win.  Their thinner jackets sometimes cause a
problem when the rifling is too deep.  As these bullets need to be spun
close to 300,000 RPM to stablize them, any weakening of the jacket by
deep grooves will let the centrifugal force tear the jacket off.  Having
seen this happen several times, I think it makes sense to have shallower

A groove diameter of .2235-in. and bore of .2175-in. has rifling .003-in.
deep.  Bullets are engraved .003-in. by the lands and this may help the
situation.  Shallower rifling would be better, but probably wouldn't be
as accurate.

If the lands have an angle on their sides instead of being vertical, the
jacket won't be stressed as much.  Obermeyer's 5R rifling has this; it was
developed several years ago for the thin-jacketed Sierra 7mm match bullets.
Seems those bullets had high incidents of jacket separation with conventional
rifling, so Boots Obermeyer figured that if the side of the land was angled,
the jacket would be less stressed at the edges of the lands.  This works
very well for bullets with bearing surfaces longer than 1.5 calibers (bullet
diameter times 1.5).  But bullets with bearing surfaces shorter than 1.5
calibers, conventional, square-sided lands seem to be the best for accuracy.


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