I am presently loading cartridges for a Swiss schuetzen rifle chambered for the old Swiss GP90 7.5mm ctg.   The throat (leade) of the chamber is ca 0.327" or so, yet the groove diameter of the bbl is 0.299" (sic).   I am loading 0.326" diameter 200 gr bullets and getting near MOA performance.  Anyone else had/having good experiences like this with what seem to be grossly oversized bullets?

CHRIS
RGChristensen

beltfed wrote on Apr 24^th, 2017 at 6:25pm:


ARNIE:
   No, 0.299 is the groove diameter.  Incredible that it should work at all, but it is probably the best-shooting fixed-ammo rifle I have.

CHRIS
RGChristensen

Question is where does the excess lead go, and what does the base of the bullet look like after it leaves the bore? Has anyone slugged their bore with these oversized bullets to see how deformed the base might be?

marlinguy wrote on Apr 25^th, 2017 at 8:17am:


Seems like relative accuracy would answer that question indirectly; if accuracy is enhanced, or at least not impaired, compared to groove dia. bullets, bases must be leaving bore in undamaged condition.

calledflyer wrote on Apr 25^th, 2017 at 3:51pm:


Assuming they ARE damaged, which is debatable.  I've been shooting 92 g, .313 dia. Midway cast bullets in a #2 Ballard ever since I rechambered it from .32 Long, Colt, or whatever you wish to call it. Bore: .306-7. Never tried any other bullet, because a friend gave me a lifetime supply of these; deplorable that anyone would be that cheap, but I'll gladly accept any donations for further experimentation.   

So-so accuracy, about 2" at 50 yds., but good enough for red squirrel hunting within peep sight range--for me, not beyond about 35 yds.

Red, my point probably was that we don't know if the bullets were damaged or not, but we go to such lengths to see that they aren't. And, maybe most of it is just unneccessary if the rifle don't mind. 
I had a friend, who like the guy with the Jap rifle, shot 'wrong' ammo for years. One of those German m88 rifles with the small bore, and a case of regular 7.9mm surplus ammo. Added a pad and kept on despite offers to help cure the problem. After a while I even lost my fear and shot it a few times.

calledflyer wrote on Apr 25^th, 2017 at 5:52pm:


That's not a serious matter. I have collected those rifles (1888 Commision) and researched them. They have a .3208 groove and the 154 gr Spitzer, only has a 8mm (.315) bearing area, that tappers from .3208 to .324. So the bullet isn't really to OS. 

Frank

My Swedish roller 8X58RD has 0,315" groove-to-groove bore. It shoots 0,329" cast GC 205gr bullets very well. Reason? Apparently they fit the throat very well and that throat tapers smoothly into to 0,325" groove-to-groove bore. What more could I want?? No pressure signs expected at the approximately 20.000 psi chamber pressures.

Only other cast bullet that shoots equally well is Ideal 250gr long nosed, 0,325" GC bullet whose long nose sticks well into start of rifling, resulting in these long bullets being well aligned with bore.

My old 32-20 Winchester LW has a somewhat oversized chamber and the throat and back end of bore are eroded to 0,316". Smoothed the once rough eroded area as best I could and it now shoots longish 135gr LBT GC 0,316" bullets nicely, often putting first 2-3 shots, from initially cold barrel, into same 1,0" square at 100 yds. Excellent for popping turkeys in just the right place for instant kills and no wasted breast meat. 10 cm or less in front of eroded area is 0,313", as is remainder of barrel. Only other bullet that I have found to shoot equally well is Hornady mantled 100gr 0,313" HP "pistol" bullet, over an ancient standard amount of IMR 4227 to give a nice HV load -- cases fall out of chamber.

Both rifles seem to be following "ancient" practice of using oversized soft lead alloy bullets big enough to "fill" chamber necks and throats nicely, giving those bullets very reproducible initial positioning for start of their journeys down long barrels. Cases for both rifles are only neck sized.

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