Cheatermk3,
Have you recovered any of those .375 bullets that you say shoot good in your gun to check for rifling marks ? I shot some .375 speer bullets in my 38-55 target and none of the recovered bullets had rifling marks, meaning to me that they did not engage the rifling in my gun.
Hope you have good results with your rifle, I will never give mine up but I wish we had more support for reloading the round.
I have not recovered any of them, but when I first shot the rifle, I tried Winchester fasctory ammo, which functioned in the action and grouped OK at 50 yds. They were making nice round holes. Then I tried .380" cast bullets which wouldn't fit right in the chamber. So then i tried some jacketed bullets, which keyholed at 25, let alone 50. I tried the Hornady .375" 270 grain interlock. When these keyholed badly at 50, I thought perhaps they were too heavy for my rifle's twist, so I got some bullets designed to be used in 375 winchester ammo, all I could find. They are 220 and 250 grain Speer and Hornady bullets, I'm at work and don't have the numbers handy. I can push a .375" diameter bullet through my bore using a cleaning rod, with no more effort than it takes to push a loosely-fitting patch thru it. "This will never work", I thought.
Since I was having other issues with the rifle, I sent it back to the factory, where they replaced the latch and fitted the barrel to the frame so that it now locks up the way it should. They also ran a brand-new carbide reamer into the chamber, which gave me a bit more room but still not enuff to chamber the properly sized bullets for my .376/.379 lands/grooves bore.
then i finally tried JPH45's suggestion to size the loaded round, which works great. I now simply decap, reprime, charge, seat a .380" hardcast bullet, and VERY lightly size the loaded round, which aligns the bullet in the case and also reduces the outside diameter by approximately .002". Note that I do not resize the fired case when loading the .380" cast bullets, my fired cases come out of the chamber at .397" and the new bullets are held pretty tightly, then they go into the sizing die for what amounts to a good taper crimp.
I still had a nagging question in my mind, which was, "Howcome the factory ammo stabilised in my bore, but my handloads, loaded with virtually the same bullet, does not?" When it comes to reloading ammo, I do not like mysteries.
The answer, of course, is that the factory ammo's bullet is obturating, or "bumping up", to groove diameter, or at least enough to engage the rifling, while mine were not. How can this be? I asked myself. Maybe the factory ammo uses a slightly larger diameter bullet, it was suggested, by JPH45, and others. The theory went that the slightly larger, .377" bullet would obturate in the bore, being just barely large enough to engage the lands, which would cause the pressure behind it to bump it up to bore diameter and spin it, thus stabilizing it's flight.
I disproved this theory by dismantling a factory round and measuring the bullet. It was .375". Since SOMETHING was causing obturation, I thought, "maybe it's the crimp; could it be that the factory crimp, which is VERY stout, is holding the bullet in the case long enough to effect the bumping up to occur in the chamber, before the bullet even leaves the case? I was quite sceptical that this could be the case, but when I got my hands on a factory crimp die, the jacketed bullets I was using began to hit the target nose-on, as evidenced by the perfectly round holes in the paper.
So, that's my theory on what's going on with .375" bullets in my .376/.379 barrel. I suppose now I'll have to build me a bullet trap and finish the experiment by miking a recovered bullet.
