A couple more tips to squeeze the best out of your M-H
1) Neck-sizing only. If you do not have a neck sizing die, then simply screw out the full-resizing die a bit (1/4 turn?)
- The unresized portion of the neck, where it meets the shoulder, helps to center the case in the chamber.
2) Do NOT crimp the bullet in any way. Instead, rather open up the mouth a teensy weensy bit, just enough that if you chamber such a case, you see a trace of a bright rubbing ring on the case mouth when you extract it. This also helps to center the case mouth (and thus the bullet) in the chamber.
The reasoning behing the above actions is thus:
A fully resized case has some play between the neck and the chamber wall. The chambered case will sit on the bottom of the chamber, and the play is all at the top. So the bullet is sitting below the centerline of the barrel.
On firing, the bullet will run along the bottom of the neck space until it is forced up by the transition/lead/forcing cone/ call it what you will.
So the bullet is inevitably engraved on the skew, leading to the corkscrew trajectory mentioned above. The above tips help to keep the bullet centered as long as possible on its way into the rifling.
A longer cylindrical portion on the bullet helps to keep it straight. If a rifle has a free flight between case mouth and transition cone, then boat-tail bullets will make it worse.
Don't forget, these rifles were designed long before boat-tails became fashionable. The bullets in those days were long and cylindrical by modern standards. I do not have a 303 M-H, but have owned (and shot) a Mosin-Nagant 91/30 sniper, Argentine Mauser, SMLE MKsI and V, Lee Enfield No.4, a friend's Finnish sniper, and they ALL performed better with the 3130s.
Try it. It won't turn the Martini into a bench-rest rifle, but it will perform better.