Short Chambered Barrel (308 Winchester)

OU,

When I was building and shooting bench rest cast bullet rifles we approached the fitting of tapered bullets to the guns's throat a different way. We used a bump press to taper any cast bullet to fit the tapered throat of the gun. There was an added benefit as by bumping the bullet in a die, the gas check and the base could be made dead square with the centerline of the bullet. The gas check was also very flat on the bottom and the amount of bump force was adjusted to still maintain a small radius at the base edge of the gas check.

Most used one degree included angle tapers on the lands with a short freebore that was around a thousanth of an inch over bore dia.. Some shot bullets up to 4 or 5 thousanths over bore but I didn't subscribe to that practice. I stayed about a thou. over on the straight cylindrical part of the base of the bullet. My idea was not to size the bullet down in the throat of the gun under the tremendous, violent conditions of combustion but to do it under much more controlled conditions in a sizer/lubricator where everything could be controlled and held straight.

I once had some of my bumped bullets checked on an optical comparator (shadow graph) and they were dead square bases to the centerline of the bullet.

I had my 30BR reamers made with the freebore dia. at .0003 to a half thou. over groove. That way they could be used for jacketed bullet guns too. Then I had a throater reamer made to the dia. and taper I used and throated the barrel to that taper after chambering it. Throating was done in a separate operation but in the same machine setup. I would throat the barrel till the bullet seated about 25 thousanths into the case mouth would just push back a little with the bolt was in battery. That way the bullet was pushed snugly up into the taper and didn't jump to the lands. As soon as it moved, the tapered lands started engraving the whole lengty of the nose instantaneously. Marks on the nose of recovered bullets showed no slippage of the bullet in the lands. And very little distortion to the bullet skin.

I also used that same throater reamer to make the bump die. As you can see, it then allows you to make a bullet that is the exact dimensions of the throat for perfect support when it enters the barrel.

Don Eagan used to use the taper of a standard taper reamer for his moulds. It was a little different than a half degree per side. They all worked well but as Ken mentions a half deg. per side gives you a max length of taper and still leave some straight side on the bullet for the match to the freebore. I made the bullets a slight interferance fit to the freebore to get the best seal and support possible of the bullet to throat.

If you're looking to maximize accuracy, you want to concentrate on bullet to throat fit. Most things that happen that at detrimental to cast bullet accuracy happen in the throat of the gun. By making a bump die, almost any bullet could be swaged to match the throat of the gun. This opened the door to a whole lot of bullets to try.

If you search the archives of this list you will find this discussed in detail several times over.

Tom Gray