Cast Bullets Under Sized....Help Please

[bluethunder]

I have been casting bullets for years. First in the 1970s for IHMSA silhouette shooting. Then in the 1980s, 90s and 2000s for USPSA, Bianchi and Steel Challenge. I've even cast for semi autos like the M1A. Currently I'm casting for a 30 cal. Extreme Big Bore. There isn't a lot that you can do if the mould is actually undersized. But you can vary your bullet diameter to some extent with alloy selection. I recently measured several of my 30 cal. bullets with the following results:

The smallest diameter will be the pure lead bullets. Also the most difficult to cast due to the high surface tension of pure lead. It just doesn't become fluid enough to fill all of the bands and grooves until it's really hot.

My second smallest diameter bullets were cast from Roto Metals 1-40 tin mix. Much, much easier to cast than pure lead.

Third largest diameter bullet was cast in 1/2 pure lead and 1/2 linotype. This gives a bullet alloy of 92% lead, 6% antimony and 2% tin. I take this bullet and harden it a little further by dropping the bullets directly into a cold plastic bucket of water. This is a very easy alloy to work with. I have not noticed any velocity loss shooting this hard of an alloy. It has produced some of my best accuracy and I can shoot entire 40 round sessions without ever cleaning the bore. Groups have as good of a chance of being "the best of the day" at the end of the shooting session as at the beginning of  the session.

The largest diameter bullets are produced in pure Linotype. (84% Lead, 12% Antimony & 4% Tin) Even though these were probably the hardest bullets that I have ever shot out of the air rifle, they are the only bullets that have produced leading in the barrel. The leading was so severe that I had to clean the barrel every 10 rounds. And noticeable small silver flakes came off of the cleaning rod brush and patch. My only guess for the extreme leading is that the bore riding portion of the bullets had become so large in diameter (.302"-.303") That they were being scrubbed off as they were pushed down the barrel.

 On a .308 diameter bullet the difference in diameter from pure lead (smallest) to Linotype (largest) is only a couple of thousandths of an inch. However you will get a bigger change in diameter with a .452 class bullet. Possibly up to .003" to .004" thousandth.  If changing alloys can't do the trick than another mould is the only other option.
There are other options but they are not cost efficient.

           In the Cast Bullet Association there is a fairly common practice of having a 7/8" X 14 die made from the same reamer that cut your rifle's chamber. You then take your cast bullet, place it in the custom die and swage it up in your press until the cast bullet forms to the exact same shape as the die -- which is also the exact same shape as your rifle's chamber. This works really well but would  require a good airgun rifle smith to build the die.