Unexplained leading

[Nocturnal Stumblebutt]

I am getting about an inch of fairly severe leading just past the forcing cone in my 10" TC contender 44 mag barrel. The bore slugged at .429. All boolits were cast from Air cooled COWW + 2% tin. First I tried a 429421 sized to .430 and lubed with WL BAC over 20 grs. of 2400. Then I tried the same load sized to .431 as well as the same boolit sized to .431 over 11 grs. of unique. No change in the amount of leading. I don't understand why this is leading at all. Thanks.

[detox]

According to Lyman Cast Bullet Handbook #4 : "Leading in the breech end of the barrel may be the result of too soft alloy, wherein the bullet is skidding excessively as it engages the rifling".

Try water quenching your bullets to make them harder or reduce velocity with different powder/charge (8.5 gr. of Unique)

BE SURE to remove all lead from barrel before next test.

BTW does a Thompson Contendor have a forcing cone? I thought revolvers had forcing cone

[Nocturnal Stumblebutt]

BTW does a Thompson Contendor have a forcing cone? I thought revolvers had forcing cone

Yes, unfortunately many T/C barrels are cut with a forcing cone rather than a throat.

[btroj]

How often do we get expected leading?

Is there free space ahead of the bullet when seated in the case? That can allow gases to ge past e bullet before it can seal the bore. That isn't a good thing.

What lube? Sometimes a lube change can help reduce the gas leakage issue.

[Slow Elk 45/70]

This problem is fairly common and can be helped/ cured, with lots of trial and error

[Nocturnal Stumblebutt]

So all it took was a change of powder. I got a nice group and no leading out of a 421429 cast from air cooled COWW + 2% tin sized to .430 over 24.0 gr of H110.

[runfiverun]

moving the peak pressure past the problem works in lots of guns, this is a good lesson to remember.

[tomme boy]

Seat the boolit out to just touch the rifling. Then give it a very mild crimp. Thats what I had to do to a H&R 45 colt. Plus, T/C barrels I have seen have been some of the worst finished on the inside.