It will act as a very simple GO / NOGO gauge. If you insert a fired case into each chamber it will tell you which chamber is tighter. 001 inch which is fine for your purposes. You can measure any discernible differences in the cases with a with a $40 Walmart micrometer It will be accurate withing. That will put them back to an annealed state. Stand them in 1/2 inch of water and heat the necks and shoulder angle to dull red and tip them over in the water. Its to hard to be stretched that quickly. The violence from being suddenly final formed is cracking it. When brass is worked slowly in steps it gets hard but it can be worked in this manor without breaking but fire forming is different. I assume you made it from 30-30 or 225 Winchester. The brass has not gotten hard because of firing, it has more probably gotten hard from being worked from its parent cartridge. Its a hard brass problem as mentioned above. You will learn nothing of value in this case. Don't waste your time casting the chamber. HOWEVER, if the cases are cracking in the neck the BRASS is too hard and needs to be annealed. It is MOST common in cases that have been work hardened by re-forming.

I have seen this problem with new unfired brass as well as once fired brass.

By standing it in the water you insure that you do not get the cartridge head too hot (annealing the head is not a good thing). Don't worry that the shoulder gets hot, that wont hurt a thing. Heat the NECK of the case to a dull red with a propane or MAPP gas torch (don't get it so hot that you melt the brass) then knock the case over into the water. Place a cartridge standing up in the partial filled pie pan. The easiest way I know of to anneal cases without a bunch of hassle is to use a pie pan deep enough to fill with water so that when you lay the case down it is COMPLETELY covered. Because this involves heating the case the bullet, powder and primer must be removed. You need to break down the loads and anneal the cases. The brass is harder than it should be causing the cracks when the brass expands at the neck. Regardless of which barrel they were fire formed in the necks expand a little when fired and you will have the problem in both barrels. Frames were 150 ish with grips.It is NOT the chambers causing the problem. Pistol barrels then were 135-150 dollars. I was lucky and got the bug for TC early. 21 in bull barrel no problem 23 inch 22 lr match not problem. Super 14 22 match stainless no problem (less made that a super 16 match also. I am looking for a super 16 22 LR Match and have been for over 10 years. Once again if it is a shooter and you can not find a TC branded one get an aftermarket. Collectors normally stick to TC branded barrels but with 86 calibers made in just contenders when the one to round out a collection comes up they can bring stupid money. Fox ridge used to make barrels and there are others also. Bull berry used to make the best aftermarket barrels and I think they are still around. Caliber length twist muzzle breaks ect all available. MGM makes barrels for if you need something made from unobtainium. I think he has about 4 8 foot tables of TC items at the Ohio gun collectors show. They normally only carry tc branded ones he said the others just don’t sell well. He almost always has Thompson center barrels that are unusual ones. They advertise themselves as a Thompson center recycling center. Since this is a public forum PM me and I can send you his email and phone number. Dixie Longshot owned by don and Dixie Paul.

Tc contender barrels used