8x56
I know that there are different opinions on this, but have you tried any standard 2 3/4 slugs in the drilling?
If it's up to shooting modern loads, I doubt that there would be a problem.

I have a few 16s that shoot well with them, and one drilling that won't shoot regardelss of what I put in it.
Standard length Remingtons have worked well (they are quite undersized though) and Brenekes have given mixed results. I plan on shooting them in a cape gun tomorrow that is cross firing, and hopefully the heavier slug will move things over.
All of the guns I'm currently shooting have short chambers.
I've never had a problem.
Cheers
Tom