I'm not 100% on the t12 but i had a muzzel can, came in large or small. small up to .22 cf i think and large up to .30cal.
It was only when i bought it off rfd that it went on ticket as a 6.5x55. It was never proofed specifically.
I would therefore think the same would apply for the t12, large or small.
My muzzel can was great.
You could phone JMS arms directly, very helpful on the phone.
Regards
Jim