Hi Tim, the pressure will be the same, but was your gun originally chambered for 2 3/4" or have the chambers been let out? The pressures for a 2 3/4" Brit gun is 9682psi.

I would NOT shoot the Winchester AA's in this gun. The pressures are to excessive.

A pic of the bbl flats would help.

Good Luck!

Greg


Gregory J. Westberg
MSG, USA
Ret