Most likely 2 1/2". However, do not translate 2 3/4" chamber into OK for use with SAAMI ammo (that is mostly what you can buy in USA without special effort like mail order). Even if it turns out to be a 2 3/4" chamber it will need care and feeding with CIP standard ammo and/or reloads of known pressure (SAy, sub 8000 psi).

DDA