Not for use with modern 3" cartridges. I have a couple so marked; one by Midland Gun Co. and one by B.S.A. The old English 3" cartridge was 1 1/2 ounce, hence the Proof Mark. Modern 2 3/4" magnums are suitable if the gun is in Proof and in good condition. I still have a few boxes of the old Eley 3" Alphamax and 3" Gastight. They are now really collector's items. Lagopus…..