What, EXACTLY, is the chamber length of your 20g gun.
British 2 1/2" chambers are (usually) just that; 63.5 mm
European 65 mm chambers are 2.56"
European 67 mm chambers are 2.64"
U.S. 16g 2 9/16" chambers are 65.1 mm
12g 2 5/8" chambers are 66.675 mm
12g 2 3/4" chambers are 69.85 mm
Some just after the turn-of-the-century U.S. maker 20g chambers were cut slightly shorter than 2 1/2".
.....2 3/8" = 2.375 = 60mm; 2.4" = 61 mm; 2.45” = 62.23 mm.
Short non-boring answer: Sure, if the 65mm load is "safe" in your gun, the 67mm will be.
