Easier fitting for the varied piston tolerances of HW. Even fits older pistons with square non chamfered edge.
Current OD 1.035"+ avg
Fits Mksmn 55-70, HW99, HW50 new model,HW57, R6,9,10, 11, HW85-95.
Hw77-97 ( over 1" bore-newer guns) For serial no.1446049 and over 77-97 guns. Below this they take a smaller seal. SN info supplied by HW.
R9 TK's all etc.
These are a modified UK design. They get high velocity in good tolerance guns. In really bad tolerance guns that vary up and down the tube badly, the soft pliable factory large size seal is best. Not great and not high velocity but it will seal better for a short time. Those types of tubes never end up doing well. Often require frequent parts and shoot harsh. Most of these come after 1999.