It doesn't fit. The O.D. on the Apex barrel is 1". The Apex barrel also has a groove that provides a positive interface, there is a ring on the Apex that fits in the groove so it can't slide off. Most barrels have a 7/8" O.D. and the Palmers has 13/16" O.D. (I believe anyway). So there is a gap and it just slides around. There are several ways mount the Apex on these smaller barrels but they all vary in cost, quality, appearance and durability.