American Rifleman ran velocity tests on .22 barrel length, and found that .22 LR high-velocity rounds maxed out at 16" to 18", with longer barels showing lower velocity. The exception was some hyper-velocity rounds, with CCI Stingers showing a slight increase out to 24". I would expect the higher-capacity .22 magnum to benefit from a barrel longer than 18", but haven't seen any articles about it. If you decide to shorten yours, get chronograph readings before and after, and let us know the results.