Is the barrel length of the winchester trapper exactly 16" or is it 16" + a fraction of an inch ? Could you put 10 38 sp in the tube if you shortened the tube spring 1/2 inch ?

Winchester barrels can show a 1/4" to 5/16" deviation in exact length, the two factory trapper barrels I have are both 16 1/4." Can't help you with increasing magazine capacity, never owned a .357.

Thank you for your prompt reply to my questions.