The book twice mentions Silas's 13 round gun, The thirteen-round Heckler and Koch USP 40 had been provided by the Teacher and again Checking the thirteen-round clip in his Heckler and Koch,
Some puzzles - at the very beginning, Silas doesn't shoot Sauniere a second time because his gun is out of ammunition. So how did he use up the previous 12 rounds - OK he'd killed three other senechaux, did he need all those bullets?
Second - when and where did he rearm?
Third - I looked at the Heckler & Koch site, all the USP pistols there have a 10 round capacity.