The Centershot brass inserts are pretty close but alittle loose.What I did on my CE 90 was to take a standard 5/16" 50 gr. brass insert and spin file it to fit.I chucked up a blunt in my drill and put on the brass insert,And spin and filed to fit,a 50 gr. insert would weigh around 42 grs. when done.I would also file the end to taper down from the 5/16" end to fit the shaft so it would not have a lip.That would make for an easier target removal with a taper.I remember when John owned Sipsey and CE 90's first came out he offered some 100 gr. inserts that were machined down to fit CE 90 and they came out to about 88 grs..