I've used the FMJ's a lot, and to get them to fly right, as with most arrows, you should bare shaft tune them. I start with full length shafts of a spine I think is appropriate with a 58# longbow, either 340 or 300's should work. 340's with a fairly heavy point should spine weak initially, so incrementally cut them down from the nock end until they fly straight without fletchings. even mis-spined arrows can be corrected by big feathers, but they are much less forgiving to form and release errors, you should be able to get them shooting great!