If you're talking lead lapping bullets, you want to be able to see them. Preferrably under 700fps. I go for 550fps. Beartooth has a great book that will tell you how to do this. I also recommend their kit... if yo use the kit put out with jacketed bullets, the bove does not apply. Lapping with lead bullets will be better for Marlins, because of the benefit of removing constrictions. The wear is insignificant on a barrel that shoots such a slow round as anything we have. (talking thousanths of an inch)