I have put alot of time in hunting load development for a M-71 short rifle . My bore is .433 land -.454 groove . Here is what I have found . First I do happen to have a set of dies made for the M-71 , not the 71/84 , and they are the correct size . You can use 45 colt dies to neck , expand and seat . RCBS are the best as they have more of an adjustment range in them . I reset my rifles headspace when I started this and it has not increased with over 400 rounds fired . I am using jacketed bullets and a "hot" load of smokeless . I have also checked my bore and it has shown no wear after 400 rounds . My best load has been the Hornady .452 dia 300 grain xtp bullet with 36.2 of I-4198 for 1600 fps in 25 inches of barrel . A lighter load of 32.6 also does ok at 1500 fps . 39.6 of Reloader 7 has done 1650 fps . In load development I have fired the 300 grain bullet as fast as 2000 fps with no pressure signs , so I feel the 1500 to 1600 fps should be fine , 1600 fps is what I settled on for hunting . I have found with the twist rate the higher velocities had beeter accuracy, much better than I ever got with slower cast bullet loads . The jacketed bullets will "bore ride" since they are a little smaller than the groove, this makes for more velocity with less pressure and the best accuracy . Cast bullets will not bore ride well , they need to be groove size or bigger . I have not had any luck with hollow based cast bullets as the bullet does not seem to blow up in the bore straight ,to ride true , plus the skirts fail now and then causeing flyers . I am getting 2 inch 5-shot groups at 100 yards with the jacketed loads, so far the best with a cast has been a .453 dia with 32.6 of I-4198 at 1450 fps , but the groups are over twice as big . I am using Bertram cases and my rifle will chamber with .453 dia bullets . The M-71 is a stronger design than the M-73 Trapdoor , which has been fired for years with jacketed and smokeless loads . If you are set on blackpowder , a case full with the 300 grain should work . Plus I would like to hear how well it does . Everything I have posted I have done ,checked and used. Check your bore size for land and groove, use common sense and do not try and make bullet fit where it does not .