I have been dabbling with pp bullets in my 40-70 SS Hepburn and am actually starting to show some improvement.  I started out with two different diameter bullets that I was able to buy online, 0.404 and 0.408.  I have a 0.409/0.402 barrel.  Blackpowder loads shot well with the bullets but I wanted to try for a little more speed.  I shot patterns with the 0.408 bullets!  After much research and numerous emails Beltfed convinced me to try a dual diameter bullet so after casting the chamber I came up with a 330 grain design to try.  Accurate Moulds made me a single cavity aluminum mold and it arrived this week in time to cast 100 keepers.  Patching with paper from BACO was something that kept me occupied but I eventually got the hang of it.  I used 20 gr of IMR 4227 with finger seated bullets and a version of Westerner's hydraulic seater to assure that all bullets were the same and shot some nice groups today.  I also shot some grease groove 336 grain bullets that I had cast for a control.  Al bullets were 20:1.  The grease groove shot better but the paper patch is more than accurate enough for hunting.  I need to play with different powders and charges now to see if I can get up to 1600 fps or so.  The 4227 load is really dirty so I am not sure if I need to add more for a better clean or just move to another powder.  I am going to try Re 7, H4198, Buffalo Rifle, and 3031 and a higher charge of 4227 next.  I wish the range would open soon as I am shooting sitting off cross sticks at 50 yards and have to limit my shooting to 30 minutes on Sunday Mornings only.  The State of California owns the range but there are rumblings about opening up soon.  Tom

Arnie, would a light taper crimp hurt the patch? Tom

what controls the oal if there is no neck tension?  art

This may or may not apply to a 40-70, but I did some experimenting with my 38-55 last week using 18 grains 4227 behind a breech seated 329 grain GG bullet....shots fired with the powder tapped back onto the primer were very dirty and showed a fair bit of vertical, as well as many case mouths dented with soot back along the case.

With the shells rolled on the shooting bench before chambering , groups were much better ( 1.25" to 2.00" ) and the burn was much cleaner as well as no case mouths buggered up.

All cases were sealed with a simple cork wad, indexed  as were the bullets.

Tried the same process with 20.5 grains 4198 and shot sub moa groups with a very clean burn with both tapped back and laid out charges.No messed up case mouths at all.The powder laid  out along the case length seems to make my old Ballard happier, just something to add to the list of things to do at the range.

I have little experience paper patching with smokeless and that all with 3031. However, I shoot a lot of PP 45-70 with Black Powder and use IMR 4227 almost exclusively for cast loads.

They aren't a good mix. Your approach to loading by seating long and pushing into the lands is probably a good one, and, in theory, should provide the initial resistance to get pressures up where 4227 is happy. As mentioned above, 4227 loads generally like a good crimp. But, a good crimp isn't great for PP bullets. 

Two things worth trying to tighten up what are good groups. Try a light taper crimp that is just enough to keep the bullet finger tight (can't be easily pulled out or pushed in), at the length where the bullet is seated into the lands. That slight additional pressure may work. 

Otherwise, try the other powders. 3031 is a treat and IMR4198 is always reliable.

Best of luck.

Thanks Doc.  I loaded some with 4198 and 3031 to try as well as a heavier charge of 4227.  3031 has served me well in my 38-56 as has 4198.  The range is still closed so have to beg to shoot at a friends ranch on Sunday morning again.  Tom