I shot my 45/70, Douglas barreled, Borchart for a few years, BSed. I shot many loads in it, both smokeless and duplex BP with very good results. I used cylinderical bullets from 430 - 500 gr and won my share of over 40 cal matches. I actually never shot it fixed. It tore my rotor cuff in 1996 and I wasn't able to shoot again for many years. 

When shooting BP, remember that you will use a lot more BP when shooting BSed and that's one reason it did what it did, to my shoulder. That load was 70 gr 1F (GOEX) and 15 gr 296 with a 500 gr bullet.

You don't have to have a tapered bullet to BS but, it can't be over .001 larger than your groove diameter. If you can modify the base band (ream), I would suggest a mold that is groove diameter or no more than .001 under groove, then open the base band to .001 - .002 over groove diameter so it will seal the bore when fired.

Frank

That's ok Joe. My sucky .459 bullets shot 5 shot groups between .60 and .84 @ 100.

I don't doubt that a tapered bullet would have done better but, they weren't available in those days (1991).

Frank

fearless,
no experience with the 45/70, but have had success with the 2.4" case and black powder.
in this situation, charge is 100 gns of swiss 1f droptubed only, and a 0.060 wad to fill the case.
airgap of about 1/8" is where things happen.
bullet is a 540 gn patched to 0.451, so it pushes in with a little firmness using a palm seater.
this load was developed for scheutzen, but the reality is that it has too much recoil for me with the accuracy requirement of that discipline.
it did come second in the Hahndorf kingship.
if I go back to using that rifle for long range, it will be a fine load. wiping is of course necessary.
we know that later in the creedmoor era this is how the matches were shot.
it would also make the capacity of the 2.1" case less challenged for long range shooting.
while the paper shreds into reasonable confetti, it is not as pronounced as with fixed ammo.
possibly the airgap allows some loss of shock transfer and therefore bumpup.
the intent is to experiment with a lube soaked felt wad to fill the airgap and compare confetti.
oh to have the time to do all the experiments on the list.
keep safe,
bruce.

Poetically speaking, outstanding replies guys. All good and helpful information. I'll try the Paul Matthews bullet, and will also get a custom mold from Accurate. I have a number of Tom's molds and I like them very much. I especially appreciate the heads-up regarding potential recoil, but I'm pretty recoil insensitive and I use a shoulder pad for the big ones. I think I'm not highly enough evolved to have normal human pain sensitivity.

the 45-70 bullet the late Paul Matthews designed for long range showing that was adopted by Lyman is tapered and suitable for breech seating.  the first version---a nose pour did not work out well for lyman----production blocks warped and they reissued it as a base pour a couple years later.  (I think using a larger block would have solved the problem)  I have both versions and both work fine for me.  However it IS a 500 gr+/- long range bullet.
when I first started shooting ASSRA all I had was a light sporter weight 45-70 Miroku 1885 (Pre-creedmoor and BPCR production rifles) and my learning curve was colored black and blue.   Once I started breech-seating  and learned a bit more about recoil-management things improved and I was regularly shooting 230 Schoyens with occasional visits to the low 240 territory.  with that bullet I didn't have to pay much attention to wind conditions @ 200 yds.  I have had that bullet scaled down for use in 38 and 40 calibers and it breech-seats equally well in those applications.

when I get back home I get the mold #s out and add them to this post.

EDIT:   the second version of the Matthews is the one listed above. my memory is the original nose pour (with a small flat meplat) one ended in "76" for the number.   mine were cast 20/1 lubes with a more brewed beeswax/lanolin lube and shot with healthy but safe charges of 5744 (or is it 5477---its been a while)----anyway it was that good but dirty burning 45-70 smokeless