A friend of mine sold his 458 Win Mag some time ago and gave me what bullets he had. I have a box of 500 gr Hornady softs and was thinking about making a 'Thumper' load for my guide gun. Nothing crazy velocity wise, just a short range sledge hammer load. Anyone load 500 gr bullets in a 45-70 guide gun? If so, what was your load and OAL?

sand the fronts flat so they wont ignite the other primers.

The hornady 500 is too much bullet for a marlin. If you crimp in the channelure-- they won't feed. If you seat them deep enough to feed, you will not get enough velocity to get them to expand.

The .458" Hornady 500 grain round nose bullets will work just fine in the Marlin 1895 45-70. They will expand somewhat on tough targets at the relatively low velocities provided by the 45-70. Otherwise, they do make a 45 caliber hole in things. You don't really need to file or sand the nose flat as was suggested. Because of the bulge in the magazine tube the nose of the bullet will not line up with the primer of the following round. You can seat them to 2.55" OAL and then a moderate crimp with the Lee Factory Crimp Die will make its own cannelure into the bullet.

For 500 grain Hornady RN 45-70 loads try:
Benchmark 43 grains
H4198 32 grains
IMR4064 41 grains
IMR4895 43 grains
IMR3031 42 grains
IMR4198 32 grains
AA5744 29.7 grains 1,230 fps (34.4 grains 1,434 fps MAX)
AA2015 40.2 grains 1,426 fps
AA2230 36.4 grains 1,306 fps (44.0 grains 1,552 fps MAX)
Ramshot X-Terminator 35.0 grains 1,320 fps (44.0 grains 1,550 fps MAX)
All seated to 2.55" OAL and crimped with a Lee Factory Crimp Die. Velocities are for a 22" barrel. The 500 grain Hornady at 1400 fps from a Guide Gun will rattle your teeth a little.

Thanks Jackfish.

I knew that it can be done. I was not concerned with filing the tips flat because the are relatively flat to begin with. Nor was I concerned with expansion. I just thought it would be a neat sledge hammer round. I keep the rifle loaded with hand loaded speer 400 gr and they seem to handle things nicely.

Thanks again everyone.

Many 458 bullets are two diameter bullets.
They are .450 or smaller ahead of the cannelure.
You can't just seat them deeper. With all that recoil they need a cannelure at the case mouth too.
RCBS makes or use to make a 500 grain mould that appeared to be correct for use in a lever action.
Mould at the bottom #82054