457132 45-70 postel 535 grain bullet

You are referring to Hodgdon H4198 powder right?

Modern Reloading by LEE circa 1996 shows a 500 grain bullet behind 25.2 - 28.0 of H4198 for 1082 fps.

Hodgdon’s 2019 Annual Reloading manual shows a 500 grain Lyman # 457125 bullet behind 42.0 grains of H4198 @ 1523 fps, to 46.6 grains of H4198 @ 1650.
Bet that last load would be a flinch-inducing shoulder buster.
 
I'd start at 26gr as the actions are best massaged lightly. If 28gr is max with a lighter 500gr, then max will be lower than that with a heavier bullet. Light bullets need a bit more powder than heavier ones to get the pressure building. I am sure 28gr is probably safe, but at least wear eye protection and gloves if you start at 28-29gr. Cheers
 
The Sharps is a strong action.
I use a 500 grain cast bullet, seated shorter, so less room for powder, and 30 grains gives me 1350 fps or so, with good accuracy and moderate recoil, with a 22 inch barrel. You are in the right zone by starting at 28grains in my opinion.

Cheers,
Stomp
 
You are referring to Hodgdon H4198 powder right?

Modern Reloading by LEE circa 1996 shows a 500 grain bullet behind 25.2 - 28.0 of H4198 for 1082 fps.

Hodgdon’s 2019 Annual Reloading manual shows a 500 grain Lyman # 457125 bullet behind 42.0 grains of H4198 @ 1523 fps, to 46.6 grains of H4198 @ 1650.
Bet that last load would be a flinch-inducing shoulder buster.

I've shot the NOE Postell 530 gr bullet in my 45-90 on top of 38.0 grs of IMR-4198 for 1575 fps, and I can attest to it's flinch-building ability. I can go higher in my gun, but have no reason to do so.

I see IMR-4198 and H4198 (two similar but different powders with IMR-4198 being slightly faster in Burn Rate) as ideal in the various 45 Cal rifle cartridges, having used it in 45-70, 45-90 and 458 Win Mag. You can shoot from "Powder Puff" loads to "Punishing" with the same powder and enjoy reliable ignition, and in my experience at least, good accuracy with every load.

I'd start low (26.0 grs), and work up depending on your gun. I doubt you'll want to go much higher than 35.0 grs if you plan on lots of shooting.
 
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Agreed with all the great input
It’s interesting that the old manuals list using a Dacron filler over 4198 and I’ve never used one and never plan to due to the possibility of ringing the chamber.
I’ve never had a problem with ignition
 
Lyman 4th edition cast bullet handbook
Trapdoor loads
Lyman 457132 535 gr #2 alloy
IMR 4198 28.0 @ 1221 fps >>> 31.5 @ 1326 fps
 
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