I am loading for a Remington Rollingblock Rifle custom built on an 1896 frame. I have come into a small quantity of 500gr cast lead bullets, but can not find a load for this heavy a bullet. Hodgdon has a load using H4198 for a 485 round of 28gr. I am currently loading 27gr of H4198 for a cast lead bullet of 405-410grs. This is a very heavy gun with a custom 32" barrel for target shooting. Would a starting load of 28grs be a safe point? Suggestions?

Get a Lyman book and go with the "1873 Springfield" data. It has loads for a 500 grain bullet and heavier.

I'm traveling right now, so my Lyman Cast Bullet volumes are at home so I can't review their data now.

As Rock Quary suggests, get a Lyman cast bullet manual. In my opinion, Volume 3 is the best of the series with many low pressure loads ideal for your RB. Volume 4, the current manual also lacks the ballistic tables that Volume 3 has which will be helpful for you when shooting your target RB at long rangs.

__________________
Keep your powder dry.

70 grains of Swiss 1 1/2F and either bullet works best in my Rollers and 74 Sharps rifles with excellent accuracy. Steven P. Garbe (SPG lube inventor) says smokeless powder is a passing fancy! I agree!

I just plink so I use the Oregon Trail/Rim Rock 300gr
Lead round nose flat point, and IMR 4198 (1700fps?).

It is nice shooting, appears accurate, and leaves two
gaping holes in old heavy metal gas barrel at 100yds.

That’s out of a Marlin 1895 SBL Jurassic Park Rifle
and Blued GBL same model 45-70Govt.

The Best to and your Endeavors. Check Speer
Bullets website too. Have fun.

I don't think 28grs is going to damage anything, but if you are shooting 27grs with 405gr fodder why would you decide to start at 28grs with 500gr pills?

Maybe you should get ahold of a manual or at least do some more research. Btw, is H4198 all you've got? Because a lot of Quigley Down Under loads were done with different powders.

I had a original smokeless RB that was barreled to 45/70. I tried 500 gr
cast and 510gr. Besides kicking the snot out of you they weren’t very accurate. I did best with original type 405gr and the 322gr Gould HP, that’s old Lyman / ideal bullet.
I haven’t loaded for RBs and Trapdoors for years. I was using Unique and Win 630. I have the Lyman manual and I bought the one cartridge manual for 45/70.

I no longer shoot my #3 Ruger or Shiloh Sharps .45-70 and I've had a number of other rifles in this chambering over the years, but did shoot .45-70s with a variety of cast bullets for many years. I've tried a lot of powders and IMR4198 is certainly among the best for all bullet weights. While I never tried H4198, I don't see why it wouldn't work as well as the IMR powder.

I'm a big fan of H4198 because of small grain size and temp stability. I've used a lot if it in my 45-70 behind 405-500 grain cast. It offers a very wide loading range. It is also a good economical powder for 55gr @2900fps 16" barrel .223 loads used for plinking.

What is a "Quigley Down Under" load?

Western Powders includes your application in their load data.

The Lyman Cast Bullet Handbook is the most often recommended source of data for this info and it is good.
I am trying Accurate 5744 at this time as it is supposedly the go to powder these days for large caliber cast bullet work. So far seems good but have not used much of it.

A "Quigley" load is a long range match load. Many people use 45-70, But the movie rifle is 45-110!

I have Four 45-70 rifles. My Shiloe Sharpe's 1874 only gets fed Black Powder loads. My Marlin Guide Gun (18") and my Browning 1886 (26") both enjoy the following load as a plinker:

WW Brass
WLP primer (Yes that is Large Pistol)
300 grain Moly Coated Bear Creek Bullet
14.0 gr. IMR Trail Boss powder

Listed velocity 1285 fps (20,900 CUP)
Actual velocity in the 26" barrel 1325 fps.

Iron sight accuracy- 200 yards, 1.25" 5 shot group off a sand bag

Source; 2006 IMR Smokeless Powder Guide

Comments, You can easily get 45-70 much faster than Black Powder Velocities! But for your 500 grain bullets, 1300 fps muzzle velocity, will be an extremely accurate load. As for Energy, BP loads killed Buffalo to the point of extinction! The 45-90, 45-100, & 45-110 extended the range, not the lethality.

Winchester & Remington sold their ammo by Caliber- Charge weight- Bullet weight. Ex: 45-70-405

Sharps sold empty brass and rifles by the chamber dimension; ammo was sold with several different systems.

45-70 = 45-2 1/10"
45-90 = 45-2 4/10"
45-100 = 45-2 6/10"
45-110 = 45-2 7/8"
45-120 = 45-3 1/4"

Ivan