IMR 4198 is an excellent mainstay powder for the 45-70. I shot this powder today but loads just under max. All grouped ok at 1 to 1.5 inches at 50 yds. 250g MonoFlex grouped best at 1 inch and SD was 5.3.

Three Shot groups taken at 50 yds with Ruger Marlin 45-70 SBL 19 inch Barrel : Hornady Manual used 22 inch barrel.

UPDATE:  I also included Momentum calculations. 

https://calculatorfriend.com/bullet-momentum-calculator

Hornady 250g MonoFlex Best Group 

Load 47g Manual says vel  2100 fps  

Avg Vel. 2080.5 fps  Diff manual vs actual 20fps

SD 5.3

Muzzle Energy 2444 ft-lb

Bullet Momentum  10.27 kg·m/s

Note for comparison: A 175g 7mm bullet traveling at 3000fps has a momentum of 10.37 kg.m/s. Identical to this load. Hmmm!

Hornady 300g FNHP

Load 46g manual says 2000 fps

Avg Vel.1900.5 fps Diff manual vs actual 100 fps

SD 34.1

Muzzle Energy 2438 ft-lb

Bullet Momentum 11.26 kg·m/s

Hornady 350g RNFP

Load 43g Manual says 1800 fps

Avg Vel 1761 Diff manual vs actual 39 fps

SD 9.4

Muzzle Energy 2445 ft-lb

Bullet Momentum 12.17 kg·m/s

I shot a 400g FP and found the MV at 1413 fps. Momentum was 11.16 kg·m/s. Lower than the 350g due to significant velocity differences. To equal the 350g momentum, the 400g bullet needs a velocity of 1550 fps.

Thus, the 350g appears better at penetration than the 400g at 1413 fps and the 350g has a flatter trajectory. 

Conclusion: 250g MonoFlex with 47g is best load of these. The 350g Load comes in second. The 300g load was worst. The 300g loads need work to find best loads with different powder.

I do like the MonoFlex all copper said to retain 95% of weight and won’t over mushroom on deer.

On bear, it looks like the 350g Round Nose Flat Point would be a great black bear round with minimal mushrooming and great momentum/penetration

Good Shooting!