The Baikal is supposed to have a working limit of 28,000psi. Speer limited their data for the Winchester and Marlin Lever guns at 28,000psi/cup - according to them- for some obscure reason. Even Marlin and Hornday say they ever guns are good for up to 43,500psi.
28,000PSI is the actual number listed by the US government in 1878 as the maximum pressure load for the 1873 Springfield Rifle and Carbine.

Note in this round, Pounds Per Square Inch (PSI) and Copper Untils of Pressure (CUP) are the same numbers - which is GREAT!

28,000PSI/CUP will limit speeds to 1,900fps or so with 400gr. - these speeds are achievable with some powders - you need good data.

Factory RP and WW 405gr. loads are limited to 21,000 or 22,000PSI, I believe. The 300gr. factory load @ 1,800fps might be loaded somewhat hotter - I don't know for sure. The factory 300's are very soft, short & light-weight bullets. If you want to shoot a 300gr. bullet, perhaps you should consider only Nosler Partitions and those, you can drive them to probably in excess or 2,000fps at safe pressures for that gun.

Get the Hodgdon Annual Mannual - comes out every year - - it has super data for the .45/70 and lists velocity and pressure data a wide variety of powders.

Note, the double gun is not limited to 1,320fps with a 400gr. bullet. Even if you loaded to only 24,000PSI, it can achieve up to 1,800fps with 400gr. bullets.

BTW - in Rod's media penetration tests involving water, wood and steer heads, a soft factory duplication load of a 405gr. Remington bullet at something just over 1,300fps matched the 300gr. Swift bullet from a .375H&H for penetration and destruction - go figure.

I only mentioned that as some here have mentioned the factory load wasn't good enough for deer.