How does the recoil compare to a regular .45 Colt 250 grain loading or a .357 magnum 158 grain loading?
They compare quite well, like apples and orangutans!
I have a 10" Contender barrel in .357 Mag, one in .44 Magnum, and .45 Colt. And a 14" in .45-70
So, I've had the opportunity to compare recoil in very nearly identical guns.
The .44MAG is the sharpest, with the most muzzle rise, but its also the lightest, with its thin octagon barrel. The longer "bull" barrel of the .45-70, and all those little holes near the muzzle keep the rise down, but the recoil is heavier than the .44 Mag. its just a slower feeling push.
And that is with a 400gr cast bullet and the old Lyman "factory duplication load". (black powder speed)
I don't shoot anything heavier out of my Contender. And I don't shoot 500gr slugs, though I could if I felt I could handle the recoil.
Felt recoil isn't a matter of velocity alone, its many factors including the gun grip, and the shooter. Big heavy bullets even at low speeds have significant recoil.
.45 Colt, a 250gr @ 900fps, shall we say? OK, now think about a 400gr bullet at that same speed. I don't know what the BFR weighs, I know its heavy, but if its not as proportionally heavier than the .45Colt gun as a 250gr is to a 400gr, its going to recoil MORE.
I've had a (modern) Marlin 1895, and still have my Contender and a Ruger No.3. So I've got experience with guns taking loads from all 3 levels.
I don't know where, in the strength group the BFR goes. Certainly group I, (Trapdoor level, standard factory loads). Don't know if it can take Group II (Win 1886 & Marlin 1895) level loads, but I suspect not.
If you CAN run 300gr loads up in the 1800fps level (rifle length barrel - no idea what a 7.5" would actually deliver), safely, I would expect that you will need to see a neurosurgeon later in life, and maybe not so much later!
Shooting a 3,4, or 500gr bullet at black powder speeds is a HELL of a lot of punch from something that is (less than?) half the weight and much, much smaller than a .45-70 rifle.
Enjoy!
One more thing, if you aren't experienced, and don't control it, my .45-70 Contender WILL wack you in the nose!! I would expect the same thing is possible with a BFR!