My 416 Rigby is a ***** cat to shoot... of course the decelerator pad, & heavy barrel help some...
I agree with the Marlin Guide gun... I have some loads I picked up from a Mag article where the author took it to Africa to shoot cape buffalo ( actually killed 2 with one shot ( to the professional hunters disproval ) shot through both shoulders of a bull, exited & killed the cow standing behind as well...
when I 1st tried these loads in my Guide Gun, by 6-8 shots, I dislodged the factory sights
another option...you could pick up the light weight H&R rifled 12 ga chambered for 3.5" magnum shells... my smaller stature buddy bought one... after the 1st shot, I held up my arm, & stopped his rearward progress about 10ft from where he 1st stood when he pulled the trigger...
another, pick up a 8mm milsurp carbine, with the steel butt plate & straight stock... I sighted in one of these for my FIL many years ago... in the fall, with an army field jacket & sweatshirt on, I still ended up with a very bruised shoulder after 15-20 rounds fired... at least this one would give you a badge of honor for a while after you got done...
the 45-70 can be handloaded hot & reasonably cheaply, the 3.5" magnum slugs aren't cheap, but way, way less than the magnum rifle cartridges listed... but the 8mm milsurp ammo is probably the cheapest route to go...