I'm on my third .45/70 and the second Marlin.
My current one is a GuideGun, but no ports, with Ballard rifling.
It wears a Williams reciever sight and fire-sight front sight. I has in recent past also worn a Bushnell 1.5-4.5 Banner.
It has the 6-groove rather than 12groove barrel. My 12 groove barrel shot cast bullets well, but needed a .460" sizer. My current isn't picky.
I've only shot deer with CAST bullets. No jacketed bullet has been fired through my GG since I bought it in '05 (it's a ~00-01 vintage gun). I've got a box of 500 Remington 300gr HP's. Never opened. Probably never will.
I also had an H&R single shot. It was too light, and not accurate with cast bullets (large bore?) but shot factory 405gr Jacketed well. It was brutal on recoil with anything besides light factory loads. I sold it.
The performace on deer with a 400gr bullet at 1,200-1,700fps is well documented. All my deer have been bang-flops or less than 20yd, even the lung shot ones. No bullets have been recovered. My 405gr HP Lee mould dosen't seem to perform any differently than Flat-nose Hollow base. With the hardness of the lead I use (air cooled as well as heat-treated w/w metal), it doesn't seem to expand much, but dosen't need to.
After seeing my .45/70 in action, my younger bought one. He uses a commercial cast 405gr FNGC @ ~1,800fps for elk, mule deer, and antelope (he lives in NV). He prefers it to his pre-64 Win Mod-70 FW in .308.
Expect a large two ended hole, with some axial damage from bone shards. Not a lot of blood shot meat. Nothing really works any better on deer. Only drawback to the .45/70 is trajectory beyond ~150yds. Inside 150yds, it point and click.... and most shots ARE within 150yds.....
My prefered load for deer is the RCBS 300gr FNGC (cast to 322gr w/gascheck) at ~1,650fps (30.0gr #2400). It is ~2moa at 100yds and will shoot through anything in N.A. except a lengthwise shot on a large moose or brown bear. The only reason I'd increase velocity would be for longer shots west of Mississippi river. If I'm hunting where I'll need longer shots, I'll use something else.
this load has tolerable recoil, is inexpensive to shoot, and I enjoy shooting it. A 400+gr bullet at over 1,500fps becomes "un-fun" quickly...from a 7lb rifle.
For hunting feral pigs on hardwood river bottoms, there isn't much better.....