Oldelkhunter: I have a Buffalo Classic 45-70 single shot and am pushing a 405 gr Rem Bulk bullet @ over 2000 FPS out of it's 32" barrel. I have the Leupold VXII 1-4 20 mm mounted on it since new with around 1000 of these rounds thru it to date. Other than tightening the ring screws every 250 shots or so it is holding up fine.
BTW I had a old Weaver K2.5 20 mm on my Handi Rifle in 45-70 with those same loads and it also held up perfect with over 2500 rounds thru it after I had it on a 35 Rem for years. sold the rig to my buddy when I got the BC and he still is using it .
That 45-70 has accounted for alot of elk for me over the years. BT