Hi Rev,
The Baikal single shot rifle IZH-18MH,which is the same rifle as Remington Sparten is factory chambered in 9.3+74R on there home site ! so one could most likely go larger.
I have a 500 NE 3 1/4 barrel on a New England SB2 same as H&R ,i have shot full Nitro 570 Grs in it recoil is savage, so just of late ( As i,m getting old and weak ) i have been loading Nitro for Black to use in it ,still very powerfull and more usefull on most things .
All started as a big quiet rifle (sub sonic) for big game .
But yes the N/E H&R is a little crude to spend a lot of money on ,The Baikal AkA Sparten is well made with some good features add some nice wood and full size barrel from chamber looks good ,just a little hard on the cocking .
Hope this is of some interest to you ?