Not too complicated, it's basically a .300 Win mag cut right before the shoulder starts to let it seat .458 cal bullets.
Advantages? Cheaper brass, cheaper rifle (than say, a .458 Win Mag), 450 Marlin performance or more from a very accurate platform, box-type magazine, capable of taking really long or even pointed bullets.

Here, see the similarity between the brass length and diameter of the 45-70 and that of the .300 at the start of the shoulders. The rifle only needs the right barrel, but even getting a barrel made or one reamed out and re-rifled is cheaper than most big bores, usually.
The end product would be, for me, an 18" or so rifle with a light plastic stock, running a new model 70 action with those perfect triggers (honestly, they're better triggers than most target rifles I've felt, I mean GODDAMN) that throws a 400gr Speer JSP at 2000ft/s or so.
Advantages? Cheaper brass, cheaper rifle (than say, a .458 Win Mag), 450 Marlin performance or more from a very accurate platform, box-type magazine, capable of taking really long or even pointed bullets.

Here, see the similarity between the brass length and diameter of the 45-70 and that of the .300 at the start of the shoulders. The rifle only needs the right barrel, but even getting a barrel made or one reamed out and re-rifled is cheaper than most big bores, usually.
The end product would be, for me, an 18" or so rifle with a light plastic stock, running a new model 70 action with those perfect triggers (honestly, they're better triggers than most target rifles I've felt, I mean GODDAMN) that throws a 400gr Speer JSP at 2000ft/s or so.