Rossi Rifleman

I'm wondering if I can use this ammo in the rio grande?

http://www.garrettcartridges.com/4570420tech.html

You might be a bit of a "test pilot" with this. Supposedly it is tough to relate CUP to PSI but the formulas in use tend to converge at 35. In other words 35K CUP equals 35K PSI. I just started shooting my RG4570 at this level.

What would concern me most is the lack of throat used in the RG chamber, there is no taper. My .458" 300-grain bullet will only chamber at a cartridge overall length of .2.520". My 350-grain bullet at 2.420", and my 425-grain bullet at approximately 2.390". The problem is that the large bullets of a fixed diameter mess with the tapered case, creating a bulge along the seated length of the bullet. As the bullet gets heavier, the in the case length increases which causes the bulge. Rifles like the Marlin use a 3° tapered throat to allow the use of heavier bullets but Rossi does not. My bet is that the cartridge will not chamber. You can purchase piloted throat reamers for about $80 with shipping. These are hand tools that you use to relieve the throat until your desired cartridge can chamber.

I ran the stated velocities of this ammo through QuickLoad and against my pressure trace data with my TLC460-425-RF bullet and I believe it is sitting at 35.0K PSI. I still suspect that it would have problems chambering. At $80 per 20 count, I think I would would get interested in reloading!

I'm also going to move this over to the Factory Ammo forum. Might be easier found over there in the future.

I just bought a throating reamer so my RG4570 will chamber a large cast bullet topped cartridge. Here is the post: Bought Throating Reamer for the RG4570. With my 425-grain bullet loaded to 35.0K PSI I'm looking at this lightweight rifle delivering 3085 Foot Pounds of Energy (FPS). I get all this settled I'm going to take it nilgai hunting.

The RG seems to be a good stout firearm that is capable of far in excess of legacy (Trapdoors, Springfields) velocities from the black powder era.
It seems to me from perusing reloading manuals that there are 3 levels of .45-70 hand loaded performance: 1) legacy.....2) 1895 (Marlin)...........3) Ruger #1.
The graphic representation above is probably not in actual fact linear. I just made it appear so for the sake of simplicity.
The RG looks lie it would fall between 1) & 2) above & lie closer to the 2) benchmark.
If I wanted to build a super duper .45-70 I'd have a Browning BLR Takedown in .450 Marlin re-chambered to .45-70.
The Browning BLR is rated around the 65K PSI mark which is 3 or 3+.
The BLR is also fairly light, comes in Takedown (which I prefer) & is box magazine fed from the bottom of the receiver with no need to use flat nose or FTX style bullets.