Have a Baikal Double-Rifle in .45-70. What I would like to do is throat it out from a 2.55 length to a 2.85 or 2.95 length for additional powder capacity within pressure limitations. Has anyone done this before, or have comments as to reasons not to do it?

Do you mean "throat" or "re-chamber". Either way, you increase case capacity. Yes, it can be done and there is no reason not to as long as you keep track of pressures. BUT, I have found the Baikals to be so light that even good 45-70 loads within current allowable pressures which I think is 32000 psi; will kick so much that you don't want or need to go faster.

Throating was what I intend. Rechambering further on the thinning walls is not an option. I plan on keeping pressures to a maximum of 28,000 psi.

I agree the Baikals are light. I have fired enough 400-450gr cast bullets through it to become very familiar with the recoil. I was hoping to bring the 300-350gr bullets up a bit in velocity.

Jeffeosso made one.
Just do a 45-90
Use 45-70 ammo or load out or use 45-90 ammo.
Starline sells good brass for 45-90.
You should get 450 BPE velocities.

Boom Stick, you are a wonder. You seem to remember things from waaaay back. I'll send a private mail to Jeffeosso to find out about his .45-90.
Thanks!

I think El Jeffe just long throated the 45-70 calling it the 45-70 Nitro I think.
Reaming out to 45-90 will get you there too but more options like I said. The 45-90 is 2.4" so that will allow you to use normal rifle bullets when using the 45-70 case. I think the ability to use 45-70 ammo AND 45-90 hand loads is a great option. I think the 45-70 or 45-90 with tipped non cons or Raptors would be AMAZING! Retain a lot of power with the tipped bullets. Being a bore rider bullet design you could push those 300 grainers out pretty fast. I wonder if the 45-90 with the 300 grain Raptor could do 2,400 and keep the gun safe around 35k psi?

With just a longer throat, you won't get much more case capacity with the lighter bullets as you can only seat them out so far and still have the bullet in the case; need to have at least .1 of it gripped by the brass. Unless you intend on breech seating them. Which would be inconvenient whilst hunting.

Boomy,
Yeah, I finally used my head and did some searches. Saw what Jeffe had done. Since I plan on using this double here in the states, the long throating could be done just enough to hold .3" or .4" of the noncon or raptor in the case. This would allow other bullets (including heavy cast) to be long seated and provide a reasonable gain in velocity.
The problem with the Baikal is its only rated to 28,000 psi/cup (one of the few times cup and psi match).

I did that with a Siamese Mauser bolt gun. It shot 400 grain Speers very accurately. It never did shoot the stubby 300 grain bullets well after that.