I have owned two of the 45/70 BFR Revolvers. My first was a 7.5" which seemed too short for efficient powder burning. The second, which I still have, began life as a 10.5". I sent it to Ken Kelly at Mag-Na-Port, and had it cut to 8.5" with the 5 port porting, bead blasted, action job, and sling swivel studs installed. Believe it, or not the extra 1" of barrel does make a difference.
It now shoots and handles as well as any other big revolver I have owned. As a note to those who looked at these guns in the past, but did not like the quality. Magnum Research (MRI) has these revolvers made under contract. The pre-2002 guns were made by D-Max, and they left something to be desired (mine is a D-Max). The new manufacture guns are far and away superior to the old ones. They are much smoother and have a better out of the box feel. I do not own one, but have shot and hunted with the .475/480 (.475 Linebaugh and .480 Ruger interchangable).
For those who denigrate the 45/70 BFR in favor of the .454 Casull, etc.. If you are quite brave you can approach and even exceed the 45/70 Govt. with the Casull (I have one, and have tried it.), but you will do so only with lighter bullets and at a much higher pressure. In the big BFR, the recoil from the big bullets is more of a push rather than the sharp rap of the Casull.
The barrel cylinder gap on BFRs is very tight. Even so, there still seems to be an approximate 15% velocity loss with comparable barrel lengths in the single shot handguns. I have a 16" contender in 45/70 and a 12.5" RPH Hunter, and that is an estimate based on the velocities from the two SS pistols. The length of the actual barrel (without chamber) is close to the barrel only length of the BFR. Not apples to apples, but my best guesstimate based on chronograph readings.
In my opinion the ideal load duplicates the old Us Army carbine load-405 gr bullet at 1200-1300 fps, which is very obtainable in the BFR. I never found a heavier jacketed bullet, which would reliably expand in the 45/70. So I settled on CPBC 405 grainers. You do have the ability to shoot 500 grainers in the 45/70 BFR, but one can only stand so much fun.
Faster burning powders and magnum primers work best for me. I really like AA-2200 with Federals 215 primer under the 405 CPBC.
For the newer shooters, you will generally get higher velocities in a 45/70 handgun with heavier bullets, as the lighter 300 grainers do not offer enough resistance for complete powder burning. This has been verified by chronograph-not typewriter. None of this is to say that the 45/70 is better than the Casull, etc., or vice-versa. It is just different experience in a revolver, and you should give it a try before denouncing the concept.
I am sure many on this post will disagree with what I have written. Please feel free to rant and rave if your PERSONAL EXPERIENCE differs, as those differences bring a smile to the SAAMI manufacturer's faces.
If you have no personal experience, please get some rather than doing your shooting through the experiences (or stories) of others.
Good Shooting