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Too many variables. Powder burn rate and density, bullet weight, caliber...... There's too short - unused gasses and there's too long - friction overtakes pressure...... Changing powder might compensate for some of that, but not necessarily the way you think. For instance Bullseye gives less velocity in a 2" 38 Special than 2400, and Bullseye is way faster burning.
I think the T/C barrels are a 1:14 twist on the Hornet. Doesn't the BFR 10" come with a 1:9 twist? If you're looking at T/C, I might be inclined to start longer. You can always shorten it.
I had typed a detailed responce but for some reason it didn't post. I probably hit the wrong button. Here is the summary
The optimum barrel length is the length that provides for the expanding gases to be most effecient.
B = L * (D*D) * .773
B = bore volume in cubic inches.
L = length traveled by the base of the bullet from seat to muzzle in inches.
D = groove diameter of barrel in inches. (you need to square D)
.773 is a industry constant
The Bore volume along with powder chamber volume can determine the expansion ratio.
R = B + U/U
R = ratio of expansion.
B = bore volume in cubic inches.
U = volume of the powder chamber in cubic inches.
This is used to help determine velocity.
As Tycer said above, there are many factors to consider. Here are a few more. On one hand a longer barrel provides for a better sight radius which should increase accuracy. That's good. But on the other hand a longer barrel has more movement, and flex because of the expanding gas. That's bad.
“Optimum” barrel length can and is calculated for a specific weapon / powder/ projectile. This is not to say that this is truly the perfect, but pretty darn close, usually within 0.0001” for some weapons. That being said, there are few, if any, practical applications for this in either target or hunting situations. But it makes a GREAT academic exercise.
Many variables not yet listed in all of the post above are to be considered: ambient temperature, barrel temperature, relative humidity, elevation above sea level. Then you get into about 13 or 14 powder parameters, about 10 projectile parameters, barrel parameters, barrel twist rate, land and grove dimensions, barrel crown profile, target distance, just to name a few.
You get the idea. This is possible and with of LOTS of data, the aid of computers and some proprietary software, you can calculate “best barrel length”. Bottom line, I believe that a weapon / cartridge combination can, and is, far more accurate than any human can totally utilize.
Find a weapon and ammunition you feel comfortable with and spend the time to know where you shots will land is all of your expected situations. RANGE TIME RULES.
