velocity differences

[TreyAzagthoth]

My dad is loading for a 45-70 with a 350 gr hornady jacketed flat nose bullet.  The hornady manual is using a marlin lever action with a 22" barrel.  He is shooting a 24" encore.  How much velocity difference can he expect with an added 2" of barrel?

[Grumulkin]

If you're asking me to guess, I would say 50 to 100 fps assuming there were no other differneces in the barrels like barrel wear, a longer throat, etc.

[Siskiyou]

A tough question to answer and most likely there is not a correct answer.  Firing the same batch of loads out of two different .270 Winchesters with 22-inch barrels showed a difference of 40 fps between rifles.  Switching loads and bullet wieghts still showed that one rifle was faster then the other.  Rifle "B" may need two additonal inches to keep up with rifle "A".  Testing was done on the same day, same location, and the same general conditions.

Rifle "A" has proven itself in the field numerous times over the years.  Rifle "B" is newer but proved itself on deer.  If I did not have a Chrony I would not know that it has the slower barrel.  If 40 fps was to make a difference it would be at a range far beyond what I normally shot.

[PaulS]

My brother and I have twin revolvers. They have the same barrel length, chambered in the same caliber and even the serial numbers are within a few digits of being the same. His shoots my ammo at 120 fps slower than mine with 140 grain bullets and mine shoots his ammo with 158 grain ammo 94 fps faster. Where is the difference? I can only guess that his barrel is slightly tighter than mine is. It is not measurable with machine measuring tools. At least not at .0015" .  I think that the bore condition and its diameter is at least as important as the length in determining the velocity of any particular load fired in it.
In answer to your question the rule of thumb says that a two inch longer bullet in the velocity range of a 45-70 will gain about 100 fps. Will it? that is why you should use a chronograph. You can get one for less than the price of a top of the line single station press.

[EsoxLucius]

26 fps per inch of barrel with 40,000 CUP loads and 20 fps per inch of barrel with 28,000 CUP loads is a good rule of thumb.