My question is about the velocity difference between a 7.5" revolver barrel and 15" Encore barrel in 45 LC. If my revolver load is shooting 1300fps what should I see in the 15" Encore?

My guess is around 250 FPS more.

You might see 300 fps but it's really going to depend on the load especially the burn rate of the powder. When I shoot a handload of 296 I see 1450 in my 4 3/4 and 1900 in my 10 inch. I recently tested a Winchester load and only saw 100 fps difference between the same 2 guns.

There are several approximates for barrel length and velocity. The most common is about a 50 fps increase in velocity per inch of barrel length. Again these are approximates and are dependent upon several factors, i.e. bullet weight and type as well as the rate of twist in the different barrels. Were this theory to hold true you'd see an approximate increase of 375 fps for a total of 1675 fps in the 15" barrel. A Chronograph is however the only true way to tell for sure.

Hardball, you have a pretty good idea!!! TC Tec’s have told me, and it has ran pretty true, that it is 33 fps per inch AFTER the barrel length is 14-15in. From 10 to 14 it is 150-200 fps difference and from 12-15 it is 100 to 150 fps difference. Gary is right on about the loads. The heavier and broader the bullet the less FPS it will gain. A 44 Mag shooting a 300 XTP will have somewhere around 100+/- fps from a 4&5/8 handgun to a 20 in rifle. A 22-250 can loose over 400 fps going from a 15in to a 26in barrel. So basically it also has a lot to do with burn rate of powders as Gary stated. The key is matching powder with barrel length (this is a general rule of thumb and not a “law”) and bullet weight.

Duane