[jbpmidas]

How much velocity gain from going from 16" to 24", 26" or 28" on .223 AR?

 

[captain-03]

Just a guess -- 100fps per inch up to 26"

 

[sigma74216]

This is the best I have found.
http://www.chuckhawks.com/rifle_barrel.htm
Look under velocity loss (or gain)

 

[jbpmidas]

Thanks Sigma!

 

[titleiiredneck]

About 45 fps per inch or so, but chamber, powder, bullet type ect play a role in this also.