Optimal barrel length for .20 cal for highest velocity

[Big10gauge]

Need some advice on what would be the longest barrel length for a .20 cal (.20 tactical, .20 practical) with a rifle length gas port before getting bullet deacceleration in the barrel? Wanting to get the highest velocity possible before deacceleration happens. I know this can be powder burn rate dependant but there must be some general rule of thumb for this. Also does anyone have a formula for any velocity gains once the bullet has passed the gas port vs before the port? Thanks

 

[venatic]

You will always get more velocity with a longer barrel at least in lengths that are reasonable.Now the slight gain from 26 inches to 28 inches might not be worth it but its there. Here is the only info I am aware of on a 20cal but its not on a a barrel with a gas port but it will give you some idea.
http://www.bullberry.com/204Rugerdata.html
I used this info to decide I wanted a 22 inch barrel. Just remember this is one test and chrono's vary and ZI know folks with 28 inch barrels that are getting 4300 fps ....