You will gain 20-25fps/inch regardless of the bullet weight. If you use an 85 gr bullet and 1680, 4198 or RE7 powder you will need to open the gas port to around .098". Many book loads only produce 45-48,000psi while factory Hornady produces very close to 54,000psi. If the barrel is ported to run Hornady full pressure ammo it will short stroke with light hand loads. If you run full pressure loads there will not be a problem.
OP here...
I only reload for certain rounds so I may not do it for the 6.8 (if I buy one)...
I assume the base velocity values are from 16 inch barrels and your figures add to it.
Also:
so off the shelf ammo and no ports - any issues there on a 20 inch barrel & function?
Yes, 20s are usually 80-100fps faster than a 16".
Some ammo like Remington fmj(cheap) and Federal eagle do not produce 54,000psi. If the barrel is ported closely to run with Hornady they may short stroke with light loads but you can have the barrel ported to run with any ammo you want or ported a little large and then run an adjustable gas block. A #40 wire sized bit is .098" a 39 is .0995" that range should run all decent ammo with a standard carbine spring and buffer. If using an H2 or A5 setup you may need a larger port.
The port in the barrel controls everything. It really isn't a big deal to open the port to tune the gas to work with the load you plan to use.
Many don't really think about the whole process. They go to the range with cheap underpowered ammo to start and then get pissed when it doesn't cycle. They open the port to run with junk ammo then it is over-gassed when good ammo is used and it isn't accurate because the carrier starts moving while the bullet is in the bore. Tune the rifle to run with the ammo you plan to use.
