What is a realistic velocity for a 6.5 Grendal with 120-123 gr. bullets from an AR-15 with a 18-20" Bbl.? (safe pressure limits) Advertised vs. published?

with my 16" barrel I am getting right at 2400fps+/- with 120gr Sierra pro hunters bullets. This is using the max load data from the Alexander website. I was told that the difference between the 16" and 18" is approx. 200fps increase. The difference between the 18" and 20" was less than 100fps difference as reported by a member on the 6.5 Grendel forum. I think they still have the information posted on the forum there under the reloading forum. With that said all rifles are different and will produce different velocities. Most of the reloading data on Alexander Arms website is with 24" barrel.