Difference in velocity between my 20" and a friends 23" is a whopping 30 fps so personally I'd not go over 20" bbl. IMHO, 18-20" is about ideal barrel length for the 6.5 Grendel. Shorter if running a can and keeping overall length down but giving up some velocity and LR performance.
I haven't chrono'ed the 14.5" bbl (1.5" of brake/adapter for suppressor so no tax stamp required for an SBR) I have in 6.5 Grendel but I'm guestimating a 120 Berger at about 2400-2440. I should be chronographing this load in next couple days, definitely by next weekend and will post velocities at that time.
Nothing against a 12.5" barreled 6.5 Grendel but OP didn't state he wanted an SBR (if an SBR is even legal in his state of residence) or pistol build. I know a shorter barreled 6.5 Grendel will get bullets to 800 or beyond but in all honesty, the are running outta gas pretty noticeably past 600 when shooting reactive steel (LaRue, old style flash targets where steel rotates to show orange, etc.) so it wouldn't be my first choice for an 800 yard set up. 250 yards and in, running a suppressor and want really handy rifle....12.5" would be a great way to go.
120 Berger from a 20" is running 2550-2575 where it shoots best. Possible to get more velocity with some of the double base ball powders but that comes with more temp sensitivity so I'll stick with single base extruded powders.
