What info I give is based on over 40 years of loading ammo for competition and hunting. That doesn't make me an expert, but I have been around a lot of them. Companies like Hornady spend a lot of time and money testing load data with specialized equipment to ensure information they put out is safe. They are usually cautious in their manuals, to protect themselves, so we get to decide how far to push pressures past that point. I have been at the underground range at Hornady and watched as loads were tested.
In the case of the 6BRA, I have been using it for competition for over 5 years now. I have pushed several barrels past safe pressures and "good" harmonics to have a really good idea where safe stops. That includes most of the cut rifled barrel manufacturers with 5R and 4 groove. If we could get the velocities out of a 26" barrel, why would we shoot 30" barrels? In my testing of barrel lengths, there was an average of 75fps difference between a 26 and a 28 inch barrel from the same manufacturer in the 6BRA chambering.
The other part of the equation is twist rates in barrels. The faster your twist rate, the slower the velocity. I can run a 105 grain bullet faster in a 8 twist than a 7 twist barrel. I can't stabilize a 109-115 grain bullet in an 8 twist barrel, so now decisions have to be made on what will work best for LR accuracy. I want to push a bullet as hard as possible to cheat the wind and other factors as much as the next guy, but know that there are diminishing returns on what will happen if it is pushed too far.
Cheers