Seems like you are on the right track to change initial pressure to 1400 or 1500 in the model, but you make no mention of accounting for friction loss in the bbl. especially say a 28 inch bbl. Its pure engineering. Lower friction loss = higher velocity. Less bullet x section contacting the lands and rifling = less friction and lower initial pressure.That is getting a bit nebulous and fudge-factory and well beyond the accuracy of the tool. In any case, pick a bullet in GRT which probably has a start pressure around 3k and run your load. Record the velocity and pressure. Now change the start pressure to 1000 or 0 or whatever and get your velocity and pressure. It will be lower. Now increase the charge back up until pressure equal and see velocity. I would bet few 10s of ft-second which is in the noise of velocity variation. Some folks are showing several hundred ft/s difference and showing higher velocity with same powder charge. That is not representative of lower pressures given the relatively small knob dealing with
It would seem youd expect more velocity gain for a given load in a longer barrel using hammers vs. standard bullets
because of friction reduction.
You consume pressure in friction loss, ie, the lower the friction loss at the end of the bbl, the higher the pressure at the muzzle. The higher the pressure at the muzzle, the higher the muzzle velocity.
Its the pressure in the whole rifle system, not just the initial pressure that matters. The whole concept of hammers is reduced x section and friction loss.