A longer barrel length will increase the burn time and the velocity.

It will not affect the max pressure as that max pressure time occurred in about the first inch of the barrel.

Incidentally, if I remember correctly; a cast bullet needs about 1160 psi to get it going (if It is not jammed up against the rifling), while a jacketed bullet of the same weight needs about 3626 PSI.

Don’t quote me… if you want the exact calculation numbers I can reference that start pressure data and post it.

To help illustrate my point on the criticality of the seating depth let us take the SAAMI Min of 1.780 for the 300 AAC Blackout

Using the same 9.2 grains of H110 powder, & a seating depth of .839 which (also happens to translate to a 100% fill) in that cartridge - bullet combination.

The prediction now becomes ~36000 psi and 1200 FPS.

The seating depth difference between .839 (SAAMI MIN) & .423 (SAAMI MAX) = ~ .416 difference in seating depth.

The max pressure difference goes from ~ 13000 PSI at max COAL to ~ 36000 PSI at min COAL.

(Both calculations I believe are valid within SAAMI specifications for the parameters utilized in the separate calculations).

I hope that this Intel helps.

Take Care & Stay Safe