65G HLB Project

[BluntForceTrauma]

I first want to understand the theory of rifling as it applies to chamber pressures.

Let's start with a hypothetical smoothbore 6.5 Grendel 16" barrel, whose bore is exactly 0.264", with zero rifling.

I load and fire a factory Hornady 6.5 Grendel 123 Amax round and get 2500 fps. Remember: Smooth bore, zero rifling.

Now I magically add SAAMI six-groove 6.5 Grendel rifling to that barrel and fire a second round.

Is the velocity going to go up or down, and why?