I am not sure what you mean by slow barrel unless you mean slower barrel twist. A twist rate is for bullet stabilization, and a faster twist does not necessarily mean more velocity.
I have SAKO M995 1:11" 24" that is propelling the 190 Berger VLD at 3043 FPS with 75 H4831SC, no slouch in my opinion.
Does Twist Rate Make a Difference in Velocity? Yes. Our tests show that, with the 80-90gr bullets, a 1:10 or 1:12 barrel will give you as much as 80fps more velocity than a 1:8 barrel, shooting the exact same loads. Fast twist (1:8) barrels have more drag and friction, which can slow the bullet down. Ideally you want to use the slowest twist rate possible that will stabilize the bullet you choose to shoot. For a dedicated "point-blank" 100/200 yard Benchrest gun, you want a twist rate from 1:13 to 1:15. But if you want to shoot both light (60-80gr) and heavy (100gr+) bullets, stick with a 1:8.
(SOURCE:
https://www.6mmbr.com/barrelfaq.html)
I am surprised that's all the velocity you're able to generate out of 168g VLD. I am propelling the 190 Berger VLD out of my .30 Gibbs 1:10" 24"
4-shot series, 57 RL17:
AVG = 2806,
ES = 15,
SD = 6