You'd have to insulate the inside of the chamber not the outside.
I'm pretty sure anything on the outside would make near to no difference on a chamber with thick metal walls with such a short burn time, the projectile will be long gone before the walls heat up and then start heating up the surrounding air.
Edit: D'oh, I obviously can't read

Well in that case, I'd say it does have an effect along with chamber geometry and exposed surface area. But you're not exactly going to tell unless you actually measure projectile speed.
Even a 10% difference would be hard to notice when just shooting at point blank range.