Buna-N will serve you well for static (non-sliding) seals in PCP rifles. Elastomers do of course exhibit a change in elasticity with temperature but the seals will have already conformed to the mating surfaces. Perhaps it would be different if you rebuilt a rifle and then placed it in the cold before attempting to pressurize it.
Actually, it also does quite well for dynamic seals but other materials like polyurethane have better abrasion resistance...at the expense of other tradeoffs like high temperature rating and compression set. So for example, it makes an excellent choice for a bolt probe O-ring.