I tend to view RAC calibers as direct upgrades to their Ultra counterparts, due to them only being a ton more and, in the case of the Ultra 5-Class, a crit more; The fact you can fire six shots if you want as well as unjam them makes it all the better.
Of course, it's also only useful if you do it right. While unjamming, you had best hope you can get to cover before your enemy gets to you, or have the rest of your lance act as support. I've had a triple-RAC custom unit get left alone when all three guns jammed over the course of two turns, simply because it had a trio of 'mechs standing around it giving it cover, and nobody wanted to deal with that triangle of trouble. I feel like it also gives the weapon the quirk of being more useful in a combined arms environment, where you can sandblast enemy armor and internals when needed, and be defended by the rest of your units when the weapons inevitably jam.