B, from the JRC website -- It should not be a problem for you to use Plus-P ammo in your JR Carbine. We initially prohibited Plus-P ammunition because the JR Carbine is a direct blowback weapon with a non-locking bolt. The added power of Plus-P ammunition caused a significant increase in gas discharge through the ejection port without giving any ballistic advantage over conventionally charged cartridges. We were concerned that the higher power ammunition could cause the bolt to move rearward too soon in the firing sequence, before the chamber pressure had dropped sufficiently, thereby prematurely exposing the case wall and causing potentially dangerous case failures. We have subsequently modified our bolt design and significantly increased the weight of our proprietary buttstock buffer. As a result the bolt stays in battery for a longer period, directing more of the combustion gases into the barrel and out of the muzzle. Plus-P ammunition may or may not provide a significant increase in performance at this point, but it should not cause any problems.

Plus p ammunition is fine but as with any firearm I'd recommend limited quantities because it can wear parts out quicker than standard pressure/velocity ammo but unless you're shooting thousands of rounds I doubt you'll ever notice a difference vs standard velocity ammo. Hope this helps.