This is a non-canon weapon that will appear later in the zombies storyline by HellHoundSlayer. It is a extremely powerful flamethrower, and is usually just known as the Phoenix. It emits high powered flames that will rapidly burn enemies. It was created by scientist at the pentagon i nthe early 1960's.
it is unknown what "R" stands for in it's name, but it is thought to stand for Raptor, as it was theorized to be created by Dr. Raptor.