Speaking of the magazine barrel gun, I am disappoint in catalyst for going through the trouble of introducing Metal Storm guns and then failing to represent how they work mechanically at all.
It combines the clip and the barrel into a single unit and has bullets fired electronically in sequence, making any gun built in this manner capable of full-auto fire at ludicrous speeds, and makes the gun more accurate when using bursts, since it empties all the bullets in the barrel at the target before it even finishes recoiling. This would be easy to represent mechanically in 5th edition with either increased accuracy, a high recoil compensation, or even just a line that says 'Metal Storm guns do not suffer recoil on the first shot, but progressive recoil still applies on attacks made on additional passes.' They included two metal storm guns in this book, and neither one has improved recoil compensation or accuracy over any other gun of their type, and the metal storm shotgun is only Semi Auto!