I like its potential. Now about that rifle. Sci-fi rifles that have an elongated stalk going past the arm and voiding their use as a recoil bracer would only be that way for holding a power source or massive bullets. If the stock is shortened to roughly the middle of his four-arm, it would be conceivable that he could press the rifle butt against his arm and use the rifle conventionally. Do what you want though. Your desire to draw what you want is important.