I used a SSAR stock on an AR-15 a few weeks ago. It was an ok novelty item. I had to use a loose hold to bump fire. With an M-16 I can hold the stock tightly to my shoulder and control it more. I was all over the 50 yard berm while bump firing; much less control than with an auto sear rifle.
It was fun, but I would not waste the ammo even if the stock was free.