I got one J4.. I know I know lot of controversy on them.. I look at it as a non reliable fun at the range thing a dinga.
Had it on my super shorty, but had to put it on one a bit longer and heavier to make it work.. Would I count on it in a SHTF situation? Hell no..
Not jamming, just stops firing if you do not hold it right.. Not conducive to a long life if you need the damn thing to shoot.. Fun at the range, but I damn sure would not want to count on it in a bad situation..
Bump Fire stock, load yourself down with a thousand rounds, 75 round mag, your called a "Target."