The 5.56 pistol length barrels are tough. It really depends on who made it. Because the gas port is so much closer to the chamber, the gas pressure is much higher. So port size is critical. A difference of 0.001” or 0.002” will make it huge difference in the violence of function. It is so easy to be over gassed, which will beat on the bolt group. Don’t run more than a couple of mags as the gas is still burning and very hot. It is easy to melt the gas tube. As I recall I didn’t get through a complete Beta-C mag before the gas tube failed. In the picture above, a fraction of a second later, there would be fire coming out of the ports in the side of the carrier. I find the fireball at the muzzle to be so bright that even in bright sunlight, I find it hard to see the target once I start to fire shooting full auto. The word best describing the function would be “violent”. I have a couple of 7.5” uppers, but I hardly ever shoot them. As Richard said, they are very loud. Double hearing protection is recommended. They are a fun novelty, but nothing that I would want to dedicate a firearm to.
BTW, if you want an even bigger fireball, spin off the flash hider. There is a device called a flash enhancer. I don’t own one, but I am sure to wear gloves when I shoot a 7.5. The fireball burned the hair off my fingers.
Scott