Nothing wrong with it exactly, just a little over gassed, meaning you are supposed to run heavy buffers. Carbine gas systems were developed for barrels under 16" such as on the military M4's. Since us peons have to stay 16 or longer the mid length was developed by civilian manufacturers to come up with the correct gas port location for 16" barrels. If you look at the distance from the gas port to the muzzle on a mid length vs a 20" M16, it's the same distance, meaning about the same force is exerted onto the bolt. Carbine gas system on a 16" barrel delivers too much gas pressure to the bolt.
Wasn't sure. I thought at one time all CHF PSA barrels were produced by FN?
