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Basic 9mm subs have about the same energy as standard loads. The most likely scenarios as I see:
Really dirty/dry gun.
They used a fixed mount instead of a booster. With tilting barrel pistols (most), you have to let the barrel float back a bit to unlock from the slide. You use a special mount called a booster to let this happen.
They used heavier recoil springs to prevent the gun from cycling - this would help keep the suppressed gun quiet. Alternatively, they may have used something with a locking bolt, but the hand motions look like your typical slide action.
It’s either a big whoops or intentional.
Basic 9mm subs have about the same energy as standard loads. The most likely scenarios as I see: