Today, we shot FN57 pistol and PS90 carbine into gelatin and at other targets to check performance. I'll post gelatin photos later. From pistol, 5.7x28 had more cavitation and 22WMR had more penetration. From carbine, 5.7 performed much better than from pistol, with some deformation and greater cavitation.
We also discovered that a can of beer can literally stop bullets.
27-grain lead-free load from PS90 at 10m failed to penetrate both sides of full Pabst Blue Ribbon can. The bullet tip made a slight indentation on the back wall of the can but did not penetrate! The can set 6" behind the first was completely unharmed. 40-grain ballistic tip penetrated both, though in fragments. Fired through a 2-3cm piece of bark, both loads fragmented explosively and made very little impression on the gelatin with their fragments. This is where rounds like 30WMR and 30 carbine outperform 5.7 and .223 -- their projectiles suffer less from intermediate barriers.
PS90 ergonomics are a whole other topic. It is commendably ambidextrous. The room for the forward hand is insufficient for many users. Backup iron sights are crude and placed high above the bore line. The integral sight sits even higher and is absolutely worthless. It's reticle is barely visible in daylight. Moreover, as it draws light from above and behind the gun, standing under a roof or wearing a brimmed hat or a cap is enough to cut off the light and the entire reticle disappears. It has to be the worst optic I've ever seen on a gun. The lack of bold lock-back or any way to distinguish an empty magazine from a misfire from a misfeed is not helpful.