Finnish Unlimited Ammo recommend to use under 4 millimeter pellets and double wads for protection of barrels. I would not use them in Purdey from 1880´s, but cheap pump guns like Remington or Mossberg are ok. My frieds use Benellis with UNA special screw-ins (light modified) and no scratches in the barrels. They use light 32 grams (1 1/8 oz)loads, but speed is over 450 m/s (1.500 fps). Unlimited Ammo produce too 5 and 6 millimeter UNA 18 pellets, but they are very dangerous, like fistful of .22 LR bullets on air.
