GTO pellets shoot great out of my FX Royals in 25 and 22. It's like shooting sub sonic rimfire ammo speed wise. I find I have more fly offs using the GTO pellets probably because of the lack of momentum of the lead pellets.
Inside a barn shooting my FX Royals around 470fps I try to get a metal beam as a back stop. I don't do headshots but aim at the center mass to keep the pass throughs down.