You're going to find a little flex on any free float handguard. It's just the nature of the design. I think that's why a lot of people just leave their FSB in place and learn to ignore it when shooting optics. Also some of that flex is the barrel. The whole point of a FFR is to allow any pressure on the handguard to flex the handguard and not affect the barrel. I get your point about the BUIS being attached to the handguard and being shifted if you're putting a lot of pressure on it, but in that case your primary sight, the Eotech, should be more precise than if you'd had a non FFR. It's just a tradeoff. The good news is you saved a little weight and you're very unlikely to need your BUIS, especially for a long shot.