If you use a quality free float rail flexing shouldn’t be an issue. Most standard profile barrels flex way more than a rail when fired or from sling pressure so a barrel mounted sight isn’t really improving anything IMHO.
I made $400 at the last buyback I went to. Spent it all on ammo. 1 revolver $200 and 2 .22s worth about $25 and got $100 for each one. The money came from a community fundraising. Suckers!