IOR had some quality issues a while ago
Super Snipers usually only break once and are great after warranty fix
NF has a lifetime of the scope warranty it dont matter if you are a pre owned owner they will fix it and quickly
USO is good but very pricey cost as much as or more than the rifle the SN-9 had the have its front mount redesigned to hold up to a 50, know some one who broke it twice then they went to redesigning it not a thing wrong with the glass
with a 50 it is a matter of When rather than If you will break any scope made on one

trust me i know i started in the 50 game long before scopes were designed for them (eye relief, and construction )