I’ve been shooting SR-25s, AR-10s, LR-308s, LR-260s, LR-6.5s for the better part of the last 30 years.
Not many scopes under $500 are going to take a beating from the large frame .308 Win.
The initial recoil resonating through the rifle and scope is the “easy” part, and that is not easy to hold together for most scopes.
Then comes the bolt impacting the breech and the carrier smashing into the barrel extension.
There’s a 3-impulse set of resonant shocks that do everything in their power to knock something loose.
Midway USA has Vortex Viper PST 5-25x50 on sale right now for $659, which is a steal. Might want to up your budget just a tad and get that. It’s a bit big for an 18” .308 though, but a great scope for entry into LR. I would prefer one of the 4-12x or 3-15x/3-18x shorter scopes for an AR-10, but most of them will be in the $800-$1200 range.
I just found the Vortex Viper PST Gen II 3-15x44mm Rifle Scope, 30mm Tube ,First Focal Plane (FFP) at Optics Planet for $585 with EBR-2C MRAD reticle. I think that’s your Huckleberry right there.
It’s not worth trying to cheat the price reaper on this with a .308 gasser. He will show up and knock your reticle canted just when you get into a groove of shooting at the most inopportune time, and that money does you no good in the moment.
I spoke with Vortex at SHOT when they debuted the Viper PST Gen IIs, and they were using CNC epoxying processes to lock in all the mechanical components to prevent things from shaking loose like on the Razors, so I consider Viper PST Gen IIs one of the minimum standards for a large frame gasser.
I personally prefer the Razors and NightForce, but those are way outside of $500. That said, I have a ton of trigger time behind gas guns with Gen I Viper PSTs, the 2.5-10x44 and 2.5-10x32 versions, with zero problems. I moved the 2.5-10x44 from my LR-260 to my 16” Grendel back in the 2000s, left it on the 16” Grendel for many years.
I’ve had the 2.5-10x32 on my 17.6” Grendel for 10 years straight, hasn’t moved from the day I installed it, and I’ve shot that Grendel regularly since 2014. It goes out almost every time I shoot LR. Zero issues, tracks still perfectly, center-punches steel at distance like clockwork.
