We have not received any complaints regarding the turrets and tracking on this optic, but many people just set the turrets to zero and use the reticle the way it was designed to be used which is with holdovers.
You would have to recalculate the drop markings for 6.5 Grendel using a ballistics calculator.
No, neither of the options listed are for a .300 blackout. One is meant for 5.56 and the other is for .308. They can be used with .300 blackout BUT you will need to do your own measurements and figure out what the hold overs will match up to.
Yes, the Primary Arms SLx 4-14x44mm Rifle Scope features exposed tactical style turrets with Zero Reset.
You would be able to use low height rings due to the height that M1A scope mounts provide. I would recommend the TRYBE Optics Advanced Scope Rings, found by using the link below:
https://www.opticsplanet.com/trybe-optics-advanced-scope-rings.html?_iv_tube-diameter=30-mm&_iv_ring-height=low
The models at the link below have the 5.56 reticle.
https://www.opticsplanet.com/opmod-primary-arms-opmod-4-14x44mm-ffp-308-223-riflescope.html?_iv_reticle=acss-hud-dmr-5-dot-56-nato
The mount or rings you need for the Primary Arms 4-14 x 44 mm Rifle Scope will depend on what model rifle you are mounting it to. The guide at the link below will help you determine what rings you will need.
https://www.opticsplanet.com/howto/how-to-how-to-measure-scope-height-scope-ring-height.html
The Primary Arms 4-14 x 44 mm Rifle Scope has a reticle designed for .308 or .223. However, the Primary Arms 4-14 x 44 mm Rifle Scope could be used with another caliber such as 300 Win Mag. You would need to sight your rifle in with the scope but understand that the suggested elevation and windage holdovers will not be as accurate since they are not calibrated for 300 Win Mag. So you would need to make your own corrections.
Here is a guide that can help you determine what specific rings would work this Primary Arms 4-14 x 44 mm Rifle Scope.
https://www.opticsplanet.com/howto/how-to-choose-and-install-mounts-rings-bases.html
I would recommend a set of rings like these:
https://www.opticsplanet.com/vortex-precision-qr-extended-cantilever-mount.html
The Primary Arms 4-14 x 44 mm Riflescope 30 mm First Focal Plane is designed for either a .308 or .223.