It appears impossible to shim as the turrets use internal push-pull action. The turret height does not change between the extreme upper and lower mil range. I'll post pics in a bit.
Diamondback Tactical FFP 4-14 turret pics
I received my 4-14 today. Comparing it to the Nikon FX1000: turrets feel more “clicky” and sharp (less refined?) but still good; glass is noticeably less clear, eyebox is smaller, and reticle is too thin at 4x. However it is also half the price of the Nikon, and seems like a great value.
This...
Not a factory load, but my 16.5” 308 loves 178 eld-x bullets. Hornady Precision Hunter line of factory loadings includes that bullet: https://www.hornady.com/ammunition/rifle/308-win-178-gr-eld-x-precision-hunter#!/