The older M118 used a 172 grain bullet vs. the current 175 grain bullet, it doesn't drop quite as much as the newer cartridge and that means that you would probably have a couple less MOA drop at 1000. I can't give you any real world numbers because I've never used that ammo, I reload. Again, I think testing in the field is the best way to go for two reasons;
- Bullet drop calculations on paper don't always match the real world (sometimes it's the calculator and sometimes it's the guy doing the calculatin' DI5).
- Like Ivan said, sometimes the scope just doesn't work as advertised.
There are several tests that you can perform on the scope, and I recommend doing them before you try shooting at long range, that will verify it's proper function. Going in to detail on each would use up a lot posting space so I'll just mention the tests that I perform and if you have questions about any of them just ask or do a search on this forum or the Internet.
Is the reticle straight up and down
Is the reticle focused sharply
Has the parallax been focused out at the target's range
Do the knobs create the same change in point of impact
Can you adjust the scope to get dead on target
And yes, I did my 20+ in the Corps. I was air winger with the Harriers out of Yuma, AZ most of the time.