gramjet,
The 3 X 9 50mm Luppie is about right for the Rem 700 PSS and the .308 cartridge. I use a fixed 10X on my M40A1 clone and it's about as perfect a marraige of rifle/cartridge/scope I can imagine.
Variables have come into vogue because of the nature of our conflicts. The ability to "dial down" the power and get a wider field of view can be helpful. The trouble is that folks envision a tactical rifle being used for CQB which is almost never the case. Besides you usually have a spotter + armed with M4's. Sometimes you carry an M4 and the scoped rifle, but i don't recommend it due to the increased load.
The scope at 6 X allows for fast shots. It's not as limiting a factor as you might think with some practice. I the doubt the trained operator will see a difference.
Too many people get hung up on the variable scopes being able to go from low power to high. Unless you use a high quality scope you will lose some accracy due to mechanism mechanics. Only the really high dollar scopes are excellent throughout the power range.
For me the perfect tactical/sniper scope, on a .308 rifle, would be a 3.5 to 10 or 12X scope. No higher magnification is needed.
Above 10X, or so, magnification only amplifies some problems. Think you've got a slight tremor at 10X? Try it a 20X! It looks like a hound dog defecating razor blades! Reading wind and mirage though a high power scope is even harder. In short high power scopes have more attendant problems than those they solve.
High power is popular with guys that like to use rifle optics for spotting scopes. DON'T DO IT! While you're "tunneled in" the scope your situation awareness goes to do-do. It's a fact of life.
Anyway, I've rambled enough. Tactical rifle is a passion of mine, so I hope not to have bored you.
If I were to pick a Nikon Monarch "perfect" variable for your rifle I would go 3.5 X 10 power. Unforetunetely, Nikon doesn't make that model in the lower power range.
I'd still go Nikon 6.5 X 20 and be happy.
Here's a picture of my GAP AR-10 when it had a Luppie M1 mounted on it...
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My M40A1 clone with Luppie MK IV M1 10X:
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Wes