I have a Ruger RSM in 416 Rigby. I would like to
mount a Leupold 2.5x8 VariX III on this rifle. The
rings supplied for this magnum lenghth action
will not accomidate the relatively short scope
tube on this VariX III. Any suggestions for after market bases and rings to mount this particular scope on this rifle? To anyones knowledge, would an extension mount be available?

Ruger does offer a set of rings that has one of the rings offset 1/2". The Ruger part number is D71MO for the model 77. They do list a set double offset rings, but only for the #1 single shot. Hopefully 1/2" will be enough for you. Those long actions can sometimes raise havok with the scope mounting!

The 2.5-8x is a fine scope and we have two of them, both mounted on No. 1s, one in .300 Wby and one in .375 H&H. On the M77 .416 Rigby, like was said, you may try the offset ring. Or, IMO, stick with the 1.5-5x straight tube scope. The .416 Rigby can be a long range round, but it is not such a long range round that 5x will not be more than enough power to get the job done, and at 1.5x, you will count your blessings if you ever have a fast shot up close. I had a hell of a time getting a 1.75-6xE Leupold to mount on the No. 1 .416 Rigby and had to resort to double offset rings. Again, just like the 2.5-8x the forward bell can cause real problems and forget about the 1.75-6x on the M77 .416. I tried and it and there was no way to make it fit for me, even with offset rings. The 2.5-8x is a bit longer and will give a bit more adjustment, but not near as forgiving as the 1.5-5x. IMO one of the biggest mistakes American hunters make is overscoping their guns. Using the 1.5-5x on my M77 Rigby, I have shot out to 500 yards, targets of course, and while you do need a bigger bullseye to shoot at, it is nowhere near as big as the kill zone of any animal you are likely to hunt with the .416 Rigby.