Find a scope with 4 or 5 inches of eye relief, and mount it as far forward as the rings will allow. That's what I did with my .458, it has a Leupold 1.5x5 VX-III. On 1.5 power, it has quite a bit of eye relief, and that's still good enough magnification to get 1.5 MOA from the bench.
Also a slip-on recoil pad can temporarily increase the length of pull and sometimes this helps also.
What we have done for the hard kickers is make up a leaning bench. you can see these in Greeners book The Rifle and its Development and some other books I cant think of right at the moment. It a "U" shaped bench at standing height, keeps you verticle and lets you rock with the punch
(you need to lean slightly into the rifle). You can shoot very well standing and take a lot more recoil than sitting. Our first one was of PVC and anchored to the ground with rebar. the second of 4 X 4's, not really as satisfactory.
If you look at the old pictures of Elmer Keith and others shooting big guns like the .50 caliber bolt actions etc. they are shooting over the top of a car or off the hood of a truck.
Mike G is spot on with lower power scopes and a good pad or coat, helps keep your mind clear for tha task at hand!
Definately go with the Leupold line and low powered scopes. I have a 1.75-6x on the No. 1 416 Rigby and a 1.5-5x on the M77 .416 Rigby. Plenty of eye relief on both and lets be perfectly honest here, if you need more power than 5x on most any big game arm, you are shooting at game way too far away. I have shot my .416s out to 500 yards with no probems, sure you might have to go to a bigger bullseye, say 8", but any game you are going to be hunting with a .416 Rigby will have a kill zone of at least that much.
I'll second the motion on the scopes with long eye releif, they are absolutely essential on these big guns, and the comments concerning what many would consider low magnification, and applicability of range to your .416 are right on the money.
Another suggestion I'll add, is the addition of a 25 lb. bag of shot placed between your shoulder and the rifle. The dead weight of the shot not only deadens the felt recoil from you rifle, enhancing better trigger control and concentration on the target, it also largely tames the violent and sudden rearward bucking of these big boomers, thus alleviating the tendency to receive a case of "magnum eyebrow" when bench shooting these big magnums.
I like to enjoy shooting my rifles, not going out for an endurance lesson, and the shot bag helps me aviod developing a flinch when shooting these guns off the bench.
As aptly observed in an earlier post, when shooting from a standing position, such as you will most likely be doing with a dangerous game rifle in the field, the body will roll with the recoil, and you'll likely never be bothered by it if you have one of the suggested scopes with a longer than average eye releif.
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