My experience with the .416 Rigby cartridge was over all positive. The cartridge is magnificent, although I found I was paying close to $200.00 for 50 Norma cases.
There are a number of things that you need to consider if you are going to use this rifle for dangerous game work. These comments are genericly directed towards any dangerous game rifle as I have no personal experience with the Ruger RSM. The first item is that the rifle must fit you well. Fitting will make the rifle easier to shoot well, and will make it quicker to get into action.
I do not believe that any mass produced rifle is bedded adequately. In the case of a heavy hitter like the Rigby, cross bolts should be added to the stock, the tang should have adequate relief, and the action should be glass bedded for as precise a fit as possible.
Until you have the rifle in hand you will be unable to determine if feeding is acceptable. Again, if the rifle is going to be used against dangerous game, good enough won't cut it. Working the action vigorously must feed and eject cartridges every time. Make sure that if the bolt handle is going to break off it happens on the range rather than in the bush. If the bolt is difficult to ####, some polishing might be in order, and the same goes for a bolt that is not smooth to operate.
One of my pet peeves is the sling swivel stud. Invariably this is mounted at a mid point on the fore-end of the stock. If you have long arms, the stud in this position will result in a cut hand, so it is better to move the stud to the radius of the fore-end. My preference is a good quality leather sling, which is suitable as both a carrying strap and a shooting sling. The Galco Safari Ching Sling is excellent in this regard.
Sights need some consideration as well. You will most likely want to mount a scope on the rifle, but if you do a low powered fixed model or a low powered variable is your best option. A low power scope places you at no disadvantage should you need to make a snap shot. These scopes offer the most generous amount of eye relief, and the ocular lens should not extend behind the rear bow of the trigger guard or behind the cocking piece of the action. Being hit by a scope from the recoil of a .416 Rigby is no small matter. In case of scope damage, the scope should be in QD rings so that you can switch to the metallic sights, or to a second scope if you primary scope is damaged. My preference for iron sights is a ghost ring and post, but many prefer conventional open sights. Attention must be paid to the rear sight in that if it is a "V" design that the "V" is not so deep that it obscures the target. Which ever you choose, the sights must be regulated to ammunition you will have with you.
A problem I needed to address with my .375 has to do with the trigger guard. In recoil, the knuckle of my middle finger would painfully bump the trigger guard, and I knew if I did not address the problem quickly, I would begin to flinch. The solution was small nylon filler at the back of the trigger guard that just painlessly directs my knuckle downwards when the rifle recoils.