Xausa,

an overly firm grip (with the hand that supports the rifle) is neither needed on a well-balanced (and typically heavy) double rifle, nor is it a good idea as it is detrimental to precision shooting. The gun needs to be firmly pressed into the shoulder by the trigger hand while the supporting hand (typically the left) should grip the gun as lightly as possible. The more force you apply on the supporting hand, the more it influences your shooting, which is difficult to keep consistent from shot to shot. All you want to do is to mount the gun the same way each time (keep the stance consistent also - this is overlooked by many people), and to keep the pressure of the butt into your shoulder consistent from shot to shot. If you apply an overly firm grip on the left hand bejond that, you introduce another variable that needs to be kept consistent from shot to shot. Obviously, you do not want to loose control of the rifle. To test this, I have shot my heaviest .470 loads (500 grain bullet at 2280 f/s) from a rest without using the left hand at all. As long as my right hand was firm (and rifle butt firmly in the shoulder) that was no problem, and so I knew I could keep the grip of my left hand as light as I wanted to. Takes a bit getting used to as the natural instinct is to hold powerful rifles firmly. I have found that my double shot like a precision rifle once I started treating it like one.