If shooting for group & impact with hunting loads, I hold exactly as Don does, gripping the gun's stock as in offhand shooting (left hand at the entry pipe) fingers gripping up alongside the barrel or barrels, and resting the back of tht hand on the front bag. I don't like a rear bag with a ML and find it doens't seem to help.
A rear bag is best used imho, for a dedicated bench position, were the off hand, the left hand (right handed shooter) reaches backand under the stock and grips the bag just below where the stock engages the shoulder. Squeezing and relaxing the grip on this rear bag raises and lowers the rear of the rifle which in turn gives very small elevation changes. The forend is resting alone on the front bag at the position that gives the best accuracy, no where else. The best positon is found through shooting.
The best position is the postion that eliminates all 'flyers' and gives consistancy for you. This is the postion that should also show the smallest groups. Some guns give this by resting as Mike noted, just behind the muzzle, while others require to be rested farther back. My .40 prefers to be rested at the second pipe for chunk shooting and the next time I bench her, I'll start there before moving back to the entry pipe where I usually rest it.
Rest shooting with free recoil, without holding, will usually give up to 2" of elevation at 50 yards. There usually isn't a windage change - but possibly could be. I'd be looking rather at form - mine - feet flat on the ground, proper height, not strained, not pushing into the gun, rest, without steering. Many left and right shots are caused with low and high velocity rifles is caused by the cheek steering the gun into being lined up. Between the cheek and the right hand steering shots abroad, more lost shots left and right - fliers if you want to use that name, occur.
Beware the position and check it when 'fiers' occur. It's probably something you did. Between form and accuracy, many complications are present for lack of a better way of saying this - ball, patch, powder charge - then there's the method of securely holding or not holding the gun on bags, then your form at the bench -solid and square. All of these 'conditions' effect how the gun shoots with fliers and all and finding what caused exactly what, can cause hair loss.
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