I have three tactical rifles with A2 stocks, Also one each McMillan light, heavy and "F class" benchrest stocks. Comparing the A2 to stocks purpose made for bench shooting the following differences are apparent. The A2 forestock has a strong taper relative to the bore line. The A2 forestock is relatively narrow, and it has a much smaller flat area compared to the benchrest stocks. The A2 rear sock also has a much steeper angle relative to the bore. The F class stock is very close to parallel with the bore for both the front and rear bearing surfaces. And probably the largest difference is that the A2 has a higher center of gravity over the rest bearing surface, especially compared to the F class stock.
McMillans description of the A2 stock states it's designed to shoot from sandbags and the taper is used to adjust elevation. That adjsutment is course for benchrest accuracy. Iwould not use the A2 for shooting benchrest matchs. Nor would I try to use the benchrest stocks to shoot from a sandbag or bipod. You can use either of course, but it just makes it more difficult to be competitive.
Benchrest stocks can be shot "free recoil", at least with smaller cartridges. That doesn't work well with the A2. It needs to be held when shot and it does not "return to zero" as smootly between shots. Each shot needs to be adjusted for cant as well as set for near neutral holding force on the target. Still, the A2 is easier to get "acceptable" accuracy for targets which move and is much faster to set up for a new point of aim than a bench rest. That's desirable for a tactical or long range hunting rifle.
Sorry Rob, I don't have an answer to your question other than to suggest you try to hold the rifle vertically stable using your trigger hand. You might get some benefit from a "bunny ear" rear bag which holds the stock in a V slot. Overall it's just not going to be as stable as a wide flat forestock.
