Gas in your face from under the rear receiver cover? Did you check for pierced primers?
If so, try to take the firing pin out and examine it for lack of radius on the edge. If necessary, smooth down the tip of the firing pin with a very fine wet emery paper. Don't overdo it.
I think your results aren't typical. When benched I can keep all the rounds in a 4" diameter circle without too much fuss, using conventional corrosive Sellier&Bellot Czech ammo. Make sure your magazine makes full contact with the bench or not at all. Keep the forearm on the rest, not the barrel.
And use a six o'clock hold for your sighting.
Choice of target can also give very different results. You need a six inches diameter black circle with a 2" white center; this will give you a good reference point to aim at 100 meters. The reduced 200meter Palma Match is a good one.
If you want the .pdf file, just PM me.
Good luck!
PP.