With regard to NPA here's a tip that I recently learned from a very experienced instructor USMC scout sniper and Hunter. Field shooting is not like pure bench shooting. So zeroing and shooting groups should simulate best case scenario shooting positions. Prone being the most stable, is where he zeros. Natural point of aim by his definition is where the rifle come to rest at the respiratory pause post exhale. The rifle will move Naturally while breathing a we should not fight or restrict that movement. Basically, the sight picture appears to shift the Point of aim/impact down below the intended zero while inhaling and as we exhale the rifle settle back to the NPA and the reticle zero rises back up to the intended POI. He posted a video of this and my ability to consistently shoot groups has improved considerably. All my shooting has by Establishing a solid NPA and allowing the reticle to move with my breathing and trusting it to come back to the NPA when it's time to break the shot. In my opinion it is much easier and more natural than trying to hold the rifle on the zero group throughout the breathing process, which creates tension that hinders accuracy.