Simple answer: You don't.
Better answer: Aim for vulnerable areas, such as the tracks, the rear, on top of the engine deck (behind the turret), and the commander's cupola. both the AT rifles have the same penetration (I believe) but the Boys Rifle is better because it fires faster. The Tiger, Matilda, and Valentine are all but immune to AT Rifles, the Sherman, Grant, and Panzer 4 are technically capable of being killed by them, but take far too long unless you are just finishing the job someone else failed at. Panzer 3 takes several (more then 5) shots to the engine deck to take out. Lighter tanks can be taken out with less than 5 shots. Panzer 2 are bait if they don't know where you are, they take 2-3 shots from the side or rear.
Soft skinned targets (Jeeps, Trucks, APCs) are taken down in 1-3 shots. Aim for their engine compartments. AT Rifles are awesome at sniping fixed gun emplacements (At Guns, AA guns, Arty). 2 Pounders are easy pickings, especially. Also, Infantry are turned into chunky red goo by AT Rifles (note that the ATRs have a small "splash" effect vs infantry, if you see a hit indicator, you may not have scored a clean hit, although he will be lightly suppressed).