The 20 deg angle serves two purposes:
1. It ensures that the projectile splatters in a downward direction and into the ground rather than off to the side or up, potentially into an unsafe zone risking injury to someone.
2. It reduces the energy that the plate has to absorb when the projectile hits, therefore less damage to the plate, therefore your plates last longer.
The issue of uncontrolled deflection of projectiles is becoming more and more of an issue as awareness improves. Lots of pistol clubs have now banned the likes of poppers and falling plates that hinge backwards because competitors 'chase the plate' with rounds as it falls backwards resulting in and upward facing plate and projectiles skipping out of the range safety zone/back drop. They now only allow forward facing targets that fall forward when hit.
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