I'm building a steel target for long-range rifle shooting. For those unfamiliar, the purpose of the steel is to produce a loud "PING" when the bullet strikes the target. It's very fun.
Since I have a variety of materials to choose from, I'd like to know the ideal shape and size of a target to produce the loudest possible noise upon impact.
The steel grade is AR500 (very hard but not too brittle). Its density is ~0.281 lb/in3 (7.78 g/mL).
I can choose from three thicknesses: 1/4", 3/8" or 1/2".
The metal can be cut into any shape, and I'd like it to be somewhere in the neighborhood of 24"-36" in any dimension. The most convenient shapes would be rectangle, square or circle. It could be bent along one axis if there's a very good reason to, but preferably kept flat.
The target will be placed 1000 yards from the shooting position (see why it needs to be loud?).
For the sake of discussion, the projectile will be a "6.5 Creedmoor" round: 6.5mm diameter, 140 grain (9 grams), 2700 fps (830 m/s) muzzle velocity, copper jacketed with a lead core. Ballistics calculations give us ~1200 fps (366 m/s) velocity at 1000 yds.
How should I build my target to produce the most audible sound upon impact from 1000 yards away?
Bonus Nachos: How should I mount the steel plate to a wooden post to reduce dampening? Options include bolts, metal hooks, chain, rubber cables, or nylon straps.