Re: Acceptable group size?
The plates are 1/2" thick standard steel, not hardened as far as I know (also have a big one that is 0.375"). We have just started playing with the big ones and have no significant impact damage from .300 Win.mags and 190 Matchkings at 700. The .308's only take the paint off and we have shot them a bunch out to 1000.
We shot the smaller plates last week at 4, 5, 6, and 700 with one shot from the .308 Win. (168 Sierra) and one shot with the .300 Win. mag. (190 Sierra) and the .300 made a significant crater at 400, enough that I wouldn't do it again. Estimate 2/3's penetration and a nasty crater that could snag a person's hands - we have to carry these things to a storage area. 500 was also significant, 6 was a distinct dimple and 700 you can barely feel. The .308 did a bit of a dent at 400 but not bad (very small crater), good dimple at 500 and only takes the paint off at 6 and 700. These plates were hanging, we also shot them sitting on the ground and got pretty similar results.
I believe that a person could use much lighter steel plate at 700 and longer - probably 1/4 inch if you had it hanging or angled forward to deflect the bullet downward.
We don't shoot the big boomers, can't say what they will do to this type of steel.
Best sound we get from hitting steel with the .308's is on a huge plate of steel that is suspended on coil springs. Steel pipe frame in the ground with three coil springs welded on, then the big buffalow shaped steel plate welded on the springs. We are pretty disapointed at how little sound we get off our smaller plates.
A tip, we just mooched an old swing-set and will be hanging targets on it way out there.