Go to ammunitiontogo.com and buy a 500 round case of the Hornady 55gr. TAP practice with a steel case. It will cost you about 155.00 which includes shipping.
It is good stuff and won't hurt your gun.
Or go to ammoman.com and buy the case of Federal 5.56 XM193 for 329.00 which includes shipping and then split it with a friend. That works out to 164.50 each for 450 rounds on stripper clips.
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