My son called home to tell me he is thinking about buying a smith&wesson M P 15, AR type rifle. Calliber is 5.45 X39 he wanted my advise on this round. Problem is I dont know jack about the 5.45X39. Is it worth anything ? How dose it compare to the 5.56 (223) as far as velocity, energy,range,etc etc? I could really use some input/information. thanx Tim !