I gotta pipe up against my better judgement...
Being the only one here (probably) with any real experience with firearms of this nature and military-style marksmanship, I gotta do some corrections and explanations.
*If you are not completely offended by the arogance, read on

*
1) It is a .223, they said that. So, most likely it is an unmodified AR15 since that is the most common civilian weapon in that caliber.
2) From the shell casing they found, they can tell fairly acurately what type of weapon it is. Extraction marks and such are fairly unique per weapon.
3) An AK74 modified for .223 would shock me. I can't see the point.
4) A hunting rifle it is not
5) Range does not make the sniper. It is about evasion and kill shots, range happens to only be a bonus for the prior.
6) Moving targets are not easy to hit at any range, and a kill shot is even tougher. The area you need to hit is small.
There is probably more that I should say but I can't think of anything
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