What are the things to look for when choosing an AR15 for target/varmint hunting?

1st thing is -- Buy from a quality manufacturer. Rock River Arms, and DPMS are excellent choices. Bushmasters are a good choice also... For critter hunting I suggest the a longer barrel, not the 16 or 18 inch M4 type barrels.... Get an upper receiver that either has no handle or a removable handle.... All manufacturers claim there upper receivers interchange, but be cautious. If your using a Rock River lower and want to add a Bushmaster upper I have seen cases where they didnt fit, needed a shim or required some other farting around.... Keep within the same brand when switching receivers and uppers.... Thats not to say they wont but if your new at this save yourself the grief..... Check out a href="http://ar15.com/" rel="nofollow"http://ar15.com//a for a lot of info --- ask what you want to know at there message boards -- You will get a lot of good advice there!