If you want a pure long range varmint rig I'd go with an 18" to 24" barrel. Generally the rifle will be heavier and more cumbersome. However, a 16" barrel is deadly and accurate out to at least 300 yards depending on the load used. I own a DPMS 16" bull barrel AR and it will easily take varmint size game at 300 yards. I hunt with V-max or ballistic Tips w/ 40 to 55 grain bullets w/ a 1/9 twist barrel, I don't use Nato ball for hunting, nor do I use my 556/223 on larger game, so w/a good hot V-max or ballistic tip load a 16" barrel will get the job done well past 300 yards on small game. If you are going for very long shots over 400 yards on a consistent basis you'd be well served to go w/ the longer barrel in my opinion, as the 16" barrel does lose a little steam ballistically compared to the longer barrels on the long, long shots.