particularly the 18"-24" models.  I am looking into making a long range varmint gun.  

Any other simliar setups you would recommend? Any advice?

Thanks  

ok then... what about the standard rra a4 varmint uppers?  i'm thinking about the 18" model.  Anyone have one of these?  pro's and con's?

I am not a fan of the RRA EOP uppers as you are limited to only certain optics.  I would much rather prefer the standard A4 uppers due to the increased versatility that setup would give you.

As for the gun itself, odds are, mechanically, it will outshoot you.  18 inches of barrel is usually enough to get the job done but if you are deadset on longer ranges, getting a 20 or 24 inch won't hurt.  The longer barrels will give you a slightly faster bullet velocity which will in turn slightly increase the range for fragmentation of the bullet.

But is the increase in weight (from 6 more inches of bull barrel) worth it?  Only you can decide that.