Before classifying your gun as not accurate
try heavier and higher quality bbs (i like KSC perfect .25s). Tighten everything, especially bits that hold the barrel in place. Clean the barrel with the stick thingie, re-adjust the hop up and try again. Most of the time people disregard the bb quality, which plays a huge role in consistancy at range.
I think the p90 is one of the more accurate (consistant) stock guns around. I played around with someone's AK once and that was extremely consistant as well. Stock m4's seem to have pretty bad groupings with the barrel wobble, though FMU'ed ones with the one piece barrel gets you some impressive results. G3's are long yes, but i've haven't seen too many fielded, and from the reviews I've read from people upgrading them for sniper/support roles, they don't get the performance they would like. The m14 also looks pretty good for long range. seen a bunch of those fielded and they do quite good at long range.
If you aren't looking into bolt action rifles, the M14 sounds like it'll fit your needs.
Also, Im curious as to what you consider 'long range' airsoft guns aren't the most accurate things in the world, what range do you want to engage people in with this gun?