I find barrel length to be largely irrelevant with regards range and accuracy, there's lads at my local site with 455mm barrels in their ridiculous M16+silencer 'long range' guns that get outshot regularly by people with MP5s, me with my MP7 (140mm barrel), pistols... you name it.
Range is dependent on power (to a point, it's not everything) and hop-up, a decent hop up and as many joules as you're allowed will maximise your range.
To that end, I recommend anything by Tokyo Marui or a G&G as both manufacturers produce rock solid reliable guns with excellent hop-ups. Marui will be short on FPS but probably not short on range, you can 'upgrade' the spring but it might not improve things significantly.
Remember, long gun does not mean long range! An MP5 will be just as good in the woods as an M16, occasionally better as you'll be able to run a lot easier with a smaller gun.