The biggest contributors to accuracy and range are:

Proper Compression
High Grade BBs .25 at the very least, maybe .28 or .30 for a baseline
and a good quality hopup.

Everything else comes secondary.

by proper compression what do you mean exactly? how does that transfer into parts ?

Cylinder, piston, piston head, nozzle, etc.

The mechbox works under the laws of compressed air creating the propulsion of the projectile. Air is compressed by the piston being slammed forward by the spring and concentrated by the air nozzle behind the BB sitting in the hopup chamber. The idea with a mechbox is to have zero air loss anywhere in the box so that compression can be had for the highest value and to remain constant. If there is air loss the compression suffers and will be translated to the BB by loss of FPS, inconsistent trajectories and overall insufficient performance.

So basically proper airseal all around as compression is only in the cylinder. Air could leak between nozzle and hop-up (common problem). What are the FPS limits where you play?

Ragboy - you are right that the M14 has much more potential to be far more accurate and have better range. I know of a couple M14s like that. They have about $600+ worth of upgrades/gun doc fees on top of the original cost and you always start with a TM otherwise it's a headache.

Same with VSR-10 sniper rifles, a couple hundred in upgrades are generally what any sniping platform requires.

Also thing with airsoft is with enforced FPS limits, the field is pretty balanced in terms of maximum range one can achieve in ideal circumstances. The only solutions are to move to heavier grade bbs and get higher quality hop-ups. All the upgrades are just to improve accuracy and not power.