You want an accurate gun, and you bought an echo1? lol
1) length of barrel means very little, stay between 380 and 500mm
2) actual bore of the barrel means very little, stay between 6.03 and 6.08
3) hop up rubber is EVERYTHING when it comes to range and accuracy. Get a firefly hard rubber with the firefly nub, will have a huge impact on range and accuracy
4) Use the right weight of BB for FPS. >380fps=.30g >440FPS=.36g >490fps=.43g
5) Make sure your airseal is PERFECT. You'll need a perfect airseal in order to get a laminar airflow down the barrel to guide the BB. Too long of a barrel increases the chance of the BB hitting dirt inside the barrel or losing pressure behind the BB.
6) Make sure your cylinder has enough air volume to push the BB out the barrel, loss of pressure and laminar flow means loss of accuracy
7) Get a proper platform for the task at hand. You don't substitute a glock18c for an M249, so why substitute a sniper rifle with an automatic rifle?
8) For accuracy and range, nothing beats a bolt action rifle. Higher quality components, better sealing, way higher repeatability over AEGs
9) If you want a REALLY accurate and long range M4 variant, and it HAS to be an M4 variant, get a GBBR. Weather permitting, gas guns are more accurate and have better range than AEGs. There's huge posts on it, but basically the BB is better stabilized faster by high pressure gas than by low pressure air.
If you have the money you could also buy a PTW lol