dependimg on what cylender you have will dictate the length of inner you can run.
Most m4 type guns have a 4/5 port so you can go up to 455mm inner
http://arniesairsoft.co.uk/?filnavn=...andbarrels.htm
that said you likely wont see a huge increase in range and accuracy.
even going from 350-400fps wont make a huge difference.
easyest cheapes option would be to buy a 455 mm tight bore and a spare hop up and a barrel extension or mock silencer and just swap barrels and screw on the extension/silence when you play outdoors.
youll gain maybe 10-20 fps a bit of range and some tighter groupings.
just dbl check what cylender you have befor you choose an inner
as some cqb length m4's will have a 3/4 port limiting you to 430 mm.
my stag15 (baisicly an m4) happend to have a 4/5 so i went to a 460mm 6.03 and that works well but again not a huge improvement over the stock 363mm aside from tighter groups at range.
most mildly upgraded aeg's between 350-400fps have about that 100-150ft of good useable range, to increaee that try using .28 or .30 bb's if you hop up can provide enough lift for them and thatll give yo
more range.
http://www.airsoftforum.com/board/lo...x.php/f85.html
check out sections 2,3 &5
this will show you that heaver bb will be beter then tryin to squeeze out 50fps more. Say you increase to 400fps and still fire .20 youll loose that 50fps in the first 20-30 feet after the bb exits the barrel swap up to .28 or .30 and youll maintain a higher speed longer, after 50 feet or so they will be goin tue same speed but the .20 will have way less power and start to slow down fast, the .28 or .30 will maintain speed and power longer thus increasing your range even with the same fps to start with.
i dont see a need for you to upgrade the spring just upgrade the barrel and you bb weight and your knowlage and youll see a spring swap isnt needed.