I was meaning more specifically to you.
Whilst parts (especially for m4's) tend to be generic, the emphasis is on compatible-ish, and a good part that works great in one gun might make another gun worse.
For range and accuracy i tend to go with the following as a generic formula (in roughly increasing order of benefit/reward ratio):
Give the barrel a good clean
Good quality heavier ammo (as heavy as your wallet can afford), i find the ~.3g range is a good cost/performance balance for assault guns outdoors
Change the hop rubber/nub combo (especially if its struggling to lift heavier ammo), i like the maple leaf macaron/omega nub combo although some guns dont like feeding with the longer feedlips so the pdi W hop would be my go to in that scenario.
Air seal, air seal, air seal, i have a guide which covers the main areas to look at:
Once youve got the air seal sorted you can think about spring changes if you're a decent chunk under your sites limit, no point whacking a massive spring in to lose all the energy to leaks. Dont get too hung up on fps, yes energy matters and all things being equal more is better but if you're 5-10fps under the limit its not really worth the effort.
And after all that you can think about changing the barrel, the overriding factor for barrels is quality, a well made brass or stainless barrel is gonna be better than a cheap "tightbore". I'm a fan of pdi stuff but zci's 6.02 stainless offerings are very good for the price.
I wouldnt bother changing barrel length too much, doesn't much for accuracy except mess up your volume ratio.
Rate of fire is a tricky one depending on how far you want to go, however if what you want is a snappy trigger response then a precocking mosfet like the gate warfet will give a nice boost without the associated problems of a high auto rate.
