If you want to get it up to 500(not recomended), then you will need to buy a couple of things.
1: upgrade the Trigger and Piston Seers
2: Upgrade the piston, which even starts to wear out in the TM pretty quickly
3: Grab a new spring guide, if you upgrade the spring it will crack your old one
4: Finally grab the spring you wanted.... you should be able to take this setup up to 550, past that grab a Zero trigger kit.
Why this isn't recommended, well for the most part every time I walk out on a field today the only other snipers are people using Clones(which is fine by me... kinda). The main problem with this is the people using clones are most likely newer to the game, which means they usually haven't invested in a gun that shoots at a lower FPS. Which again means there shooting a hot gun(400-450), and really don't know how large their MED's are. When they go out they always go with a squad(again fine... for now). But sooner or later that squad is going to be cornered into a situation where they need to assualt, and this is where I have the problem with High FPS. people don't abide by their MED's, which causes them to either
A: injure someone by not folowing their MEDs
B: get kicked out
C(most common): both will happen.
On top of that, I ran both of my primaries under 400FPS for a long time.... I would consistently get center mass shots at 200ft.
For reference, the longest "kill" shot I got with my G-Spec was a little over 200ft(according to the map), I was shooting 383 every time on the chrono.
Longest "kill" shot with my TM M4 was at 210ft(again according to the map of the field), and My TM M4 was shooting an average of 392.
In both cases I used high quality .25BB's and a nice KM barrel. You should be fine shooting at 400FPS, if I can get 200ft out of my rifles I'm sure you can too. Besides at 500FPS you would need a .3G+ BB to get any sort of accuracy, really I would use .43G but I like my BB's accurate not fast. Really in my opinion, grab a TBB or polish your stock barrel(recommended before you buy the TBB), some nice .25's(or even .28's), and have some fun.
sticks