Originally Posted by surebet
My target for general use guns is sub 350, so yeah, 25$ of parts.
It would be nice if it were that simple.
The trouble with a spring upgrade in a TM is that it leads to you having to upgrade other parts to make up for effects of the new spring you installed. Any time you install a higher energy spring than the stock one, you've modified that TM design and that "precision balance" of everything is thrown off.
TM are designed for the Japanese market, where they have a strict fps limit (I believe it's about 320 fps). Their guns all shoot about 280 fps which is under their limit. You can boost their velocity to about that 310-320 fps by installing a bearing spring guide and tightbore barrel. That doesn't require a new spring that increases stress on the internals.
So if you want to get 350 fps, you also have to install metal bushings, which requires reshimming as well. And if you want to preserve your mechbox, a sorbo pad is a great idea as well.
While none of these upgrades are particularly expensive, it requires more than just a $25 spring to upgrade a Marui if you want to maintain its reliability and longevity.
Now that's mostly true of V2 gearboxes. V3 that are also quite popular tend to be a lot more robust. and the shell isn't prone to breakage. So generally, little more than a spring and metal bushings are required to upgrade it significantly without requiring other parts, although generally it's a good idea to install reinforced gears if going past the 360-ish fps mark...