PDI has a lot of Upgrade Springs,it's called "fat-SP".Do you know that ? In this page, we explain the difference.Please read below texts.
You'll find the difference of those outer Dia.size when you check the right picture.It's a picture compared Dia.size.
Slim Spring is Dia.11mm(left),Fat-Spring is Dia.13mm(right)
So if you may have already known,all normal spring is just 11mm like a APS-2,VSR-10,Type96 & M40 ...almost rifle series.
As I mentioned,a Slim Spring is Dia.11mm.So it's available to some normal parts.So if you hope improving muzzle velocity with the cheapest way,we recommend Slim-Spring.But normal parts are weak parts,it may break some points when you use the strong Slim-Spring.
※ PDI products are XB Spring, VSR Ezy Spring.
Fat-Spring
On the other hand,Fat Spring is Dia.13mm.So it's Not available to some normal parts.But you get more effective merits.
1.High durability : If you have same power springs of Slim & Fat,a Fat spring is stronger than slim cause enlarged size.So you keep good condition for aged.
2.Use a VC system :
Do you know PDI-VC(vacuum) system ? It's a better idea to go straight bullet. Please read this page. The VC system is installed inside a Piston head.A Fat Spring is available to VC system,but a Slim-Spring interferes in the VC system.So you need to choice the Fat-Spring,if you want to install the VC.
※ PDI products are APS Spring, VSR Spring,SS Spring & SPR Spring.
It's available to Normal piston.(This picture is VSR-10's normal piston)
The piston is VSR-10 B↑ Ver.That inner Dia.is enlarged.