Best spring for 350fps?

[murph]

Need some insight on what the best spring on the market will be for my ISC M4 to get it as close to 350fps as possible. The barrel length is standard carbine length, 363mm and to my knowledge it's not a TBB (it's the stock ICS one).

The reason I'm looking to upgrade is that the stock spring is only pushing around 280fps and I'm looking for a wee bit more range.

Thanks in advance folks!

 

[Lozart]

M105 should get you there assuming your air seal etc are good.