The standard CZ 550 spring is too long ( the M98 spring works with CZ 550 Standard without modification), but since the ZKK 308 has a shorter action and magazin box the spring that could work is the one that comes in the detachable magazines from the CZ 550 308, this rifles are fitted with a shorter and detachable magazine to acomodate the 308 lenght cardtridges.

Hope this helps.

Best regards.

Ernesto.