Swiss Arms Tac1 main spring replacement with Gas Piston question

[labyukv]

Hi,
I like to replace Swiss Arms Tac1 main spring replacement with Gas Piston, can you point me to the right gas piston model?
Thanks a lot in advance,
Vic.

[lefteyeshot]

The Crosman Optimus is the same gun as yout Tac 1. The Crosman Vantage is the gas ram version of the Optimus. If it were me I'd also order a wood stock for a Optimus and a trigger guard and then a GRT trigger from Charlie the Tuna. Actually I'd order the stock and trigger and leave the spring in it. I don't care for gas ram.

[labyukv]

Thanks Tim.
Vic.