Interesting about the B50 ... I bought one several years ago and had a wonderful time adjusting it ... different transfer ports and hammer spring load. Really very easy to work on once you have stripped it a couple of times. I wanted to use a Daystate Huntsman (MK1 / 2) firing valve but could not get one from Daystate. I tried a few times, even asking directly at the Daystate stand at the Shooting Show. All details taken but heard nothing. I assumed I heard nothing because they were not available. They can be found in Canada but take some finding. Filler valve is no problem as it it a standard fitment.
Cheers, Phil
