FT-12 problems when cold

In all fairness, virtually all standard paintball o-rings fail to keep a proper seal when subjected to sub-zero ambient temperatures combined with even colder C02.

As a tech for many years, I can't tell you how many leaking cup seals I've dealt with on cold days with virtually every marker out there (Tippmann being the exception)

Have you tried lubricating the o-ring in the grip with a white-lithium grease? That should help quite a bit with the issue. Also, CO2 is hardly an ideal propellant at low temperatures, using remote lines, anti-siphon tanks, or HPA would prevent the issue completely.

That's interesting because I've seen no troubles at all when using HPA on my FT-12 in very cold weather. My guess is that if you're repeatedly mounting and de-mounting the assembly from the o-rings that you'd potentially put wear on them that would cause them to fail more quickly, but without that added factor, I doubt the cold would affect them that much with HPA.

If I were you, I'd contact Tippmann Customer Service by phone directly to get the new o-rings to you faster. I don't know about over in the UK, but stateside, they're extremely responsive to customers who call up and explain their needs.

How long have you had the guns?  I know there were a few issues with the first release.  Tippmann figured out the fixes and adjusted their production.  Anyone who had an issue got parts (like you are).  They are on top of these things as fast as they can be.