Thanks for the clarification. More comments but again most likely rubbish:

Can you detect exactly where it is leaking from? Is there any chance someone has screwed a gauge in too tight and cracked the housing? Is it the correct gauge?

Can you try a few winds of PTFE tape ... I note you intend to and it would be interesting to see the result.

If you compared the two rifles, what differs in the gauge/housings? Could you try the other 'good' gauge in the problem housing? And vice versa?

Cheers, Phil