When you fitted the replacement barrel, did you make sure all the spacers were in the same place in relation to the hammer spring or add any spacers. Excessive spring tension can cause safety failure (the hammer release catch part of the trigger mechanism not being strong enough to hold the hammer forward).

I would suggest removing a spacer from the hammer spring group and see what happens. If that sorts the problem then you can either leave it alone or increase the spring effective pressure on the appropriate spring in the trigger group (the forward-most one I think) to bring the forces back into balance when you refit the removed spacer.

Make sure you confirm your power levels remain legal after doing any tinkering with the efficiency of your stealth - it would be very easy to earn 5 years at her majesty's pleasure if you don't treat this brand of rifle with due respect.

David