P.S.....When you re-commence power testing, you might do well to try a good selection of pellets, especially those from the JSB stable, like Jumbo RS or Express. The Superdomes can often rob power, most especially in set-ups employing a very light piston and soft spring. What might be "comfortably under" with a 'Dome might not be with some of those JSBs.

If going for a slightly more stout spring, and especially if introducing some piston weight, you might find that things even out a little with a wider range of pellets?

Spring wise, I'm wondering if the TX200 Mark 3 spring would suit / be enough spring for this rifle? Failing that, what about one of TbT's HW97/95 springs would fit okay (wondering about the OD here). And also worth looking into the recently introduced Welsh Willy spring to check the specs etc.