The chrony results kinda rule out the reg as causing the issue, because we're looking at a 6fps spread here -- except for the first shot of the string, which is 15 feet out.
The original reg in this gun (the air-sprung AA job) seems quite unreliable... but my rifle already has a reg made by Dave Welham.
Anyway, thanks! The issue is still under investigation.
The Filthy Pig
: Air Arms 100 (1990) + AA Olympic trigger + Welham reg + Custom Shop CS1000 stock. Status: leaking.
The Unknown Quantity
: AA EV2 Mk4. + BSA Platinum 10-50x60 + Jon Harris wheel. Status: sorted up to 25 yds.
The Limping Dog
: Air Arms Shamal (1989) + unknown origin sporter stock + Optisan Cobra 3-9x42. Status: slowly leaking; needs work.
: HW97k + Venom Lazaglide. Status: working just fine; needs scope.
with apologies to NJR100