Sort of got the same issue as you as trying to find the cause of why there are the occasional pellets 'dropping out' of a decent group. I was testing various pellets 2 weeks ago but the my rifle decided to throw its own spanner in the works as well
with a broken firing valve.
At the time I was shooting groups with JSB's 4.52 and 4.53 and Bisley Superfield. It was during the Superfield group shots that the valve went. The 10shot bench groups I got at 50yds were( Height by Width edge to edge)
4.52's 20mm by 40mm
4.53's 21mm by 42mm
Superfield 10mm by 24mm (only 6 shots of which 4 hit the horizontal line I was using as a reference and the other2 were above and below the line)
All of these were pellets straight from the tin. The only thing I noticed was that the Superfield were much tighter when pushing the bolt forward (Ripley AR5s).
Will be carrying on some more testing on Monday when I get my rifle back. Also intend to use a Combro with a serial lead to record the velocities and a card to mark the fall of shot. If i had something that would allow me to measure the windspeed at target I would be using that as well.
Just from this I am beginning to think that the actual pellet size critical, as measuring JSB's with a vernier the head sizes seemed to be the same for 4.52 and 4.53 at 4.49mm and the skirts at 4.65mm.
Will be measuring a sample of each pellet tomorrow for reference and using this bunch I hope on Monday weather being all good.
JSB 4.52's, 4.53's, Bisley Superfield, Long Range Gold, H&N Silhouette, FTT, RWS Superfield, Superdome
Falcon Accuracy Plus, Crosman Accupell, Premier (also have an unopen box of Die#2's), Webley Mosquito, Lazadome
On a side note I had the same issue with a Rapid 7 in .22 about 15 years ago nothing would group on a sheet of A4 paper indoors at 30yds. An old blue tin of oxidized Eley Wasps shot a 1/2" group rested with H&N FTT coming in at 5/8" rested. The next tin of Wasps I got were like a shotgun so stuck with the H&N.