Pretty much exactly what Rob has said above. Probably 90% of HFT shooters are using JSB Exact 8.44's or Air Arms Field equivalent. The head size printed on the tin is almost meaningless as every tin has quite a large range of actual measured sizes. What you are really interested in is the die number which is either the first 1 or 2 digits of the 7 or 8 digit code on the sticker on the back of the tin. Comparing different die numbers you can see variations in pellet length, shape of waist, shape of dome etc.
For my springer (TX200) almost any die number group the same to 35 yards. Very slight changes at 40 yards. And a noticeable change at 45 yards (+/- 20% group size). However I have to shoot a load of groups and take the averages over time because shooting consistent groups at 45 yards with a springer is HARD.
I would suggest the following to people new to HFT:
- Pick up any die number for now
- As you shoot more comps, ask around for known 'good dies' (for example #4, #8, #54 are known to be good at the moment)
- When you get a chance, get some of these known good pellets (half the time people at your club will give you some to try)
- Experiment at longer ranges
- practice practice practice
In my springer, die 54 are my current choice, with die 4 close behind. But to be honest at 45 yards its still more down to me than the pellet. Having the right pellet is maybe worth 1 in 20 of the 45 yard shots for me