Yeah, that's the trouble with posting up group size claims on the internet - you're always going to get a bit of one upmanship, a bit of bullshit and a bit of rose tinted shooting glasses. Ultimately it becomes a bit of a willy waving competition.
That's why I don't pay much attention to claims in the internet, you just don't know whether they're telling the truth or being a bit delusional. I prefer to go by actual competition results.
So look at it logically - if you're shooting FT and say your average group under ideal conditions (let alone competition conditions) is say 35mm at 55 yards as opposed to 25mm at 55 yards. If you're hoping to hit a 40mm killzone at 50-55 yards and get the wind right too.. you MUST be putting in sub 1 inch groups otherwise if you do manage to hit that 40mm kill, there's a healthy dose of luck involved.
Why do I say that?
Well.. ok for starters, you don't know the actual range in FT. Let's suppose you have an awesome scope and you're able to get the range right to within +/- 1 yard.
That means your pellet could land a little bit high or a little bit low, and if your average group size is 35mm, you've got 2.5mm leeway to hit that 40mm kill. The slightest mistake on range finding or the slightest breath of wind and it means there's going to be quite a hefty element of luck involved in hitting that 40mm kill.
So either... that shooter is able to rangefind to within half a yard AND judge the wind to within 2.5mm of accuracy at 55 yards... OR they're able to put in sub 1" groups and have a bit more leeway for getting the range and wind wrong. I know which one I believe
It's even worse over on your side of the pond, where you have 1" killzones in your FT matches out as far as 50 yards, so even if you get the range spot on and there's not a breath of wind - if you aren't grouping within 1" then it still becomes a lucky shot to hit that 25mm kill at 50 yards.
Now look at actual results - Brad Troyer recently put in the highest score at one of your Nats with a sub 12 fpe springer! He beat every PCP on the course including those 20fpe rigs that people shoot strapped up like a mental patient. Was he just lucky?
Also take Steve Privett who won the FT World Championships in Lithuania last year - a very large proportion of those targets were over 50 yards - yup they were 40mm killzones, but he was still having to contend with getting the range right and coping with a small amount of wind. He still managed to knock over 45 of the 50 targets on the last day to take the title. Was that great shooting or was it luck?
Regardless of whether your friend believes internet forum posts (and I don't blame him in the slightest for not believing them) he can't ignore actual results in competition - both on home turf and in international competition. and if you look at the requirements of those competitions and the limitations of range finding equipment, it simply isn't statistically possible for shooters to put in the sort of performances they are doing if they aren't putting in sub 1" groups at 50 yards.
Have a word with some of the excellent springer shooters you have over there - Brad Troyer, Paul Cray, Kevin Yee, Ray Appelles.. there's a big list of them, ask them if they think grouping under an inch isn't possible.
Once you've done that - you might want to reconsider just how much respect you have for this tuner's abilities Maybe he should book in for the next Springer Bash over here - now those guys have got my respect.