experiments i've done with my own gun seem to suggest they leave the muzzle at the same speed within quite a varation say from 7.6-8 of the same pellet type... indeed mozzies and exacts seem very close, with barely any change. H's gun is set for exacts and runs at 784 with mozzies.
My change with other guns though, and of course, I don't always run 7.9, most mozzies i shoot are between 7.7 and 7.9 by my scales. Does raise an interesting point though... and I know stated manufacturers weights have been rounded up to the nearest decimal when converted from grams to grains. Think mozzies are something like 7.85 when converted.
It's the old problem of what variation in the measuring device is possible against what is accepted... i'd hate to think what a cheap set of scales in a field combined with the chrono's error could provide in reality. Perhaps the BFTA buffer needs to be bigger to accomodate it? I did some playing in chairgun last night, and winding up H's walther from 784 to 815 with mozzies gives her barely one less click at 55, and shaves 1/2" off a 20mph broadside wind taking 6.5" of wind at 55...
Think the rise in temp may have been the reason... the car said 14 degrees when we arrived late morning, James W had said he saw 6 on his car on arrival in the morning, I saw 32 on the scope (albeit in the sun) in the afternoon. H's gun seemed to have gained 10-15 over another chrono tested a few weeks back, and 1&1/4 MOA of rise... which is way more than chairgun indicates, but in the same direction. Certainly it appears than when we have these sudden changes in heat upwards, there's a few caught out on the chrono... i remember Wales a few years back where it ran into double figures.
I'm still getting to the bottom of a possible temp shift on the walther, but it needs those hot sunny days to see it... when we're in the wood shoots, it's a lot cooler... the scope is a known factor, but my rig seems to flick up now and then, and has been slowly growing in speed over the past couple of months, albeit by only a few fps.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep