Air Arms Owners Club

Hi,

I was recently given a shotstring to produce a graph for. Rifle was an HFT500 firing 8.4 grain AADF pellets. Filled to 190 and shot down to 110 bar.

Graph looks like this:-



Something strange has happened here and it has once happened to me. The graph seems to be in two parts with shots 1-38(ish) having a lower recorded velocity than the rest. Its still a typical curve shape but lower than the rest. When I had the problem I relocated my Combo chrono and low and behold higher readings were obtained. With further experimentation I have found that the Combro is quite position sensitive when measuring .177 pellets and not at all finicky with .22 .

Anyway rather than have the full string done again (although I am happy do another if required) I have cheated somewhat by adding 7 fps to the first 38 values. I have picked 6 fps as it looks 'right' and is in keeping with other shotstring curves I have seen.

What you get then is this:-



...and what a result! The orange bands show the sweetspot which in this case is a 12fps band (1.5% of maximum recorded velocity). there are 90 or so shots in that band which I would be well chuffed with. Improving the shot to shot consistency (it is a little spikey!) would help things along. This may be improved to using weighed/selected pellets and/or a polish of the hammer rail.

Hope this info is of some use. Must rush as Wednesday is quiz night (and ale )

Mark

Thank you very much Mark,i am well pleased with the result, I have polished the hammer rail infact I have polished everything I can think of.

Mark,and for anyone interested, Iv'e just had an explanation as to the cause of the lower fps on the first 40 shots courtesy of g.l.w over on STB .
When the firing pot came loose and ended up rattling around the cylinder, Geoff advised me on the setting of the firing pot, I should have set it to 57.4mm but because I`m a numpty I set it to 57mm thinking that .4mm wouldn`t make a difference !. Unfortunately it means that the exhaust valve spring is holding the exhaust valve closed for just a fraction of a second to long so it produces a lower fps until the pressure drops to around 160bar and the valve can then open for longer and ups the fps .
So now I have to remove the cylinder again and adjust the firing pot to balance the fps and hopefully get 3 fps variation, well that's the target ( pardon the pun )
Thanks to Geoff g.l.w I will let you know how I get on, or not !!!