HFT500 Shotstring

[Mark112]

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:-

HFT500.jpg (152.94 KiB) Viewed 1950 times

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:-

HFT500Adjusted.jpg (155.19 KiB) Viewed 1950 times

...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

[gordon]

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.

[gordon]

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 !!!