Creating a shot string

[chrismdd]

Hi,

Just got my comnbro chronoscope and want to make an analysis of the shots. Would welcome recommendations for doing this i.e. chart every pellet or one every five?

Also as its a pre-tamper S410 what size is the torx bolt if I need to make any adjustments? (I already have the correct size allen key for adjusting once the torx is removed).

Its a standard S410 that I currently fill to 190 and won't let go below 100.

At the moment according to the combro its chucking out 8.4 grain AA fields at slightly over 11.4 ft/lb. Going to try some other pellets and so want to make sure I don't go over the legal limit.

Regards

Chris

[Timmytree]

Hi Chris, I usually fire off batches of 12 shots, then discount the highest and lowest to allow for differing pellet weight. Weighing pellets means you can just do batches of 10 but my way is easier!

[chrismdd]

Cheers Timmy,

Would you recommend I go all the way from 190 down to 100 to find the 'sweet spot'? And any idea on the torx bolt size?

Regards

Chris

p.s. crops are starting to ripen so pigeons will soon be on the cards hopefully.

[gary martin]

when i owned a 400, i filled to 190 bar, and recorded every shot down to 100bar. i worked out the average FPS and as a tollerance +- 10fps then concluded for my sweet spot every shot that was 10 fps above - 10 fps below the average.i had a classic in .177 and got approx 60 shots in that range.
Gary.

[chrismdd]

Cheers Gary, I'll give that a go.

[Mark112]

Hi,

Personally I would do a complete string from 190 bar down to the point where the velocity drops to your start velocity. If your initial 190 bar shots are 700 fps or so then keep shooting until they are about the same (this was after 150 shots in my classic .177 and about 100 in my .22 carbine though so it's not a job for the faint hearted )

Record all of the velocities and the final cylinder pressure and if you can enter them into some sort of spreadsheet. Chairgun has a simple means of showing shot strings in its 'Toolbox' then 'Statistics' function. A spreadsheet will enable you to do other stuff though.

S410K_Sheet.jpg (187.43 KiB) Viewed 3137 times

In the above graph of my own S410K I have set the sheet up to give me data about a specific FPS range .. in this instance my sweet spot which I have determined to be to part of the power curve that gives me a difference of 8FPS or less. From the graph shots 28 to 90, a total of 63 fall within the 8 FPS 'window' and the pressure at shot 28 (i.e what I could fill it to to be straight into the sweet spot) was 162 BAR. My sheet tells me that average velocity of those shots was 565 FPS or 11.46 ft.lbs.

I derived my 8 FPS value in this instance as I use 3 main levels of tolerance for my shooting needs:-

1). 1.5% of the maximum FPS in my string is my 'sweet spot' (my maximum velocity was 569 and 1.5% is 8.5 so I rounded down to 8. So my sweet spot is the shots that fall within 561 to 569 FPS. This is what I use when longe range (50 yards+) shooting as it will provide the best accuracy.
2). 2% is the value I may use for hunting as ranges are typically limited to say 40 yards on a dead calm day and less with wind. Still pretty good though.
3). 2.5% is what I use if I am plinking at tins or shorter range shooting. In the case of my .22 carbine this is pretty much all of the shots!

Hope this is of some use.

Mark

[chrismdd]

Cheers Mark, you guys are a great help. I've already got Chairgun so will find a nice quiet day when the kids aren't around to get some data.