DIY Chronograph

di11on

Hi all,

Recently treated myself to an SRC RPK... really delighted with it. I was curious to know the FPS but haven't been able to get out for a while because of a busted knee.

So I came across these instructions for a DIY Chrono (this was the only link I could find which isn't blocked by the internet filter in work).

http://www.instructables.com/id/how_to_measure_the_FPS_of_an_airsoft_knex_or_oth

Anyway, it's amazingly simple. Simply measure the distance from where the piston hits the bb to your target, record the sounds using a laptop and mic and then measure the time between bb being fired and when it hits target.

I also have a JG G36C and have the chrono measurements from its last outings, so was able to estimate the error. Turns out, the DIY method was within 4% of Chrono readings!

Some things I did differently to the instructions in the link above:

1. Place the mic half way between gun and target, to cancel the effect of the time taken for the sound to travel... probably negligible source of error, but hey.

2. Measure the distance from where piston strikes the BB rather than the muzzle, as this is where the BB will be when the sound is generated.

Question
Interestingly, it was way easier to detect when the shot was fired from my G36C. It was less obvious with the SRC. Could someone briefly explain the mechanism so I can identify the correct sound. When you fire in semi auto, does the AEG have to pull the piston back first and then release, or does it release and then pull the piston back again for the next shot?

Puding

using a mic is also the best and most accurate for measuring rof

the method is slightly off as the bb will start moving as soon as a seal is made in the cylinder when it is moving forward not when the piston comes rest (the piston never strikes the bb )

di11on

I'm thinking...

Does the piston make contact with the BB? I.e. is it contact with the piston that propels the projectile, or is it the air pressure that the piston generates, that expels the BB?

Basically, what I want to know is this, where is the BB when the "clack" firing sound is heard?

Puding

bb will start moving as soon as a seal is made in the cylinder when it is moving forward not when the piston comes rest (the piston never strikes the bb )

air pressure that the piston generates in the cylinder, the piston never makes contact with the bb

Witcher

Here's a very colourful animation of an aeg gearbox which should explain the workings:):

http://www.fiassco.net/files/gbv2.gif

di11on

Puding wrote: »

air pressure that the piston generates in the cylinder, the piston never makes contact with the bb

Thanks Puding. Based on that, where would you say the BB is when the Cylinder is fully extended, corresponding to maximum air pressure, and when the clack sound is generated?

My measurements have a fairly constant error of about 4% which corresponds to an error of 13cm in the distance measurement... this would seem to suggest to me that the BB is closer to the muzzle end of the barrel when the clack sound is heard. This would also make sense in that, if the bb is still in the barrel when the piston is being pull back for the next round, the bb would be sucked back?

Edit: Thanks Blay - that's perfect. Definitely seems as though the BB would be closer to the muzzle end when the Cylinder head is fully forwards?

Puding

This would also make sense in that, if the bb is still in the barrel when the piston is being pull back for the next round, the bb would be sucked back?

yes this could happen if the volume of the cylinder is less than the volume of the barrel

the animation posted above i think could help you more than trying to explain the timing in words