I have just received my new ACM Chrono from RSOV (bought mainly to ensure my gas M2 Carbine and chinese Thompson aren't over limits).
However I can't help but feel it's reading a touch high because my No.4 measures around 425 fps and the Thompson 465 fps, as opposed to 395 and 352 according to Gunman's chrono at D-Day).
To test it I've dropped bbs vertically through it from a height of 0.5m and 0.75m (1m would have made the maths easier but I couldn't hit the hole at that distance).
Using s = ut + 1/2 a t squared and v = u + at
where s = height dropped from
u = initial velocity which is always 0 m/s
t = time to drop to the centre of the instrument
a = acceleration due to gravity which is approx 9.81 m/s
for height of 0.5m
0.5 = 0 + 1/2 x 9.81 x t squared
0.5 = 4.9 x t squared
t squared = 0.5 / 4.9
t = sqr root (0.5 / 4.9)
t = 0.32 secs
so after 0.32 secs
v = 0 + 9.81 x 0.32
v = 3.1 m/s
for height of 0.75m
t = 0.39 secs
v = 3.8 m/s
The actual readings we got were remarkably similar to those predicted i.e around 3.0 m/s and 3.7 m/s and I would have expected some losses obviously.
Sorry for really boring post but if anyone knows about this kind of thing and can see a glaring flaw in my reasoning above please let me know. Otherwise I'm forced to conclude my No.4 is marginally better than I thought (hurrah) and my chinese thompson is unusable without chopping the spring due to it being way too hot (boo).
Any advice would be appreciated.
edited to define "a"
which model did you buy barrie? some are configurable for ammo weights and thus yours could be set wrong
Yes, yes, * nods sagely * flawless math Barrie. 
Heh, yes you baffled me but I can see your logic!
Clearly something is wrong - your readings are 7.5% and 32% higher than Gunmans, you would have expected consistent variation at least!
I've got a lovely new mad bull chrono, which Kermit reckons are the mutts nuts . I'll drag it along next game & you can compare.
The model I got was an ACM Speeder 558 which apparently is a direct copy of the Guarder Speeder.
http://www.rsov.com/product/105_133/137 ... r-558.html
Chomley - lol the Thompson fps is a typo sorry - it should have read 365 (not 465) as opposed to 352. Taking your approach this is just a 3.6% difference so not wildly inconsistent with the No.4. results so I'm kind of thinking it's not that far out.
Old 'Un - yes please if you can bring yours along I'd be interested to see how the two compare seeing as I can't think of any way, other than the above, to show if it's reading right.
I've that one also barrie.
can confirm it doesnt need any configuration that I've seen. however mine has shown as consistent with the othersites I've played at. perhaps gunmans was off? have seen some chronos give odd readings in odd low or high light conditions. also when their batteries are low
can u tell me what is chrono calibration is as my thompson i brought is surposed to be 330fps
ive never heard something like that before
some chronos can be set for different weights of ammunition marsha. most however are just automatically configured for .2g ammo
To calibrate an instrument basically means to compare what it is reading with something that you know is correct, to check that it is measuring correctly.
For example, you could check an egg timer by comparing it to a clock.
All I want to do is somehow check my new chrono is actually giving me an accurate reading of the fps of my thompson because I think it might be reading slightly wrong.
Rhys is referring to setting up the chrono to make sure it's measuring what you want because if I'd set it up by pressing all the wrong buttons that would have explained my problem too.
If your thompson is around 330fps you've got nothing to worry about.
Barrie I'll bring mine along to haggard if you like. plus GZ will chrono I imagine so we can ascertain if one or other or both are out!
