FPS chrony measurement - automatic
 

If someone can humour me on why this occurs...

Using the usual cheapy XCORTECH X3200, single shot will measure the FPS just fine. When you switch to auto or burst, it will measure the ROF but the FPS is inaccurate. For example, single shot I'd get 260 FPS, but switch to auto and the FPS reading is 385. Now I understand the FPS is incorrect but why is it so? Just curious to understand where the failing is from a technical/physics(?) standpoint.

Your chronograph/gun is broken.

That would be the simplest answer but I'd push on to be broken how? The chrony itself is accurate when measured against another. The gun does in fact shoot ~260 single shot... I'm just wondering if such a low end chrony cannot accurate measure FPS when measuring ROF (huh?!)

I should check the manual ;)

Your gun is not working properly.

Is your hop off? Your BBs may not be locking into the chamber.

I like that answer though the hop is at ~50%. I should really add some context...

The gun is the P90 I have other threads on and is using the stock CYMA spring with just over 1.5 coils cut. Given the stock FPS was ~400, I tend to think that the ~260 FPS on semi is accurate. But if that's the case, how in the heck would it do 385 (readout)? Unless of course the semi shooting is hosed and firing much lower than it should consistently shot to shot.

To rule out the chrony itself I'd have to test with my other AEGs on full auto. I should do that -- will report back.

I've never seen any errors on XCORTECH chrono's when shooting full auto, I would say it's most likely your gun.

Thanks doc, the answer will be more clear when I run the other three AEGs through it as well. Having said that, I'm wondering out it could run at 260 on a cut down spring in semi and 385 on full auto :banghead:

The fact that you cut the spring has nothing to do with your FPS inconsistency.

My cut spring comment is more for the very improbable 385 FPS. An original spring doing 400 with over 1.5 coils cut (forward end) only dropped 15 FPS? That's from my armchair reading about cutting as little as possible and not to go beyond 2 coils mind you. Your thoughts?

Every spring is different, but a coil and a half dropping 15 FPS sounds normal.

Ok thank you for your check. I have absolutely NO idea how many FPS even a ballpark drop might be. Given people say don't do more than 2 coils (risk of PME?) I thought the drop might be drastic. I'm going to start another thread since I've done a sanity check and the chrony is fine with the other AEGs semi/auto.

The problem with reading that stuff online is that every moron has something to say. The thread you read might be filled with morons.

It doesn't matter how many coils you cut off as long as the spring is still longer than the distance between the spring guide and front of the piston. As long as it is at least that long you won't get PME caused by the spring.

Thank you pestobanana (and others). I've continued the discussion in a more targeted way here (http://airsoftcanada.com/showthread....wpost&t=174648)

Wait people are still cutting springs instead of spending $10 on a replacement one that is rated for a fps range?

Only when they are in a time crunch :) Funny thing, wasn't able to use it anyway so it was a moot point. A proper spring is on the way.