Spare spring in box... M-1??

[slink182]

To the point:

The VFC SCAR-L I picked up came with a spare spring. The rifle's velocity is stated at 430+ fps on the manual. I can only assume that the fps applies to the stock spring in the rifle.

Can I assume that the spare spring is a downgrade spring? Guesstimates to what rating it may be - M100, M120...?

(I really wish I had access to a chrono... *sigh*)

[VooDooPeteK]

Prob a 120 but the only way to be able to really tell is to toss it in and chrono the gun.

[L473ncy]

Probably M110. IIRC it should be around 360-ish FPS.

PS: @ slink, technically that spare spring is the "stock spring". I can't say any more than that.

[slink182]

Originally Posted by VooDooPeteK

Prob a 120 but the only way to be able to really tell is to toss it in and chrono the gun.

I was afraid of that.

I did stumble across a "coke can" test that could provide some basic estimates:

http://www.redwolfairsoft.com/redwol...il?bulletID=34

Still, I'm ordering spare springs from airsoftparts on the offchance...

[L473ncy]

Coke can test is unreliable and I'll tell you why.

Different batches and different bottling companies can have slightly different thicknesses or other properties in the can leading to slightly differing structural properties. Don't get me wrong it's an alright test but should never be used to get anything more than a precursory glance as to what kind of speed the gun is shooting at. I'd say at best you're looking at a potential variation of up to +/- 20 FPS give or take.

Instead it might be a better idea to do the "microphone test" which I am a big proponent of. Basically setup 2 microphones a known distance apart and use a recording program to record the "woosh" (at it's peak) of the BB as it passes by the mic.

Using grade 10/11 physics formulae you can then solve for speed knowing distance between mics and the time the BB passes them. (v=d/t and assuming you set distance in metres then you get metres per second (obv), then to convert from m/s to fps it's velocity*3.281).

Example: Say you setup your mics at a known distance of 5 m apart and it takes 0.05 seconds for the BB to pass by both mics as measured by your computer/recording program you can then solve it to be shooting 100 m/s or approximately 328 FPS. This way is a lot more accurate than the coke can test, although not as good as a "real" chronograph test.

[slink182]

Originally Posted by L473ncy

Coke can test is unreliable and I'll tell you why.

Different batches and different bottling companies can have slightly different thicknesses or other properties in the can leading to slightly differing structural properties. Don't get me wrong it's an alright test but should never be used to get anything more than a precursory glance as to what kind of speed the gun is shooting at. I'd say at best you're looking at a potential variation of up to +/- 20 FPS give or take.

Instead it might be a better idea to do the "microphone test" which I am a big proponent of. Basically setup 2 microphones a known distance apart and use a recording program to record the "woosh" (at it's peak) of the BB as it passes by the mic.

Using grade 10/11 physics formulae you can then solve for speed knowing distance between mics and the time the BB passes them. (v=d/t and assuming you set distance in metres then you get metres per second (obv), then to convert from m/s to fps it's velocity*3.281).

Example: Say you setup your mics at a known distance of 5 m apart and it takes 0.05 seconds for the BB to pass by both mics as measured by your computer/recording program you can then solve it to be shooting 100 m/s or approximately 328 FPS. This way is a lot more accurate than the coke can test, although not as good as a "real" chronograph test.

Yep, thought of using sonic peaks to estimate velocity. Only problem is that I don't have the equipment (mics) or software to run that test.

The coke can test is a "quick & dirty" to guesstimate the velocity and install an appropriate spring. The test can only determine a range of velocities (your +/- variance) and I'm ok with that for the time being. Until I can get a quality chrono, it's better to be low than high, imo.

... Or I could just install a M100 spring and leave it at that.

[slink182]

Originally Posted by slink182

Yep, thought of using sonic peaks to estimate velocity. Only problem is that I don't have the equipment (mics) or software to run that test.

The coke can test is a "quick & dirty" to guesstimate the velocity and install an appropriate spring. The test can only determine a range of velocities (your +/- variance) and I'm ok with that for the time being. Until I can get a quality chrono, it's better to be low than high, imo.

... Or I could just install a M100 spring and leave it at that.

Gah! I'm being an idiot today!! I DO have pickup mics lying around. Thank you, Mom, for leaving all your karaoke crap behind when you moved to the Left Coast.

Which leads to the next question...

At what point for the gun is the initial sound generated - piston chamber or muzzle? This would affect the setup. (I would think sound would initiate at the piston, but I could be wrong.)

[Styrak]

Don't worry about any other sound than the BB. You should be measuring the BB, not the first sound generated.

[slink182]

Originally Posted by Styrak

Don't worry about any other sound than the BB. You should be measuring the BB, not the first sound generated.

Hmm... some back-of-the-envelope calcs indicates that, to closely estimate the muzzle fps, the audio rig I'll be putting together will use two sheets of paper, with paper mounting frames 1 m apart, and muzzle set a 30 cm from first paper mount. Pickup mic would be set at the halfway mark of the mounts to pickup the impacts from both sheets. The proposed 30 cm separation distance would be used to help isolate the bb/paper impacts from the initial piston noise, and to avoid potential tearing of the first sheet from the muzzle blast (not expected, just being cautious). A baffled bb trap is proposed for the far end, with possible dense foam applied to the baffles to absorb kinetic energy as the bb is being trapped. The software I intend to use is AudioEdit.

Does this setup sound reasonable?

[L473ncy]

Sounds quite reasonable actually. If Autoedit is a pay to use program there's a program called Audacity that I use. It's quite well renowned and is hosted on Sourceforge, it's also free.

[slink182]

Finally managed to chrono the rifle consistently. Averaging 398 fps with 0.25 g rounds (average of 12 shots) - so, yeah, it's a little hot. Looks like I'll need to drop in the spare spring and see where that puts it.

I love tearing stuff apart... my only worry is correctly putting it all back together. Fortunately, there appears to be some great guides floating around the site.

[m102404]

The sad truth about that spring in the box...and the one in the gun is that you'll never know what they'll shoot until you install it and shoot it through a chrony.

It seems a bit of an investment...but buying a chrony pays off in the long run...and if you decide to sell it, well...it's pretty obvious that there's always someone out there looking for one, right?

The doubly sad part is that buying a M90/100/110/etc and installing one of them won't really make sure you're shooting under this or that limit. It's a pretty good GUESS...but it still sucks to show up at a game and not be allowed to play with a hot gun.

I had a knock about CYMA AKS74U (itty bitty AK) that shot 480fps with a M110 spring. I finally got it DOWN to just under 430 with a M100 spring!! (with a M120 it sounded like it'd punch right through your gear...LOL)

So...NEVER simply say "I'll toss this part in and that part in and I'll end up with this". Get a chrony and know for sure (or within an acceptable range of error).

Personally...I'd rather use a chrony vs fiddling with mics and timing sound charts. It may not be as accurate...but it's quick, repeatable and easy.

[slink182]

Originally Posted by m102404

Personally...I'd rather use a chrony vs fiddling with mics and timing sound charts. It may not be as accurate...but it's quick, repeatable and easy.

I agree, and I have one on order - it should be delivered today (I hope; apologies for omitting this comment). My calcs are rough, I know, but they indicate that my measurements can't be that far off if the original calibration in the rifle was 430+ fps with a 0.2 g round (conservation of energy, after all).

Anyway, will re-chrono the rifle tonight to recheck velocities, and may have a weekend project learning about gearboxes. :wink: