Post by Shift on Aug 15, 2012 11:42:45 GMT -5
I was originally going to post this as a response to the "FPS Limits" thread in the "Airsoft Discussion" section, but I think it really needs it's own thread. The other thread is here and I reference it several times in this post: kalamazooairsoft.proboards.com/index.cgi?board=questions&action=display&thread=1009&page=1
This is pretty much the crux or thesis of my post: I think that the established format of chronographing airsoft guns using .20g BBs is not in the best interest of safety.
I was going to suggest to Loony and Infidel that they follow the "speed limit" sign they have right by the chrono that Juggernaut posted about, and strictly have people chrono with the BBs they plan on using that day. I'll explain why.
As Gunny said in the other thread, the energy is really what matters. If your gun is shooting hot and you just use a heavier BB to slow it down, the projectile still has the same amount of energy when it hits you (we're not taking into account loss of energy as the projectile travels).
A gun shooting 380 fps with .20g BBs produces 1.33 Joules at the muzzle. The same gun with .25g BBs should fire at 338 fps but produce the same amount of energy (we're assuming a perfect system with no air leaks, inconsistency, etc.).
If we take Boomer's example of a gun with a muzzle velocity of 400 fps, and assume he was chronoing with .12g BBs (no idea if he was or not), that gun would produce 0.89 Joules, which translates to 277 fps with .25g BBs. So it is conceivable that his brother's gun dropped 150 fps when he switched to .25s if he doesn't have a good air seal.
The primary reason for the above examples and why I'm bringing this up, though, is that if a gun is over-volumed - GBB or AEG - (and has a spring strong enough to effectively push the piston in said cylinder if it's an AEG) and you chrono with .20g, using heavier BBs might produce more energy.
A personal example of this happened when I played in a field in Indiana that insisted we chrono with .20 and they provided the BBs. My gun came in right around 340 fps/1.07 Joules. When I came to Kalamazoo Airsoft a couple weeks later and chronoed with the .30g that I actually used in the gun, it came in at about 308fps/1.29 Joules, or the equivalent energy of a .20g traveling at 372 fps. I had done nothing to alter the gun between uses - I didn't even clean it (bad Shift). Luckily I was using a Polar Star and I could just turn the pressure down until I was below 308 fps with the .30g BBs. (I could also have probably just adjusted the poppet dwell down to allow less air out per shot but that's another story).
Another example of this from another forum:
"...My M4 is running a CLOSED cylinder, Promy hard piston, SP140 spring, and is short stroked by 4 teeth, with a PDI .05 187mm bbl.
It chrono's with the following:
.20 400 FPS for 1.4J
.25 360 FPS for 1.5J
.30 340 FPS for 1.6J
.40 301 FPS for 1.6J
..."
I know this could be a huge pain in the arse for Looney and Infidel, as well as people that might have over-volumed AEGs that they don't even know about (although this would be remedied pretty easily by just sticking to .20g BBs if they chrono under 350 already), but if safety is our #1 concern, I think this needs to be addressed.
Thoughts, comments?
This is pretty much the crux or thesis of my post: I think that the established format of chronographing airsoft guns using .20g BBs is not in the best interest of safety.
I was going to suggest to Loony and Infidel that they follow the "speed limit" sign they have right by the chrono that Juggernaut posted about, and strictly have people chrono with the BBs they plan on using that day. I'll explain why.
As Gunny said in the other thread, the energy is really what matters. If your gun is shooting hot and you just use a heavier BB to slow it down, the projectile still has the same amount of energy when it hits you (we're not taking into account loss of energy as the projectile travels).
A gun shooting 380 fps with .20g BBs produces 1.33 Joules at the muzzle. The same gun with .25g BBs should fire at 338 fps but produce the same amount of energy (we're assuming a perfect system with no air leaks, inconsistency, etc.).
If we take Boomer's example of a gun with a muzzle velocity of 400 fps, and assume he was chronoing with .12g BBs (no idea if he was or not), that gun would produce 0.89 Joules, which translates to 277 fps with .25g BBs. So it is conceivable that his brother's gun dropped 150 fps when he switched to .25s if he doesn't have a good air seal.
The primary reason for the above examples and why I'm bringing this up, though, is that if a gun is over-volumed - GBB or AEG - (and has a spring strong enough to effectively push the piston in said cylinder if it's an AEG) and you chrono with .20g, using heavier BBs might produce more energy.
A personal example of this happened when I played in a field in Indiana that insisted we chrono with .20 and they provided the BBs. My gun came in right around 340 fps/1.07 Joules. When I came to Kalamazoo Airsoft a couple weeks later and chronoed with the .30g that I actually used in the gun, it came in at about 308fps/1.29 Joules, or the equivalent energy of a .20g traveling at 372 fps. I had done nothing to alter the gun between uses - I didn't even clean it (bad Shift). Luckily I was using a Polar Star and I could just turn the pressure down until I was below 308 fps with the .30g BBs. (I could also have probably just adjusted the poppet dwell down to allow less air out per shot but that's another story).
Another example of this from another forum:
"...My M4 is running a CLOSED cylinder, Promy hard piston, SP140 spring, and is short stroked by 4 teeth, with a PDI .05 187mm bbl.
It chrono's with the following:
.20 400 FPS for 1.4J
.25 360 FPS for 1.5J
.30 340 FPS for 1.6J
.40 301 FPS for 1.6J
..."
I know this could be a huge pain in the arse for Looney and Infidel, as well as people that might have over-volumed AEGs that they don't even know about (although this would be remedied pretty easily by just sticking to .20g BBs if they chrono under 350 already), but if safety is our #1 concern, I think this needs to be addressed.
Thoughts, comments?
