.29 De-accelleration and other oddities

[XavierMace]

So, I was test firing the M700 AICS (pre-ban) in a friends back yard to tweak the hop-up and chrono it before I upgrade.  We found a couple oddities.  We think part of it at least was the can of green gas.  The can sucked.  It was not filling the mag well.

1) Gun chrono'd higher with .29's then .20's.  The gun was chronoing inconsistant and low to begin with, but it was chronoing higher with SGM's then .20 AE's.  390fps - 470fps with SGM's, 350fps - 415fps with .20 AE's.  This was with the PCS bolt at about 70% open.  While the range varied a lot, and was lower than it should have been, the SGM's always chrono'd higher.  Why is this?  Not enough back pressure with the .20's because they are too light?  I assume the higher quality of the SGM's would provide better accuracy and consistancy, but not magically provide velocity as well.

2) How fast do SGM's de-accellerate?  I ask because the gun chrono'd at 430fps, but there was a appx 2 second delay between firing and hitting a target at 220 feet.  The impact sound still seemed like it was hitting pretty hard, but how could there be that long of delay?

3) Adjusting the PCS valve between 50% - 100% seemed to have little or no effect on velocity.

Could these issues all be caused by sucky green gas, or is there something else going on here that I am not aware of?  Can a chrono be (consistantly) wrong?

[Vince]

There's a theory that some call 'energy creep' that dictates that a heavier projectile attains more velocity under certain circumstances. This is why OPLC series events are chronographed with .25s.

[Ganef]

Time to crunch some numbers, I will be back tonight with some findings.

[XavierMace]

Thanks Ganef, that is what I was hoping for.  I am too lazy to do that much math, plus I wasn't sure if there was a short answer.

Vince, that would explain the higher chrono results but not the time to target.  Time to target would indicate an average velocity of +/- 110fps.  Once again, I am too lazy to do the math, but we determined muzzle velocity was +/- 430fps and it took approximately 2 seconds to travel 220 feet.  Rough guess, that would mean impact velocity would have has to have been under 100fps by quite a bit, correct?  It doesn't seem like it could have possibly lost that much power without a noticeable drop.  The target was appx 3ft x 3ft meaning smaller than a man, so it was still reasonably on target.  If the BB was slowing down that fast I don't see how you could possibly hit a man sized target @ 300 feet which from reading is not abnormal with this gun.

This confuses me.

[Kurn]

I would check the gun with another chrono.  Chrono's can be inconsistent.  What type of chrono was it?

[XavierMace]

Not sure what model it was, it's not mine.  However the other guns chrono'd were chronoing within expected ranges.

However if the chrono was the problem, based upon time to target it doesn't seem like it would have had the energy required to reach the target.

[alex]

Here is a great site with lots of calculations and tables pertaining to the physics of arisoft: http://www.cybersloth.org/airsoft/trajectory/index.htm

From that site:
 "As was explained in Section VI-A:  Determining Muzzle Energy, heavier projectiles spend more time transiting the length of the barrel, leading to a greater overall energy transferred to the projectile (to a certain extent).  In other words, using heavier BB's generally leads to higher muzzle energies.  For instance, a rifle that chrono's at 500 fps with 0.20g BB's will, at the very least, chrono at 408 fps with 0.30g BB's.  Since the 0.30g BB spends more time transiting the barrel, it is able to absorb more energy; consequently a rifle that chrono's at 500 fps with 0.20g BB's might chrono much higher than 408 fps with 0.30g BB's."

So it seems that your .29g bb's spend more time in the barrel and absorb more of the expanding gas's energy, hence the higher chrono reading.

Just my .02 cents

[Ganef]

Great find alex!
Wow, that was what I was working on. Looks good to me. Would like to see the max range data when he finishes.

One thing that would be interesting to see is how different length barrels affect the energy imparted to the bb, and what the optimum would be.

Other than that, note in the section Mass Comparison of Projectiles Fired at 1.49 Joules

and then compare that to the section for 2.32J

i.e. compare the distance vs. weight bb data of a typical upgraded AEG versus fps limit of sniper rifles.
for a .25g bb:
AEG limit to hit a person in level flight = 197'
Rifle limit = 230'
 you gain a whole 33' by using a sniper rifle versus an AEG, that isnt that much, proving that sniper rifles arent effective versus tactics change.

[Whiskey11]

Here is a great site with lots of calculations and tables pertaining to the physics of arisoft: http://www.cybersloth.org/airsoft/trajectory/index.htm

That is the exact webpage i was looking at in the "Metal BB's" thread.  Good Find Good Find!

[XavierMace]

Yes, the heavier BB will absorb alot more energy, however if you look at what they said (and any chrono result I have ever seen) a .29 is still shooting a lower FPS then a .20.  It is closer than expected and is still providing a higher amount of muzzle energy but the FPS is still somewhat lower.

That was somewhat along the lines I was thinking of.  A heavier BB would buildup more pressure before leaving the barrel.

However that is all good and well but that still doesn't explain the 2 second 220' travel time.  That is what is bothering me the most.  You are going to have to do some serious shot leading to hit a moving target at 200+ feet that way.

[alex]

Ganef, I actually really looked into this area of the sport before splurging on my AICS, as I've found that sniping isn't viable under the current safety restrictions, and more specifically with the power available from spring rifles. Ive used that site to develop a new energy limit and accompanying minimum engagement distances for snipers, and am testing its viability in private games with my team mates, I do not plan on using it at any other events. It has worked well and isn't any more dangerous when proper safety measures are utilized. The advantages are that the gas system can deliver much more power and range, at least that is my hope. The drawback lays with the responsibility required and level of trust to let someone operate a rifle at those power levels, hence I'm not sure these rules should be implemented on a large scale. Currently my AICS chrono's 550-ish with .33g bb's, but its running on an external HPA rig and palmer regulator, so its adjustable from like 200fps on .20 to like 900 on .20g. I would, of course, be using a laser range finder to make sure all targets are outside the MED. So basically Airsoft Sniping on the whole isn't a viable option, only when very very very specialized and dedicated players develop the patience and maturity to wield a rifle of significantly higher power and accuracy: one which requires much custom work. Basically if you are interested in Airsoft Sniping, I have a lot of theorizing and practical data stacking up as i test and tweak my rifle. if your interested i can work on a more official representation of my thoughts and put them up here, this was an unorganized ramble. :oops:

To Xavier mace: Apologies for hijacking your thread, and as for the 2 second delay, its not going to go away unless you speed up the bb. Basically if I were to give you one piece of advice about the AICS, it would be to solve the power fluctuation problems. You are never going to get the full potential out of the rifle running off of the stock Tanaka magazine, it's just too small of a tank (i.e. not enough room for expansion) and outputs inconsistent velocities.

[XavierMace]

Alex,

Similar situation here.  We are currently limiting our group to 600fps with bolt action rifles and 350fps (400fps with MED) for full auto AEG, which seems to be enough to still give snipers a significant range advantage.  However at that velocity you have a 150' MED.  So far, this has worked great.  One guy in the group was using a Kar98k at the last game we had up in Prescott.  Even with the outside temp at 18 degrees, it was still shooting pretty well.

That is why I am trying to get back into sniping.  500fps for bolt action wasn't enough of an advantage, so I stopped.  So, this is why this is concerning me.  If the problem is actually the chrono we need to know because at that point it could be a safety issue.

On that note Alex, what BB's are you using in your AICS?  I have heard people claiming the .30+ Straight BB's don't work well in it.

[alex]

I have tried .43g digicon bb's, .36g digicon, .30 maruzen, .29 maruzen super grand masters, .30g marui precision grade, .33g SIIS.

There is no comparision between the high grade (TM Precision and Super grand masters) and the rest. Those two are undoubtedly the best ammunition you can buy.

[Ganef]

Alex,
I hear what you are saying and agree. The data clearly shows that above the leagal limits long range (300'+) can be achieved. I was hit with a sniper rifle at op RSD a while back at probly less than 20' shooting wayy to hot for legal play, it hit me in the forehead and went straight to bone, I have never felt such pain in airsoft. People in general have no real idea of distances and I don't trust them enough to respect MED in a game.  Most players who want to be "snipers" don't know and don't care about the kind of responsibility that entails.
Higher fps AEGs destroy themselves and Gbbs arent consistent without a lot of money invested in them. But this discussion is going wayy off topic.

[alex]

Yeah, i could only see it working if there was some organizing body that could train and certify players for the position....

Im just going to take this discussion to the other "Snipers" thread and leave this one for its original purpose