A study in airsoft ammunition (primarily BB Bastards)

[Spawn28]

Well ive said it before and ill say it again nuthin but Bastards in my guns and you DO! get what ya pay for ive tried all kinds of other ones before and now im hooked i swear Scarecrow sprinkles them in a addictive residue before he sends them out. Ive also noticed that Bastards dont shatter nearly as often as say KCS Perfects or Excel.

[Naglfar]

That's a really good test you got there m102404, if you get any other barrel lengths be sure to post them up, and thanks for the info.

[seabass]

Diffrent hop ups will slow down your BBs diffrent amounts, aswell as how much "hop" your using. Also this doesnt have much to do with a BB study as it does a barrel study?

[CDN_Stalker]

He just pointed out the same 25-30fps drop with 0.25g BBs as I posted up.

[seabass]

Originally Posted by CDN_Stalker

He just pointed out the same 25-30fps drop with 0.25g BBs as I posted up.

oops missed that last line, lol. The barrel thing really could be in its own thread, thats pretty interesting

[CDN_Stalker]

I'll toss in one interesting fact regarding different BB weights................. there is a distinctive pitch change the heavier you go. Is something that Testie pointed out in my basement when we were testing (i'd shoot and call out, he'd write down) but he noticed a more of a thud with heavier weights as opposed to the thwack with lighter ones. Is something I noticed years ago when I started loading the first dozen rounds of a mag with red 0.12g BBs (reminds me, I need to buy more), then fill it with either 0.25g or 0.28g. Used that as a low ammo indicator (reduce dry firing of the gun, but also got some surprise kills with them too), not only see the red BBs overhopping, but the sound of the gun firing changed to a higher pitch as well.

[Scarecrow]

Shorter barrels means the BBs spend less time in the confined space of the barrel and thus less time with backpressure behind it accelerating the BB to its ultimate destination, the crown of the barrel. The same principle applies to firearms - you do get velocity drops if you compare the same bullet weights/powder/primer loads to lesser barrel lengths.

The pitch change makes sense because as the weight of the BB increases there is greater backpressure, or blowback. In fact, I would posit that if you test with a longer barrel, you may actually see the opposite effect (even greater acceleration) because the backpressure is higher and the BB spends more time in the zone of acceleration, ie: the barrel.

I don't have stats on that, its more a rule of thumb, but the ballistics of real bullets are very well known and calculatable. In fact I have a program called Quickload, which calculates velocity, chamber pressure, etc and takes inputs like barrel length to do this.

[Bontic]

In fact, I would posit that if you test with a longer barrel, you may actually see the opposite effect (even greater acceleration) because the backpressure is higher and the BB spends more time in the zone of acceleration, ie: the barrel.

I think this may be stickied somewhere I remember reading, but just wanted to check. This is true up to a limit, right? Beyond a certain length of barrel, the volume of the barrel gets close to the volume of the cylinder stroke. Then the "suction effect" will reduce fps - the backpressure runs out before the bb has exited. The air rushing down the barrel has expanded to its normal atmospheric volume, so will not expand anymore. Now the exiting bb has to "pull" on the air behind it, creating a lower pressure behind it than the pressure in front. The bb is decelerated again before exiting and fps drops. Presumably there is an optimum barrel length for every cylinder/piston, where speed is at its maximum.

[Scarecrow]

Originally Posted by Bontic

I think this may be stickied somewhere I remember reading, but just wanted to check. This is true up to a limit, right? Beyond a certain length of barrel, the volume of the barrel gets close to the volume of the cylinder stroke. Then the "suction effect" will reduce fps - the backpressure runs out before the bb has exited. The air rushing down the barrel has expanded to its normal atmospheric volume, so will not expand anymore. Now the exiting bb has to "pull" on the air behind it, creating a lower pressure behind it than the pressure in front. The bb is decelerated again before exiting and fps drops. Presumably there is an optimum barrel length for every cylinder/piston, where speed is at its maximum.

Yes Bontic I should have probably qualified that but I thought it was evident. Once you hit the end of your accelerate (ie: positively pressurized air) if it is still in the barrel, friction will begin to decelerate your BB, and the recycling piston will suck air into the barrel, further decelerating the BB. So it is critical that your bore size on your cylinder has enough volume to push the BB to the crown of the barrel.

[The Saint]

I don't know how much air a cycling piston is actually capable of drawing down any barrel. Between a vented piston head and the nozzle being retracted at the start of a piston's rearward travel, it should be little to none.

[CDN_Stalker]

The suck is exactly as Bontic described it, and it doesn't have anything to do with a cycling piston. A single shot bolt action rifle with too long a barrel will suffer barrel suck as well.

[Bontic]

Yes, that's why I did not mention the cycling piston as it will not have an impact due to the porting, not to mention that the sector gear whipping around is probably slower than a bb accelerated to upwards of 300 fps in a split second. I will do the math later and see what rof is required before the piston could begin retracting before the bb has reached the end of the barrel.

[Bontic]

Hmmm... was not as simple as I'd thought. Need to know som more things about airsoft springs, but as an approximation, probably slightly overestimated:

You need a ROF of approximately 28 rounds per second for a gun firing at 330 fps down a 450 mm barrel for the sector gear to cycle fast enough to begin retracting the piston before the bb has exited. Higher velocities and shorter barrels than this require higher rates of fire before the gear can make it round in time to engage the first teeth.

In general, long barrel, low fps, and high rof means the piston will begin withdrawing before the bb has exited.


If anyone wants to check my work: (if you hate equations and stuff don't read)
Assumptions: 1) that the sector gear releases the piston teeth at an angle of 45 degrees to the vertical and engages again at 45 degrees, passing through 1/4 of a rotation while the piston gets forward and the bb is exiting.
2) the acceleration is near uniform
3) The time taken for the spring to push forward is negligible before the bb begins its acceleration. That is, the bb begins accelerating before the piston head has reached the cylider head. The time for the piston to slam fully forward overlaps with the acceleration of the bb. This may be a source of error in the calculation. The true result could be a lower rof, if there is a significant "compression time" delay. Anyone know how long this could be?

Knowing the spring constant k for a typical airsoft spring (I know some are progressive, but k can be approximated) we can get the force of the spring on the air mass, and minusing a bit for friction and the compressibility of the air, we can get the acceleration, and thus the time taken, and thus can add some of that time on. But I have no idea what range we are talking about.

the math:

using d=1/2 (Vi + vf)t

time taken for bb to accelerate from 0 and exit 0.45 m barrel at 330 fps (100 m/s) is 0.009 seconds.

1/4 of a cycle in 0.009 seconds implies 27.78 cycles/s.

EDIT: true result could be lower still if the sector gear completes the "unengaged" portion of its circuit faster than the engaged portion. Very likely.

Thus, I would lower the estimate still. Perhaps on stock TM AKs and similar barreled guns with low fps the piston does indeed begin cycling back before the bb has exited, especially with a battery upgrade. Porting and nozzle retraction takes care of any potential suck.

[CDN_Stalker]

Fuck I hate math.

[Bontic]

Lol.

Oh, sorry for thread pollutin' by the way. Back to the bbs. You're getting some good info out, it's appreciated.