FPS higher with heavier bb's on AEG???

[HeReigns]

I recently built a project HK416 AEG with Lonex parts and a Modify Baton Ryusoku AEG Flat-Type Hop-Up Rubber Set. I am using a M110 lonex spring and everything works tickity-boo.

When I chrono a .20bb it shoots at 399-402fps. When I load a mag with .28bb's it shoots at 351fps(which translates to 415fps on a .20

Has anyone heard of this before? What would cause this?

[wind_comm]

overvolumed. it's called energy creep.

incredibly useful for using heavier ammo.

[Wrath144]

Yeah, that happens on all guns. It's way more noticeable on GBB, but still happens on piston/cylinder guns. In most AEG's the muzzle energy with varying BB weights is approximately linear with mass but like wind_comm said, the behaviour you are experiencing is coming from your cylinder having a larger volume than is required for your barrel.
Let me guess, 10.5" barrel and type 1 cylinder? Or 14.5" barrel and type 0 cylinder.

[HeReigns]

Its got a type 1 cylinder with a prometheus 455mm inner.

[Hectic]

I was also going to say that lol if you have extra volume in the cylender when you use heavier ammo it allows pressure to buld more behind the bb creating more energy thus giving you "more fps" in realation to lighter bbs.
In gbb youll notice this even more as the gun will use more gas in a cycle and allow the gas longer to build pressure as it expands behind the bb.
If you have a 4/5 ported cylender try throwin a 455 mm inner barrel in youll notice you will have the 415 fps and when u move up in weight youll have a more even decline in fps.
Most m4's can use up to a 430mm inner(3/4 port) some a 460mm 4/5 port)
http://arniesairsoft.co.uk/?filnavn=...andbarrels.htm
And a closer matched barrel to cylender will make more efficent use of the air.
This is somethin in the sniper world with vsr 10 gspec. Itll shoot 100fps lower with its 330mm inner then the same setup on a 500mm inner.
By porting the cylender to make the compression stroke shorter the fps will raise untill you hit the sweet spot then you will notice the same fps as with a long barrel and non ported cylender with all other things the same
Ill see if i can find it and ill paste it here.

[Hectic]

Originally Posted by HeReigns

Its got a type 1 cylinder with a prometheus 455mm inner.

When u change bbs are they the same brand or different? Bio, silica ect..
Some bbs have much beter size then others you could bebloosing alot of air around the bbs.

[MaciekA]

You can also interpret an over-volumed cylinder as a signal that you're wasting spring energy on slamming a piston into a cylinder head (edit: this previously read piston head). This may be acceptable if you're trying to avoid pre-engagement, but if you're not, and don't usually use heavy ammo, you may want to explore the possibility of downgrading your spring.

The other side of this is the realization that your spring is capable of more velocity at 0.2g, if only you had a longer barrel to give your BB extra time to accelerate.

In extreme cases, this situation can limit a very short barrelled gun's ability to gain velocity without resorting to tricks.

[pestobanana]

Nothing is wrong. Using heavier ammo with the same propulsion system will always result in more kinetic energy. There will be more air pressure behind the projectile, and due to the slower acceleration of the air involved it will present a more efficient transition of energy to the projectile.

[▼Stripes▲]

Originally Posted by pestobanana

Nothing is wrong. Using heavier ammo with the same propulsion system will always result in more kinetic energy. There will be more air pressure behind the projectile, and due to the slower acceleration of the air involved it will present a more efficient transition of energy to the projectile.

Oh yeah it's not all dremels and teflon tape. There's SCIENCE in this shit!

[Drake]

Originally Posted by pestobanana

Nothing is wrong. Using heavier ammo with the same propulsion system will always result in more kinetic energy. There will be more air pressure behind the projectile, and due to the slower acceleration of the air involved it will present a more efficient transition of energy to the projectile.

Won't always produce more.

If the inner barrel is extended enough that ~100% of the potential energy is transferred from the piston/Gas to the 0.2g BB then the heavier weight BBs will have a proportionally lower velocity (equal in joules).

Similarly, if you have a very short barrel (as in a pistol) heavier weight BBs will also be slower than 0.2g, although not proportionally as they do accelerate more slowly, thereby staying in the barrel longer and acquiring more energy, but still leaving the barrel very early.

There's also a difference in the power curve between piston (AEG, spring) and gas systems: pistons have inertia as they begin to move forward and generate power through the entire stroke (usually with a bias towards the first half, when the spring is more compressed; how much depending on the spring). Gas, OTOH, is released with maximum energy (sharp spike) which tapers off quickly as it expands. This also depends on how much gas is being released, what type/how pressurized it is, environmental factors (such as temperature), etc.

[MaciekA]

Drake is right, this is all about surplus or deficit of volume ratios between your cylinder and your barrel.

You can design an AEG that has an absolute maximum limit of exit energy (determined by the spring). I know this because my @#(%^!#@% Diablo and MP5K refuse to cross their respective invisible barriers..

[ThunderCactus]

Our limit is 420fps anyway =D

[Rabbit]

INC THREAD DERAIL.

While we're on the topic of cylinders, why are cyclinder types selections becomming so confusing? My brain took a shit when trying to figure this out

Modify states a Type-1 ported cylinder is optimal for 300mm-450mm

SHS states a Type-2 ported "M4 Hole" cylinder is optimal for 229mm-430mm

references:

Modify:
http://www.airsoftparts.ca/store2/in...roducts_id=777

SHS:
http://www.airsoftstore.ca/index.php...9u9ug769fo85o2

Side note: To dumb it down is this only a concern for FPS - or ultimately does this affect range in some way

[lurkingknight]

the SHS is either a gross factory misprint, or you can take into account that you CAN use an over volumed cylinder on a short barrel and it won't make much of a difference. You can still achieve your target fps with more air pushing, you just aren't being efficient about it. In most setups, you wont' run into any weird issues when mismatching. Probably only in the most extreme builds will things like that start becoming factors.

and by extreme, I'm thinking more the 40+ rps setups.

edit:

of course, you can verify what you need by doing the math yourself... buy a full cylinder and cut your own port in it in the correct location to match your barrel volume. the sweet spot ratios are all in a chart out there somewhere.

[ThunderCactus]

Its also a matter of what BB you use.
Say the cylinder barrel matches are meant for .2s, so you could use the next size up of cylinder if you ran, say, .36s, to build higher pressure behind the BB
And if you have a sorbo cyl head, it really doesn't matter if you get a bit of extra slap from the piston
I've always tried to use the largest volume possible for the barrel I have