Is this Joule-Creep?

[Tad]

So I've been doing this a fair while now, actually went dark on the web because life. This is just theory, but actually curious.

Was having a discussion with some ref's last weekend, and it seems like this is just "spirit of rules vs written"

Say I have an M4 platform. It has an m110 spring, which means it should be targeting 360fps, or 1.23J, using .20, and when the hops set properly.
- If you increase that BB weight to 0.25, the fps comes down to ~310 fps (1.14j). So there is a slight loss of power, but still in line with site regulations.
- If you increase the BB weight to 0.36, the fps tanks to ~240 fps, well under the site restrictions (260 / 1.14J)

Using that 0.36 weight, if you then put a m115 spring in, because you have that headroom your FPS starts hitting 260 on site limits.

If you then stick a 0.20 weight back into that now upgraded system, your FPS is going to be ~380, which is well over the site limit for Joules rating. But because the it's fine under the 0.36 weight, it's not a problem to play with.

Is that right? Is this what Joule creep actually is?

 

[Impulse]

No, but also yes. Sort of.

Joule creep typically refers to the phenomena where the power increases alongside the BB weight. This happens predominantly in gas guns, spring sniper rifles with large cylinders (SRS, Tac41, bore-up kit VSRs etc.) but can also affect HPA guns when set up properly, and also can affect AEGs, but often it'll be the reverse for AEGs due to air volume.

What you want in an ideal world is for the propellant, be it gas or air, to constantly be used up until the BB leaves the barrel. As you can imagine, lightweight BBs accelerate very quickly, and so they spend less time in the barrel than heavy BBs and therefore have less time to accelerate. With an infinite amount of air, you'd get the joule creep of a gas gun, as gas guns push the BB until it leaves the barrel so it is under constant acceleration. This means that if you have a long barrel and heavy BBs in a gas gun, you're getting way more time for that BB to accelerate through the barrel than if you put lightweight BBs through it, so the resulting power is lower as the lightweight BB accelerates a lot less than the heavy BB. My 640mm inner barrel Tanaka m700 can hit near to 1.14J on a .2, then I put a .48 in and it's suddenly well over 2J because of this. My MTW 308 is the same, because I've set the dwell (how long the valve is open) to ensure that the heavy BBs I use in it as a DMR are under constant acceleration; if I put .2s in it, it'd be well below the 1.88J DMR limit that the .43s are around.

However, AEGs have a limited volume of air. In the vast majority of AEGs, they are volumed for .2s or .25s at most, so heavy ammo tends to cause the power to drop. Why is this? It's because the heavy BB is accelerating through the barrel, then it's running out of air and starts to decelerate before it leaves. To more efficiently run heavy ammo in an AEG you usually need to replace the cylinder with a full one, as a lot of AEGs come with ported cylinders, meaning they only have a certain fraction of the total air volume possible. However, overvoluming can also cause inconsistency with accuracy, as if you have too much air, then lots of air escapes with the BB and tends to affect it with a tiny, but random spin on the projectile.

So... it's related to joule creep, but what you're experiencing is cylinder to barrel ratio. Your AEG doesn't have the air volume to efficiently propel heavier ammo.

I would go into more detail, but I'm heading out the door right now. But I love this nerdy shit

 

[TheFull9]

No that's not what it is. Your gun is either within the energy limit for the weight you're shooting at that moment, or it isn't.

The phrase mostly refers to GBBs, in that their muzzle energy can creep up quite a bit as you put heavier ammo in them.

 

[Cannonfodder]

Is that a real world example or theoretical? Joule creep usually puts the muzzle energy up as you increase BB weight. This is because the slower moving heavier BB spends slightly longer and n the barrel with the gas/puff of air pushing it

In my experience joule creep tends to affect gas guns more than aegs.

How long is your inner barrel? You might be experiencing suck back if the barrel has a greater volume than the cylinder

 

[Tad]

Is that a real world example or theoretical? Joule creep usually puts the muzzle energy up as you increase BB weight. This is because the slower moving heavier BB spends slightly longer and n the barrel with the gas/puff of air pushing it

In my experience joule creep tends to affect gas guns more than aegs.

How long is your inner barrel? You might be experiencing suck back if the barrel has a greater volume than the cylinder

Click to shrink...

Nah it's theoretical, my breacher has a 203mm barrel, so the current cylinder is fine

Was just trying to understand what this actually is. The amount of air being pushed is the same volume, just the force applied is larger.