GBBR’s - Why a heavier weight bb?

[Enid_Puceflange]

Right,

Something I have never understood, but have practical experience of

Why do Gbbr’s work better with a heavier bb?

If green gas or air compressed with a piston is coming out of a nozzle, then putting the bb through the hop up no matter what variant of replica (aeg or gbb) why does a gbbr prefer heavier weights

Ive never understood the mechanics/physics involved ??‍

My gbbr’s both happily Chuck .36’s a nice flat trajectory 

(and send lighter weights into orbit with zero hop applied) 

My only aeg doesn’t really like above .28’s (with an upgraded rubber)

Whats the story please?

 

[Adolf Hamster]

In general heavier flies better if there's enough gas to push it and enough hop to spin it.

Unless you're wondering about joule creep?

 

[SSPKali]

okay...this might be a long post!

With AEGs you have a fixed cylinder volume and spring speed. This is usually matched to barrel length / diameter and BB weight you want to use (others can chime in with formulas and typical set ups)

With a GBBR when the trigger is pulled the gas is vented down the barrel, and as there is a BB in the way it is at a high pressure. This keeps the rocket valve open and gas going down the barrel. Once the BB leaves the muzzle the pressure drops, the rocket valve closes and vents the gas backwards to cycle the bolt.

Now BB weight influences how much energy can be imparted to it (assuming barrel, gas, temperature, etc remain the same) If someone gave you a pingpong ball and a golf ball (same diameter, ignore the dimples, you are not throwing that hard!) and asked you to throw them as far as possible, which would go further? The golf ball, being heavier, allows you to transfer more energy into it from your arm. Light BBs tend to leave the barrel too soon before maximum energy is transferred to them. 

So a GBBR with light ammo would be better with a much longer barrel (as it will have longer time to be accelerated, remember gas continues to vent until it exits the end of the muzzle) but would require more gas volume and still end up with less range.

So a short barrel will limit time to transfer power (which is why TM use 250mm barrels to suit Japans 1J hard limit and 0.25g BBs used)

Heavier BBs work better as they have the same cross section / drag as light BBs, can get more angular momentum (hop) and lose energy slower than 0.20g so go further. Time to target is negligible difference until much further out where 0.20g probably won't reach.

I have 0.28g and 0.32g for my GBBR, the lighter ones to "Joule creep" the power down on hot days. This is why you chrono on the weight you use as if I used 0.20g BBs it would be 0.6J, not the 1.05J the 0.32g give me!

 

[Enid_Puceflange]

okay...this might be a long post!

With AEGs you have a fixed cylinder volume and spring speed. This is usually matched to barrel length / diameter and BB weight you want to use (others can chime in with formulas and typical set ups)

With a GBBR when the trigger is pulled the gas is vented down the barrel, and as there is a BB in the way it is at a high pressure. This keeps the rocket valve open and gas going down the barrel. Once the BB leaves the muzzle the pressure drops, the rocket valve closes and vents the gas backwards to cycle the bolt.

Now BB weight influences how much energy can be imparted to it (assuming barrel, gas, temperature, etc remain the same) If someone gave you a pingpong ball and a golf ball (same diameter, ignore the dimples, you are not throwing that hard!) and asked you to throw them as far as possible, which would go further? The golf ball, being heavier, allows you to transfer more energy into it from your arm. Light BBs tend to leave the barrel too soon before maximum energy is transferred to them. 

So a GBBR with light ammo would be better with a much longer barrel (as it will have longer time to be accelerated, remember gas continues to vent until it exits the end of the muzzle) but would require more gas volume and still end up with less range.

So a short barrel will limit time to transfer power (which is why TM use 250mm barrels to suit Japans 1J hard limit and 0.25g BBs used)

Heavier BBs work better as they have the same cross section / drag as light BBs, can get more angular momentum (hop) and lose energy slower than 0.20g so go further. Time to target is negligible difference until much further out where 0.20g probably won't reach.

I have 0.28g and 0.32g for my GBBR, the lighter ones to "Joule creep" the power down on hot days. This is why you chrono on the weight you use as if I used 0.20g BBs it would be 0.6J, not the 1.05J the 0.32g give me!

Thanks for that

I understand the physics of joule creep, I maybe worded my original post badly, but I’m wondering why gbbr’s can hop a heavier bb than aeg’s can (without heavy upgrades to an aeg)

make more sense? 

 

[Impulse]

Essentially no, it comes down to the hop up. If you have a GBBR and an AEG, both capable of hopping .48s and both shooting at 1.1J, they will perform about the same. However, there's usually issues when it comes to voluming for AEGs which I'll go into in more detail. If you're interested in me absolutely nerding out over tiny details, continue reading. If not, you have been warned!

I think it's probably because gas when it comes to propelling the BB works differently and most AEGs have voluming issues for heavy weight BBs. I've recently been properly delving into cylinder to barrel ratio stuff for my SRS, so I actually have a good understanding of it now. If I'm wrong, someone please correct me, but this is my understanding from my work on my own guns though my knowledge on AEGs is all theoretical because I just can't work on gearboxes; I've tried and failed way too many times, but I'm well-practiced on gas guns, from gas bolt actions, to GBBRs and GBBPs (and mk23s...)

I'll start by citing my sources (ooooh, look at me being all professional and stuff) because we stand on the shoulders of giants

http://airsofttech.dk/Guides.cshtml?Page=CylBarVolRatio_Page1

https://www.airsoftsniperforum.com/threads/cylinder-barrel-ratios-explained.14303/

Basically, a heavy BB requires more pressure to push through a barrel and takes longer to get from the start point to leaving the barrel. A .2g BB accelerates incredibly quickly through the barrel and leaves a lot quicker than a .48, assuming the same power level, so your cylinder to barrel ratio for heavy BBs tends to be higher to make sure you're pushing the BB appropriately through the barrel; you want a lot more air in the cylinder to push the BB through the barrel and also around the BB as it travels, creating a sort of air cushion around it. As an example (it's not an AEG, I'm sorry, but the mechanics are the same) I've got my SRS at about a 3.36:1 cylinder to barrel ratio for chucking .48s, so my cylinder volume is 3.36x the volume of my barrel and that seems to be about right for it (though not 100% accurate without me getting a barrel and testing it and cutting it down 10mm every time to see where it's the best.

Most AEGs really struggle to volume properly which causes inconsistencies. Funnily enough, the best guns AEGs for chucking heavy BBs are things like mp5s or short m4s with shorter inner barrels. If there is too much volume it can push your shots off course after it leaves the barrel, causing inaccuracy. If there is not enough volume (a lot of AEGs using heavy BBs) it starts decelerating inside the barrel, which causes inconsistencies.

With GBBs, the gas constantly expands as it pushes the BB through the barrel, so you have constant acceleration much like a real firearm; a 10.5" AR-15 firing the same 5.56 ammo as an 18" AR-15 will struggle to hit ranges that the long barreled one will hit with ease because the long barrel is giving the bullet more time to accelerate through the barrel. GBBs work in a similar way in that when the hammer strikes the magazine and releases the gas, it's constantly expanding and accelerating the BB through the barrel, so there's no issues with voluming in the same way AEGs have issues, and it compounds with heavy ammo as heavy ammo spends longer in the barrel, so it has more time to accelerate. This is why you get so much joule creep in long barreled gas guns. One of my gas m700s has a 640mm inner barrel and I find it tough getting it shooting the correct power on .48s and 144a during the summer; fortunately it's the one I don't use and is more of a collectors item  

However, let's assume that we have an AEG correctly volumed for heavy BBs for the sake of argument. In this instance, no, there is no inherent advantage that a GBBR will have for hopping heavy ammo.

Thanks for that

I understand the physics of joule creep, I maybe worded my original post badly, but I’m wondering why gbbr’s can hop a heavier bb than aeg’s can (without heavy upgrades to an aeg)

make more sense? 

And I just saw this so you may disregard everything I typed (but I'm not deleting it because it took me time to write and I like nerding out!).

I think GBB hop rubbers tend to be a bit better than AEG rubbers for hopping heavy ammo. Longer hop patches thanks to unbridged barrels certainly help getting a more even and consistent spin on heavy ammo. I put Flamingo buckings in my GBB hop rubber stuff and the patch is noticeably longer than the patch in the Maple Leaf MR rubbers I put in all my AEG rubber stuff

 

[SSPKali]

Thanks for that

I understand the physics of joule creep, I maybe worded my original post badly, but I’m wondering why gbbr’s can hop a heavier bb than aeg’s can (without heavy upgrades to an aeg)

make more sense? 

Ahh OK, I think that might be to do with the hop chambers, barrel cut outs and general build quality. Plus pushing 0.36g BBs in an AEG needs some BIG cylinder volume and spring pressure (same for HPA set ups, higher PSI and volume) and then you are still using a hop unit designed for 0.25g BBs at 1J....

 

[Adolf Hamster]

Thanks for that

I understand the physics of joule creep, I maybe worded my original post badly, but I’m wondering why gbbr’s can hop a heavier bb than aeg’s can (without heavy upgrades to an aeg)

make more sense? 

Typically gbbr hop design allows longer hop patches to a certain level, but frankly given a simple macaron swap can get a .48 spun up it's kinda moot these days.

 

[Nick G]

It's quite simple GBBR's are just so much better ??

 

[Sewdhull]

Gbbrs have the advantage of a reservoir of gas to keep the pressure up until the BB has left the barrel. This means that a heavier BB will continue to experience the same pressure as a smaller BB but for a longer period of time and so gain more energy.

Since the pressure is maintained after the hop, the proportion of the energy used by the hop to spin the BB is less relevant, so more hop can be dialled in while maintaining BB forward energy.

With an AEG you have a fixed amount of energy to impart to the BB. If you use a heavier BB and have pressure left over at he end of the barrel when the BB exits you will still see more energy in the heavier BB.

The amount of pressure increase in an AEG will decrease the moment the BB starts to move and then pressure will peak and then reduce. Getting the reduction in pressure in the right place/time is the goal for voluming.

For both types of gun, hop quality matters but for AEGs in particular tuning to hop and shoot for a BB weight needs to be done at build time.