GBBR higher than expected FPS with heavy BB's

[rjb]

Hello Chaps,

Just a quickie really.
A lot of folk swear by heavy weight BB's in GBB's in particular and it does seem to increase accuracy (up to a point) but more interesting relative FPS. You can expect an AEG firing at 320fps on 0.2's to fire at about 260fps on 0.3's. With GBB's its completely different and GBB's seem to kick out FPS way above what you might expect (>260FPS). Any ideas what's going on here?

R

[MarkPulling]

Re: GBBR higher than expected FPS with heavy BB's

Interesting ...ill check fps on my we g36 tommorrow with .2s and .3s and report back

[CKinnerley]

Re: GBBR higher than expected FPS with heavy BB's

Can't remember the exact physics but I'm sure someone who can will come along. Sufficed to say a heavier BB stays in the barrel longer with a GBB rifle and hence receives a greater amount of propulsion energy from the expanding gas, therefore your muzzle energy will creep up as you use heavier ammo. So you GBBr might only be doing say 280FPS for a fairly hefty BB but the energy behind it would not be safe for skirmishing.

Sites say (for example) 350 with a 0.2 for a reason, it equates to a certain level of energy. If your AEG was shooting 330 with a 0.4 it'd obviously have far too much energy output.

[pacey]

Re: GBBR higher than expected FPS with heavy BB's

This has been covered before and was a for long time the BS reason sites didn't like letting GBBR's play.
If I remember correctly its because the propellant is going from liquid to gas, this expansion along with the additional mass of a heavy BB makes for a higher pressure (more mass, more pressure required to shift it in the same amount of time) I believe this is why NPAS's get used as they will stop the over pressure once the desired pressure setting is reached.
Im sure those with more knowledge will correct me soon.

[Viper Scout]

Re: GBBR higher than expected FPS with heavy BB's

You also have a bit of f=ma. With a heavier bb or a longer barrel, the greater the pressure build up behind the bb before it states to move.
Oops, just read previous thread.

[Danny_boy85]

Re: GBBR higher than expected FPS with heavy BB's

GBB fps is seriously affected by barrel length.

Hence the Hi Capa 4.3, P226 and Desert/Foliage warriors coming in at a nice 270-280 fps on propane but the Capa 5.1, 1911/MEU and 5-7 come out about 310-320 on propane.

The gas system is affected by so much its untrue. temperature, amount of silicone in the gas itself, how full the mag is etc etc etc. I have never had an issue with using heavier rounds in all my gas stuff, i see a very rough average of 30 fps drop from a .2 to a .25g bb and thats accross all my guns as they get chrono'd on .2's just becasue its much easier than trying to explain the other way lol.

But barrel length only works to a certain length before the valve in the nozzle closes and then sends the bolt/slide back. If your barrel was so long that the valve had shut before the bb had left the barrel, it would in theory slow down before it leaves and thus give a lower fps than a barrel thats been cut to have the bb exit right before the valve shuts.

This is my theory by the way. Before people argue with numbers and shizz, this is my experament and my findings.

I built a stupid length barrel'd capa and had a 18 inch barrel installed, and i cut off an inch each time and then chrono'd the gun and the fps got higher (slightly) as the barel got shorter to around. roughly 11-12 inches was the sweet spot of the fps (clocking about 420 on a .25) and then once cut down to the silencer i have (about 10 inches) its around 405ish on a .25

Also the BB has left the barrel before any of the bolt or slide have started moving, so the gas is still flowing. The gas then shuts the valve in the nozzle, this in turn sends the gas into the nozzle chamber which then expands against the piston head and boom, blowback is created. NPAS valves edit this gas released into the barrel system by making the gap smaller or larger.

Smaller = Less gas to accelerate bb along barrel

Larger = More gas to accelerate bb along barrel

What you say about AEG's giving different FPS on different weight bb's is the same principle. The piston always pushes the same amount of air forward, the bb is still in the barrel longer as the air pushing it out has to work harder so same thing. Im sure an AEG bod will say about ported pistons for shorter barrels and such (i know very little about this)

Hope this helps? if not, meh, im at work and bored lol

[Sir Stephen]

Re: GBBR higher than expected FPS with heavy BB's

as has been mentioned above better than i can explain, GBB's propel the bb using relativly unpredictable expanding gas.
change the bb weight, unlike in an aeg the gas will behave differently.

what sites should do while chrono'ing anything is simple:
ask what weight bb is being used, get chrono reading, used mobile app to work out energy.
would take an extra 5 seconds per player and everything would be safe.

[Danny_boy85]

Re: GBBR higher than expected FPS with heavy BB's

im going to test this when i get home and work out the variations on averages.

Personally i dont think theres a difference. the drop and gain in FPS on heavier and lighter bb's will be the same give or take the normal varience in bot AEG and GBBR. If im wrong though thats new to me lol

Ill do some tests and post ones findings.

[JCheeseright]

Re: GBBR higher than expected FPS with heavy BB's

im going to test this when i get home and work out the variations on averages.

Personally i dont think theres a difference. the drop and gain in FPS on heavier and lighter bb's will be the same give or take the normal varience in bot AEG and GBBR. If im wrong though thats new to me lol

Ill do some tests and post ones findings.

There is a definite difference, energy creep with a GBBis a documented and provable thing which is relatively easy to explain.
An AEG pushes the volume of air in the cylinder into the barrel, at no point is it really compressing and it's just like someone blowing really hard into a straw to force the BB out, the amount of force behind it is constant as it's dictated by the spring.
With a GBB instead what happens is the valve on the magazine is opened and a certain amount of liquid propane (we'll use that as an example here, the principle is identical regardless of the gas you use) will boil off to become a gas which expands into the barrel and blowback unit. Because a heavier BB accelerates slower it remains in the barrel for longer (marginally) and so is pushed by the gas for longer before exiting, the amount of push is not linear because the gas will continue to expand until it reaches the same pressure as the ambient air around it, it's not restricted in the same way an AEG cylinder is.

[Danny_boy85]

Re: GBBR higher than expected FPS with heavy BB's

Ah ha i see!

Right then, i shall still do my tests later (as ill be bored with nothing better to do lol) and post it up.

I have .2, .25 and .3g bb's so i shall post them using a Hi Capa 5.1, a KWA MP7 and a TM Scar Recoil.

[MarkPulling]

Re: GBBR higher than expected FPS with heavy BB's

Here are my results on my WE G36...
0.2 G .....395 FPS ..1.45J
0.25 G.......370FPS...1.59J
0.3 G....340 FPS......1.61J
Using propane at room temperature
interesting as said the energy goes up with the heavier bb ,ill try this test again on my aegs .

[rjb]

Re: GBBR higher than expected FPS with heavy BB's

Here are my results on my WE G36...
0.2 G .....395 FPS ..1.45J
0.25 G.......370FPS...1.59J
0.3 G....340 FPS......1.61J
Using propane at room temperature
interesting as said the energy goes up with the heavier bb ,ill try this test again on my aegs .

I'm glad its not just me imagining it... Some great explanations BTW. Love a bit of debate stimulation!

R

[dadio]

Re: GBBR higher than expected FPS with heavy BB's

in short you need to crono with the weight of bb you are using and have a crono that can measure for different weight bb's , if you do this you can't go wrong and don't need to do complicated math's.