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Old 01-29-2017, 01:36 PM   #16
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PDI Palsonite HD
PDI 520 spring
EdGi unbridged barrel
Maple Lead
PDI V Trigger
AA hopup

I could buy a VC piston
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Old 01-29-2017, 02:36 PM   #17
1tonne   1tonne is offline
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I was going to suggest using either a VC piston or go off this chart of mine.:

0.3gm bb use a 554mm barrel
0.32gm bb use a 530mm barrel

0.36gm bb use a 500mm barrel
0.4gm bb use a 470mm barrel
0.43gm bb use a 430mm barrel
0.45gm bb use a 400mm Barrel


There barrel ratios are near perfect. Depending on what bb weight you are going to use, these barrel lengths should give you the most energy from your cylinder without any "barrel suck"
The above cylinder to barrel ratios should also mean that you would have very little excess cylinder air that may or may not effect your results.

If you said an AEG I would have given a different advice but I will not worry about that.
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Old 01-29-2017, 04:37 PM   #18
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I have a Krytac LVOA I could use.
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Old 01-29-2017, 07:35 PM   #19
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Vsr10 would be better
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Old 02-01-2017, 02:54 AM   #20
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Quote:
Originally Posted by 1tonne View Post
One question. Can cylinder to barrel volume have an effect on backspin?
The reason I ask is that if you under volumise your system the bb will most likely hit the ground around 30-40 meters even if it has high energy when it leaves the barrel. (I did personal experiments and found this) My guess that this happened is because of "barrel suck"
How will you know that your cylinder volume ratio is correct and that it does not effect any of your results for backspin?

(I understand this is not about airsoft. But you need to make sure you have the perfect system to project your bb. Or maybe it just needs to be over volumised)
Two questions:
1) Did you under-volume via cylinder or barrel?
2) Did you adjust the energy input to match both tests' outputs?
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Old 02-01-2017, 03:35 AM   #21
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Quote:
Originally Posted by newcomer View Post
Two questions:
1) Did you under-volume via cylinder or barrel?
2) Did you adjust the energy input to match both tests' outputs?
1. I changed out the barrel length to get the results. Also, years ago I did the same experiment by porting the cylinder and I also got similar results.

2. Note sure what you mean?
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Old 02-01-2017, 03:46 AM   #22
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I mean if you changed the volume balancing it would change the FPS output. Did you increase/decrease the spring rate (air pressure/flow if gas/HPA/Co2 etc) to match both tests FPS?
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Old 02-01-2017, 03:53 PM   #23
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No I did not change the spring.

The first experiment was a number of years ago now (about 5-6 years). I used a M170 spring and just extended the port back further but did not change the barrel length. I used a 430mm long inner barrel. (It would have been good if I have changed the barrels too and then tested it)

The second experiment I had a M170 spring and then used a long 590mm inner barrel. I then fired it with every bb weight (hopup set for the bb to fly straight). Once finished firing, I then cut the barrel down 10mm and redid my experiments with the shorter barrel. I then kept cutting and repeating this until I got the barrel down to about 300mm.
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Old 02-02-2017, 03:03 AM   #24
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The first test would have introduced another variable because as the ports go further the piston traveled slower, thus lower bb speed as it passes under the rubber. This does have an effect on backspin applied. In addition, the longer barrel increases internal friction hence taking away some of that backspin, albeit very marginal. On top of that, the change in FPS with different ports distance alters the V/U ratio (rotational vs linear velocity) which also changes the lift coefficient. It's a 3 dimensional problem really.

The 2nd test is interesting because the bb acceleration under the rubber is held constant. So with the 2nd over-voluming test the bb flew progressively higher as the barrel shortens? I would presume with a shorter barrel the FPS drops, hence more lift due to higher V/U ratio. Not to mention lesser friction along the inner barrel.

Generally I find that faster acceleration generates more backspin, but the law of diminishing returns applies here. Too fast (above 450FPS regardless of bb weight) and the gain in backspin diminishes. Too slow (below 300FPS) and lift drops. Same hop up settings, same barrel length. For the former I attribute this to either, 1)Reaching static friction limit of rubber(as evident from patch pinching, patch blown out, etc), or; 2)Change in V/U ratio. For the latter it could be due to lesser angular momentum from the slower accelerating bb under the rubber.
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Old 02-02-2017, 01:05 PM   #25
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Quote:
Originally Posted by newcomer View Post
The first test would have introduced another variable because as the ports go further the piston traveled slower, thus lower bb speed as it passes under the rubber. This does have an effect on backspin applied. In addition, the longer barrel increases internal friction hence taking away some of that backspin, albeit very marginal. On top of that, the change in FPS with different ports distance alters the V/U ratio (rotational vs linear velocity) which also changes the lift coefficient. It's a 3 dimensional problem really.
True but the variables you speak of only make a very minor difference that that I would not worry about them. Anyway, this is the old way of matching cylinder to barrel ratio. The new way is just to match the barrel to the bb weight. So the second experiment is the better one.

Quote:
Originally Posted by newcomer View Post
So with the 2nd over-voluming test the bb flew progressively higher as the barrel shortens?
The bb only gained height when shortening it from the very excessive length. So using a 0.45gm bb's with a 590mm barrel will go downwards quite fast. But if you had a 0.3gm bb and the same barrel, the bb would look as though it was acting how it should, so fly straight for a long way, although the barrel is still too long for the 0.3gm bb's as 550mm is best for this weight. So it did not ark downwards all the time when under volumed. Only with excessive under voluming did it ark downwards.

Quote:
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Too fast (above 450FPS regardless of bb weight) and the gain in backspin diminishes. Too slow (below 300FPS) and lift drops.
The only time I have ever really noticed this diminishing return would be with extra heavy weight bb's. This is why we use longer patches for extra heavy weights. But a rifle firing high fps, with a standard bucking is still capable of creating enough backspin for a heavy weight bb.
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Old 02-03-2017, 06:58 AM   #26
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Anyways,
I cannot have this in time for my science fair project. If I ordered yesterday it would come on Tuesday. Now it will come on Friday.

This is for the PDI rifle.

I need to use my LVOA. It should work, right?

I am already going to buy some BB's and a chronograph.

What weight do you guys suggest?

How can I perfect the airseal?

Barrel is staying the same as I would like to spend the money on the PDI barrel.

Thanks.
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Old 02-04-2017, 12:54 PM   #27
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Bump.

I can't have the PDI gun in time. I need to do it now.

My experiment is now, "Does more spin increase distance?"

But that creates an issue.. How will I measure the distance?

Another viable option is, "Does more spin increase accuracy?"

What do you guys say?

I'll use .33 gram for this. White BB's. (or should I use black, there is snow)

I realized I do not need a chronograph because I am doing the FLIGHT of an airsoft BB, not the energy. So to me, I do not need to spend 60$ on a chronograph just yet. The only thing it would tell me is the energy/speed, which really isn't needed. I already know it's 400. So transfer that over to .33. Boom.
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