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Piston Weight

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I have an interesting story, and have found precious few posts anywhere on this subject.

 

My son has a G&P M4 Marine, with a Prometheus 6.03 x 363 barrel and Systema hop up unit and bucking. It shot 375 fps with .25g BBs and 410 with .20g BBs. The AEG was shooting somewhat inconsistant, and periodically dropping velocity way off. We traced it to an inproperly sealing piston head, which we changed to a new Sytema one. When we chroned it with the new head, we were shocked to see the velocity dropped off to 335 fps with .25s and 365 fps with .20s. We were like "what the heck"? The inconsistancy was fixed, but we lost more than 10% in velocity.

 

The only difference was a metal weight that the stock G&P gun had in it. We did not put the weight back in the gun when we changed the head, for two reasons. First the old G&P head (the leaky one) had the fastening screw through the head, and into the weight, which was threaded to accept the screw. The new Systema one fastens with a screw from inside the piston, into it's back. Secondly when asking in our local airsoft store (where we bought the new head and who does upgrades, etc) he said the weight did not matter.

 

We decided to put the weight back in to see what it did to the velocity. We were able to find a M3-.5x10 screw in the local Lowes store. I cut off the head, making a stud. This allowed me to screw one end of the stud into the weight, and the other end into the back of the Sytema piston head, succesfully fastening it in place with the help of a little thread lock.

 

Guess what, with the weight back in the piston, it shoots 370 with .25s (5 fps slower then the stock head), and *very* consistantly. Now the punch line.

 

While we had the gun together without the weight in it, my son played a bit. He put the stock barrel with the stock G&P hopup chamber and bucking back in the gun. It chroned 325 fps with .25g BBs. Guess what, that is about the same velocity that it chroned *before* we put the prescision barrel in. Which means that a stock G&P M4 Marine chrons about the same with or without the weight in the piston. When putting the precision barrel in, it shoots 335 fps without the weight, and 370 fps with the weight, which is a 35 fps (more than 10%) increase.

 

Summary for .25g BBs:

6.08 barrel 6.03 barrel

Weight in: 330 fps 370 fps

Weight out: 325 fps 335 fps

 

It appears the piston weight does little with a stock (6.08) barrel, and a lot with a precision (6.03) barrel.

 

BTW, the G&P weight is a solid brass cylinder, 9/16 inch in diamter, 5/16 inch deep and weighs 12 grams.

 

I recenty bought and upgraded a Marui M14, which includes a Prometheus 6.03 barrel. I got a steel bar, and am going to make a similar weight to the G&P one (I've seached all over the place, and can not find "piston weights" for sale anywhere). I can't *weight* (get it?, duh) to see what it does to the velocity!

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Are you sure the weights aren't just acting as spacers within the cylinder. I havn't read any literature on varying weights within a gearbox outside the realm of high ROF upgrades. In the FPS category, I know that spacers can make a difference such as you are talking about, but I guess it's possible. As far as the difference in velocities with the tight bore vs. 6.08, I got nothing. Interesting though.

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Guest djohnston

I have not heard of anyone using a weight in the piston to affect the fps of an AEG. But without testing the idea and using your experience as a starting point I can see a couple possible reasons for the increase in fps. One was mentioned by Spartan_Child where the weight acts like a spacer in the piston and this adds compression by squeezing the spring farther than normal. The other thought I had is that the weight might cause the piston to be thrown forward harder (more inertia) thus creating more compression. Especially with the tightbore, the smaller bore would cause more of an air constricting bottle neck than the stock barrel would.

 

This would definitely be an interesting theory to test in a controlled manner.

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I have not heard of anyone using a weight in the piston to affect the fps of an AEG. But without testing the idea and using your experience as a starting point I can see a couple possible reasons for the increase in fps. One was mentioned by Spartan_Child where the weight acts like a spacer in the piston and this adds compression by squeezing the spring farther than normal. The other thought I had is that the weight might cause the piston to be thrown forward harder (more inertia) thus creating more compression. Especially with the tightbore, the smaller bore would cause more of an air constricting bottle neck than the stock barrel would.

 

This would definitely be an interesting theory to test in a controlled manner.

 

actually, there was a "Swiss Cheese" piston, which supposedly increased fps by allowing the spring to decompress faster...or something like that. I don't remember where I saw it, it wasn't a common online store.

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time to add my nerdy speculation:

 

I beleive that what this does is that by adding weight to the piston, it adds energy in the most critical part of the piston stroke.

 

let me explain: when the piston is all the way back, the spring is applying the most force, while air pressure (ahead of the piston) is applying the least force. when the spring decompresses, the amount of force it is applying decreases, and the inertia of the piston as well as the air pressure increase. if you have a light piston, it will not conserve it's momentum against that air pressure as well as a heavier piston. though the heavier piston will not reach a higher velocity, it will have a greater mean velocity than the lighter piston, meaning that it will impart a smoother, (possibly stronger) amount of air pressure to the bb.

 

of course, this is all speculation...

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Guest McHitman

Hey, I've seen this before. MY CSI AK SVD had one of those piston weights in it. It seems as if that is what's fastening the spring to the piston, as opposed to my CA M15A4 rifle, where the spring not connected to the piston. I noticed that the spring in the AK SVD had a little hook on the end of it(to connect it to the weight), and seeing upgrade springs, I think this is pretty much proprietary(sp?) as upgrade springs have flat ends.

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time to add my nerdy speculation:

 

I beleive that what this does is that by adding weight to the piston, it adds energy in the most critical part of the piston stroke.

 

let me explain: when the piston is all the way back, the spring is applying the most force, while air pressure (ahead of the piston) is applying the least force. when the spring decompresses, the amount of force it is applying decreases, and the inertia of the piston as well as the air pressure increase. if you have a light piston, it will not conserve it's momentum against that air pressure as well as a heavier piston. though the heavier piston will not reach a higher velocity, it will have a greater mean velocity than the lighter piston, meaning that it will impart a smoother, (possibly stronger) amount of air pressure to the bb.

 

of course, this is all speculation...

 

why wouldn't a light piston conserve it's momentum? because the spring is pushing it...A spring can push a light piston faster than it can push a heavier piston, agreed? The force of the spring is so many more times greater than the resistance to it that it completely blows over it (pardon the pun), if I assume correctly. :a-jester:

Edited by hydralover

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time to add my nerdy speculation:

 

I beleive that what this does is that by adding weight to the piston, it adds energy in the most critical part of the piston stroke.

 

let me explain: when the piston is all the way back, the spring is applying the most force, while air pressure (ahead of the piston) is applying the least force. when the spring decompresses, the amount of force it is applying decreases, and the inertia of the piston as well as the air pressure increase. if you have a light piston, it will not conserve it's momentum against that air pressure as well as a heavier piston. though the heavier piston will not reach a higher velocity, it will have a greater mean velocity than the lighter piston, meaning that it will impart a smoother, (possibly stronger) amount of air pressure to the bb.

 

of course, this is all speculation...

 

I think that sounds quite logical in nature.

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Just FYI, the reason the G&P piston head lost seal would be because of the bad o-ring. There is nothing wrong with the G&P piston head itself, but the o-ring is a bit too small. G&P piston heads work great with a replacement o-ring.

I replaced the weight in my G&P M4 with one the same size but custom made. Machined from aluminum with six holes in it for light weight. It chronos fine with a stock barrel, but every time I switch to one of my 509 tightbores, it barely gains any fps (about 5 to 10). I'll see if I can take my G&P apart today and switch out the weights.

 

EDIT - No significant difference. Velocity changed from 335 with .28s to 340 with .28s. I also relubed and put in a fresh o-ring, so the fps boost could just be from that.

Edited by Nerd

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