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Old 03-18-2012, 07:33 PM
Travis Travis is offline
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strange fps occurance?

so i have two guns, an m249 and an m16, both are fully upgraded as far as compression goes, cyl head, piston head with #14 o ring, air seal nozzle just everything, the works, both also have prometheus tightbores that are the same length (533mm, 6.03) the m249 (140 spring) shoots about 450 fps and the m16 (stock spring) shoots at about 425 fps, they both have nearly identical internals except for the spring (249 requires a larger spring to shoot same fps) but with .25 the saw shoots around 390-95 and the m16 shoots about 385-90, i am curious if anyone knows why my m249 is dropping 50-60 fps where my m16 is only dropping 40? and also is that normal to drop that much fps?

all results were verified with a chrono with ics .2 and .25 bbs, my chrono does not have a .25 setting so i am a little curious as to if that effects the readings i got from the .25 thank you
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Old 03-18-2012, 07:49 PM
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Honestly, it may just be your springs, since they are different sizes, they may be different brands, which differ a little bit in actual force they output.

Another thing that may cause it is your piston itself, if you have teeth cut off (1-2) on your m16 it won't take full tension on the spring. I think I saw somewhere it was 5% fps drop per piston tooth cut off, something like that.
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Old 03-18-2012, 07:52 PM
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I agree with Scrapper. Pistons are a big part too. Make sure your AOE is set correctly.
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Last edited by Private Snowball; 03-18-2012 at 07:56 PM.
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Old 03-19-2012, 08:26 AM
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I'm going to point towards your compression parts as being the cause of the problem. It would be helpful if you listed the following parts being used in each system:

Piston head
Cylinder head
Cylinder
Air seal nozzle
Hopup chamber
Hopup bucking

There could be any number of problems affecting that drop in FPS. The o-ring could be too small for the cylinder and may need to be stretched out and heated. The cylinder head may be leaking some air out and may have to be sealed in with some silicone. The air seal nozzle's o-ring may be too large for the cylinder head and might have to be replaced for a larger one. The tappet plate may not be chambering each round far enough into the bucking and could have to be sanded down a bit. The hopup bucking might be torn, old, or too soft for the gun's power output.

Check these things over and report back when you get the chance. I'll be here to help you in any way I can.
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Old 03-19-2012, 10:38 AM
Travis Travis is offline
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ok here is the parts i have thrown into both guns, i did the compression test prior to installation and was very satisfied

bravo double seal cylinder head
ICS polycarb orange piston
bravo aluminum piston head with an x head o ring
modify cylinders with no ports, type 0 i think is what they are called
jbu m140 irregular pitch spring in the saw and not sure in the m16 but i believe it is a 125 or 130
modify o ring air nozzle
stock a&k m249 hop up and hop up bucking modified nub
i am not sure of the hop up system in my m16, it was just one i had around, i built that m16 from parts of like 10 guns

my personnel chrono is a guarder 2000 gen 2
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Old 03-19-2012, 12:54 PM
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For the compression test, did you test it with the nozzle retracted or forward? A common misconception about the compression test is that you have to have the air nozzle pushed back against the cylinder head in order to test it. This is not true. The air nozzle needs to be able to seal when in the forward position as this is where it will be whenever the piston begins its stroke. Let's go through your parts list and address a few things:

Bravo double seal cylinder head: Check for leaks around where it seals in the cylinder. If you find a leak, use a bead of silicone sealant around the first o-ring (i.e. the one that sits furthest from the air nozzle), reassemble, and let cure.

ICS polycarb orange piston: Not my favorite choice in pistons but check for interference along the piston rails. With the entire cylinder assembly removed, the piston head and main spring removed, the piston itself sitting in the rails in the gearbox, and the gearbox screwed together, see if the piston can travel freely back and forth in the gearbox. If there is much friction from the rails binding in the shell, you may have to file the rails on the piston a little wider and/or deeper.

Bravo aluminum piston head with an x head o-ring: Ditch the x-ring and use a regular ol' o-ring. X-rings do nothing to help improve air seal. Take a regular piston head o-ring, stretch it around a cylinder, and heat it gently with either a heat gun or a torch. Be careful because it's easy to apply too much heat and make the o-ring snap. Wait for it to cool first before you remove it from the cylinder. If you did it correctly, you should now have the best sealing o-ring possible. Grease it minimally with some silicone grease after it's installed on the piston head and don't grease the inside of the cylinder as the nublets on YouTube will tell you. Let the grease you put on the o-ring coat the cylinder.

Modify type 0 cylinder: This should be fine, just check to see if the chrome plating is worn off on the inside. The piston head may have cut some lateral grooves into it.

JBU M140 spring: This should be fine, just make sure you orient the coils with the close-pitched end nearest to the spring guide.

Modify air seal nozzle: This is not so good. You can try to swap out the o-ring for an SHS one or one from McMaster (I have tons, PM me if you need some) to try to get it to seal right, but I've yet to see a cylinder head that they work properly with. You may want to swap that out for a Guarder, Dream Army, or GURU one if you can't change the o-ring out and get it to seal.

A&K hopup: If you're using the oddly-shaped A&K bucking without the molded lips, you should look into getting a regular one. I've found that those don't always seal properly against the air nozzle and they leak out the back like nothing I've ever seen before. The hopup chamber itself might not be tight enough to prevent air from escaping between the barrel and the bucking and out the back of the bucking, so after you've slid it in place on the barrel, peel the back edge (towards the muzzle) of it back and put a bead of silicone sealant between the barrel and bucking, then install it into the chamber to let it cure. It's like the "floss mod" but much better.

Let me know how this works for you.
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Old 03-19-2012, 01:54 PM
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eck A&K hopops are horrid, the stock one for my M249 wasnt even long enough
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Old 03-20-2012, 12:16 PM
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It could be your B-Bs! I have noticed a inconsistency of ICS .2 and .25 B-Bs. plus the fact that they are 5.95mm in diameter. Some of the most consistent B-Bs that I know of are Javelin .2 B-Bs.. They're definately closer to 5.98mm in diameter. They even beat out Airsoft Elite B-Bs when it comes to diameter consistency.
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Old 03-20-2012, 03:09 PM
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seems like you have got plenty of help, just wanted to say I dont like ICS pistons... SHS full rack is the way to go. With you guns up there in FPS make sure you have your AOE correct, and sorbo too.
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