P90 - wierd fps stuff ;d

[CJay]

Ok basically I have a p90 and I'm not the original owner - the guy that first owned it did something with the piston or pistonhead and I'm having trouble finding out exactly what's going on. With the stock barrel the gun fires at a rather slow 280ish. I also have a lengthened madbull 6.03 TB that I use. With the tightbore the gun hits a steady 375-380. The TB is for an ak47 so it's significantly longer and basically I was wandering how the he'll the barrel makes almost 100fps difference.

Worth notin that the stock inner barrel also makes a hell of a noise compared to the TB but I'm assuming that's just the barrel not fitting as tightly round the BB and allowing noise and air to be expelled. Also sure he said something bout an unported cyllinder. I've never touched a V6 mechbox before and I'd rather not go digging to see what the he'll is going on LOL

[ujiro]

Just sounds like the inner barrel is allowing a good amount of air to be expelled due to not sitting properly in the hop up unit or something. Whats the make of the P90

[CJay]

It's the marui p90 tr. And it's not that the gun ain't functioning right. 280 is about right for a stock TM but I just refuse to accept that doubling the barrel length and tightening the diameter makes 100fps difference. I'm more perplexed than worried LOL! I've used lengthened tightbores on other guns and it made maximum 35fps difference :S

[Styrak]

A P90 is not supposed to have an unported cylinder since it's such a short barrel.

With thje shorter barrel it's having very "ineffiecient air usage" I would call it. The BB is leaving the barrel before the piston completes it's cycle and there is wasted air expelled after the BB leaves the barrel and then the piston slams into the front of the mechbox with no BB in the barrel as resistance. That explains the loud sound with the short barrel.

The longer barrel fixes that, as an AK should have an unported cylinder. The BB stays in the barrel longer and get more energy transferred to it, hence a higher FPS.

I don't think it's necessary, might save some wear and tear on your mecvhbox (V6 are pretty beefy anyways) but if you want you can choose which barrel you want to use, and if you want to use the short barrel get a gundoc to switch out your cylinder.

[chaosnemesis]

With PORTED cylinders there is a hole or "port" in the cylinder allowing air to escape as the spring pushes the piston down the cylinder (for p90 i think the hole is 1/4 way down the cylinder) with no compression resistance the piston accelerates very quickly until it hits the unported section then there's no where for the air to escape but out the barrel. UNPORTED cylinders do not have a hole in the cylinder so the spring pushes the piston with compression resistance from the start. Basically it doesn't get a running start ramming itself into a cylinder of air. When your bb leaves the barrel the piston had only accelerated about 75% of the max speed. That`s why you get a slower fps with a shorter barrel and maxed out fps with the longer barrel. The bb is allowed to accelerate longer.

[Amos]

Holy crap Styrak actually said something right!!!

But yea, What he said

[CJay]

Originally Posted by Amos

Holy crap Styrak actually said something right!!!

But yea, What he said

lol ^^

well that clears that up then, thanks for the info guys - ive actually got the stock cylinder sitting here along with a stainless cylinder head so I'ma gonna read up on the V6 Mechbox before trying anything but changing it should be easy enough...hell it looks way easier than the V2 mechboxes i'm used to :S

[Styrak]

Yeah, V6 mechboxes are really nice to work with. The wiring/switch assembly can be completely removed easily and you just work on the mechbox.