Version 2 200mm FPS build

[mrpugster]

Hi guys

I had some spare parts left over so thought I make a short M4 however I'm having real trouble hitting 330 FPS or so with my M100 spring (spring works fine with other setups). The inner barrel is 200mm for which I'm using a MP5K/PDW Modify cylinder http://www.airsoftglobal.com/shop/im...E0304-AG-1.jpg, SHS aluminium nozzle paired with SHS cylinder head and SHS piston head. I get good compression etc.

Problem is I can't get the FPS over 280 FPS with this set up. Is this the correct cylinder for this setup? I've tried a number of other cylinders that's I've got, but each gives the same result.

Any help greatly appreciated

[Crude Skinny B]

Re: Version 2 200mm FPS build

Have you tried a non ported cylinder and just seen what fps you get from that? It will rule a few things out.
Try putting a small piece of paper over the feed hole of the hop up in the mag chamber and fire. If it gets blown around thats where you will have a potential leak.
You could also try removing a small amount of material from the front of the tappet plate.
Also check that the nozzle can move freely back and fourth when the gearbox is screwed together.

Also how consistent is the fps?

[seansamurai1]

Re: Version 2 200mm FPS build

Short barrel, half stroke cylinder=low FPS. Won't hurt to check around the nozzle hop interface though.

[mrpugster]

Re: Version 2 200mm FPS build

Cheers for the suggestions guys.....

I've tried a non ported cylinder and get pretty much the same FPS, I'll double check the nozzle but it appears to be working fine. I've already removed a small amount of material from the tappet plate also, had no effect. Maybe I'll just have to give in and put in a M120

[Pezza]

Re: Version 2 200mm FPS build

You'll struggle with an m100 and a 200mm barrel.
The fps on my mp5k jumped from 265 to 410 going from stock barrel to a 300mm jobby.
Full or 3/4 ported cylinder made next to no difference.

[Clot]

Re: Version 2 200mm FPS build

I got one to 330fps.
It was less than 200mm, it was closer to 100mm. If I remember, it was a full cylinder and an M130 spring.
What a cracking noise!

[dave38x]

Re: Version 2 200mm FPS build

I've got an m4 with a 14" barrel/an m100 @328fps that drops to 290fps on a 8" barrel.

i've just rebuilt a mates 7" SBR an thats doing 320fps with an m120 spring/sorbo

[sj_asc]

Re: Version 2 200mm FPS build

If you look at it this way, MP5 barrels are 229mm, that's only 3cm longer, and with basic seal maintenance will hit 290fps w/.20g with a stock marui spring. You will struggle for fps with .20g efficiently on anything shorter than 150mm.
Stick with the 1/2 port cylinder, you are on full stroke i'm assuming? And assuming you have ruled out compression assembly leaks My bet is that it's either seal between the airseal nozzle and hop bucking lip, the size of the exhaust hole on the airseal nozzle(if you have a vented one try that), or the seal between the hop bucking and barrel.

[mrpugster]

Re: Version 2 200mm FPS build

If you look at it this way, MP5 barrels are 229mm, that's only 3cm longer, and with basic seal maintenance will hit 290fps w/.20g with a stock marui spring. You will struggle for fps with .20g efficiently on anything shorter than 150mm.
Stick with the 1/2 port cylinder, you are on full stroke i'm assuming? And assuming you have ruled out compression assembly leaks My bet is that it's either seal between the airseal nozzle and hop bucking lip, the size of the exhaust hole on the airseal nozzle(if you have a vented one try that), or the seal between the hop bucking and barrel.

If you're only getting 290fps w .20g with 229mm with stock TM spring then I'm not that far off that. I've tried the same setup with a 280mm barrel and get 345 fps so does indeed look like a physics limitation - even with the half ported cylinder. Thanks for all the suggestions...

[Sir Stephen]

Re: Version 2 200mm FPS build

double check all of your air seals and use a full cylinder. my m4 cqb has a full cylinder, a 200mm inner barrel (not even a tightbore) and an SHS m90 spring. im getting 315fps out of it.

[North West Outpost]

Re: Version 2 200mm FPS build

You will be better using a ported cylinder as you are giving the piston some acceleration time before there is enough pressure to push the bb through the throat of the hop rubber.

If you use a full non ported cylinder as soon as the release tooth has slipped over the sector gear the piston is moving at it's slowest but under full sealed pressure pops the bb through the throat of the rubber and on it's journey, meaning the piston is still accelerating after the bb has left the barrel.

Also if you are looking to build a high speed gun the vent will help when the piston is retracting.

Obviously if the fps is pretty constant it is very unlikely it is an air seal problem.