Weird upgrade result - Low fps

[Karhgath]

First time upgrading a gun, followed mechbox.com videos and tips and tricks around this site, had no problem (I'm pretty good at those things). It's an ICS MP5A4. (V2 mechbox, 100% tm compatible as I found out, not the split one) I also kept the stock barrel (no longer/tighter barrel)

First time I opened it, it already had metal bushing and a minor spring upgrade (MS110SP I was told), rest was stock. I didn't chrony it as I had no chrony at that time, but it was supposed to be about 320-330.

So I put in a MS110SP spring, metal bearing spring guide (guarder), air nozzle, and Angel silent piston head/cylinder head. By my calculation, it should shoot at around 360-370 (the angel set should cancel out the spring guide gain).

Went to a game and it was shooting at 290!! Weird...

Then, the gun failed, I though I had stripped something (but wasn't the case as I found out). I bought a chrony and went testing with new gears (guarder), piston (systema) and a MS120SP spring. That should logically set me above 400. Just finished the upgrade, gun seems to shot a lot harder, but I chrony it at... 320-330. This is consistant compared to the previous 290 tho, but no where near what's expected.

What could be the problem of such low but systematic FPS? Gear timing is perfect, shimming looks right, the shot is really consistant. Could it be the cylinder (the only stock part now) that I should change for one made for a longuer barrel(m4)? Maybe the spring is too strong/fast to compress properly with the mp5 cylinder?

I don't know, I'm all out of ideas. The chrony works fine, tested with other guns with known FPS and it reads fine. I'm also using high quality .20.

[Kerbosa]

i had a similar problem with my mp5 after upgrading. My problem was an issue of air loss around the barrel due to the hop-up rubber not fitting correctly around the barrel. Make sure all your air seals are tight ( place your finger over parts and create pressure) to systematically look for areas where you may be leaking.

[Capt. T/O]

i had similar issues with my M4 though
turned out that...
1. nozzle wasn't sealing tightly and resulted in air leak
2. hop up bucking was worn down so caused inconsistent FPS
3. hop up unit itself was shot and resulted in inconsistent FPS and BB feeding

check out your nozzle, hop up bucking and then hop up

[Kerbosa]

MS110SP is an irregular pitch spring i believe. Make sure the tighter coils are at the the spring guide end, as that could be the cause of your problem as well.

[The Saint]

I thought the tighter coils are suppose to be facing forward.

[CDN_Stalker]

Originally Posted by Kerbosa

MS110SP is an irregular pitch spring i believe. Make sure the tighter coils are at the the spring guide end, as that could be the cause of your problem as well.

Was just gonna say, but he got to it before me. Expect an easy 30-40fps drop because of that (meaning, if you put it in with the tighter coils on the piston side, the fps will be a lot lower, same as if you put the port on the cylinder at the front end and not the back, double droppage if you did both backwards!!!!) so make sure you put it in tight coils at spring guide as he said. The rest of the drop................ not sure, I assume you are using 0.20g BBs to test it? Dumb to ask, but hey, at least it wipes one possible option for the drop in fps (seriously, some new-to-airsoft people expect the fps to stay the same at the muzzle no matter the BB weight, they just expect it to drop off faster per distance.)

[CDN_Stalker]

Originally Posted by The Saint

I thought the tighter coils are suppose to be facing forward.

Nope.

One of those things that causes different answers when posted, yet when numerous people back up tighter coils backwards, mods don't catch it and make a stickie of it, which they should. I opened Droc's AK47 the other night, he put in the Prometheus SP110 in backwards and had been experiencing 300fps out of his gun with 0.20g, one problem I picked up on after opening the mechbox is the backwards spring. I told him, he said he asked on ASC which way, most said it didn't matter when using non-linear springs. Nope, fps loss for putting it in backwards. His other problem was a busted clip that holds the inner barrel and the hop up together.

[ILLusion]

Another problem is:


... an MP5 barrel.

Simple as that.


Short barreled guns have diminishing gains for velocity.

Although your gun SHOULD be shooting a bit higher than your results, your estimates are way overestimated. You shouldn't shoot more than 360fps with even an M120 level spring installed, even with a completely tuned gearbox, systema head set, spring guide and tightbore inner barrel.

One thing you can try is to get a larger cylinder. The Angel's silent piston head takes up more volume than a Systema one.

Your results almost seem on par with your setup.

Want more velocity? Get a bigger gun. Simple as that. But you'd have to upgrade your gun pretty hard just to hit 400fps. Much more so than a longer barreled gun.

[Karhgath]

Thanks all.

I'll check the hop up and the air seal and put back the previous air nozzle and see if it makes any difference.

The spring is put correctly (tighter end at the spring guide), cylinder on the right side, .20 BBs.

Illusion, so if I put in a m4 barrel and cylinder (with a silencer over), that would help? Actually, that was my main concern, I was thinking of going that way anyway but wanted to take it step by step.

For now, is there any risk of using it at 320 fps with a ms120? I guess premature worn and all.

[ILLusion]

Yeah, that'd help. You'd actually get a considerable velocity boost. I've seen an AK beta spetz go from 360fps to 420fps with a barrel swap to an AK-47 length inner barrel.

That's a 60fps increase!

No real concern with your current setup... Just that your battery life won't be great and there's a slight chance you'll shoot over 400fps once you put the longer inner barrel in.

[CDN_Stalker]

My back up MP5 has a Prometheus 110 spring in it, it is shooting around 355fps pretty consistantly. And that with a stock barrel and no other fps enhancing upgrades. Funny thing is, I get about 3-4ft groups at 200ft using 0.28g BBs!

[Kerbosa]

Originally Posted by CDN_Stalker

My back up MP5 has a Prometheus 110 spring in it, it is shooting around 355fps pretty consistantly. And that with a stock barrel and no other fps enhancing upgrades. Funny thing is, I get about 3-4ft groups at 200ft using 0.28g BBs!

0.28s are the greatest thing since sliced bread!