TM MP5 - Low velocity - FIXED

[Avenging Angel]

Hey guys, I need some help with a problem I'm having but first, here's the stats of the gun.

TM MP5A4 HG
M120 Spring
Metal bushings
Poly Carb Piston
Poly Carb Piston head
Metal Spring Guide

The problem I'm having is that the gun is only shooting at 150fps with the current modifications. Before the upgrades were installed, the gun was shooting 285fps on average.

I can't figure out what the problem might be. I've replaced the O-ring on the piston head, I've tried 2 other springs (an M120 and a stock one) with the same effect of 150fps. The nozzle is also fine.

I guess what I'm asking is, what's the next step? What else do I need to try to get this thing working again?

[brently0725]

Did you change the piston head seal at all?
Also the cylinder might be to big for your new piston (had it happen with a hurricane kit I bought)
Did you check to see if your hop up rubber is not folded or creased.

I love fixing my own stuff like most but sometimes after trying for a while you might want to seek the help of a proffesional before doing something you regret and make a $500 paperweight.

LOL GOOD LUCK MAN!!

[mcguyver]

Is the cylinder in the correct way, with the port towards the rear of the mechbox?

[Avenging Angel]

Hop-Up is in working condition and the O-Ring on the piston head is making what appears to be a good seal with the cylinder walls.

I'm fairly familiar with the internals of the MP5 series as I have owned 5 of them through out my career. Also, just for information more than anything, I have taken it to a gun doctor and he has no idea what's going on either. Everything "appears" to be fine.

[Avenging Angel]

Originally Posted by mcguyver

Is the cylinder in the correct way, with the port towards the rear of the mechbox?

I'll have to check on this... I do believe that the port is currently near the front of the box. I recieved it back that way after I sent the gun out to have the upgrades done.

[mcguyver]

If it's to the front then it's wrong. You lose about 2/3 of your available cylinder volume and alot of FPS as well.

[Prowler]

What I do is I take the cylinder, piston, air nozzle system out. Then I take my finger and plug the airnozzle so no air can escape. If you push the piston forward and there is no air escaping then its a good seal. You can use this method to determine if your piston is pushing enough air. Try different levels of force to the piston too.

If your getting low fps i would also look at the air seal in the hopup. Otherwise it might be the spring. *shrug* Can't really tell without looking at it myself.

Hope that helps

[Avenging Angel]

Originally Posted by mcguyver

If it's to the front then it's wrong. You lose about 2/3 of your available cylinder volume and alot of FPS as well.

Thanks man! That was the problem! The gun is now shooting a healthy 375.

[Avenging Angel]

Originally Posted by Prowler

What I do is I take the cylinder, piston, air nozzle system out. Then I take my finger and plug the airnozzle so no air can escape. If you push the piston forward and there is no air escaping then its a good seal. You can use this method to determine if your piston is pushing enough air. Try different levels of force to the piston too.

If your getting low fps i would also look at the air seal in the hopup. Otherwise it might be the spring. *shrug* Can't really tell without looking at it myself.

Hope that helps

Thanks for that bit of information as well! I'll store that away in my brain for later reference.