Massive loss in FPS

thatguy2695 · October 9th, 2007, 14:42

I have a Classic Army M4 and I have assorted internals but mainly i have a
m120 spring and it used to fire over 400 with .2 but last time I got it chronod it was 280 which is rediculous. is it a Seal problem or the spring?

HellRanger · October 9th, 2007, 14:44

Weak spring...
Did you leave your spring compressed the last time you fired it?

Quote:

Originally Posted by thatguy2695

I have a Classic Army M4 and I have assorted internals but mainly i have a
m120 spring and it used to fire over 400 with .2 but last time I got it chronod it was 280 which is rediculous. is it a Seal problem or the spring?

jerryliang2k · October 9th, 2007, 16:33

Maybe its the chrono...

Other than that, check to see the seals on your cylinder are nice and tight. Air leaks can cause massive drops in fps as well as inconsistency.

Other than that, it may be the spring.

Zi-Xiao

CDN_Stalker · October 9th, 2007, 16:46

Last year my MP5 SD chronied 387fps with 0.20g, this year at two different games it sat between 265fps and 300fps. I figured I didn't properly uncompress the spring (always end of the day fire it in semi a couple times, but now I've found I have to fire it in bursts and check the nozzle to make sure it's full forward to ensure the piston is forward as well). Anyways, I thought my spring had weakened (a PDI 150% that was a good one) since I've been using my more compact MP5 all year, and seems the spring was fine. My Hurricane reinforced mechbox on the other hand, was seriously cracked on both sides.

You might have a cracked mechbox, take off the upper receiver and look at the front of the mechbox to see.

If not, could just be your piston's O-ring has worn out or just needs some fresh grease applied.

It COULD also be your hop up unit, I troubleshot an AK earlier this year that had a large fps drop, I was baffled by it, then though to check the hop up rubber. I cycled it to make it sit with the nozzle full forward, shined a flashlight into the magwell and looked down the barrel to see a sliver of light coming through the bottom. I pulled out the hop up unit to find the plastic C clip had broken in two places. That C clip holds the barrel in the hop up unit, if that clip cracks the barrel will slip forward a little bit every time the gun is cycled, which will end up causing large fps loss.

Hope your problem is one of those, good luck!

kalnaren · October 9th, 2007, 17:18

Kindof a similar issue...Last time I chronied my G36 it was reading 205 with .25's on an MS100SP... seemed a little low to me. Brand new spring too.

ThunderCactus · October 9th, 2007, 18:59

could have installed the spring backwards, most likely it's an air loss. In some guns the hopup needs to be pushed backwards via a spring tightly against the mechbox, the hopup rubber could be torn resulting in loss of air, your air nozzle could be damaged, mechbox could be cracked, or it could be as simple as there being too much grease around your piston head!
If you take your gun apart, remove the cylinder assembly, remove the air nozzle from the cylinder head and plug the hole with your finger, then ram that piston into the cylinder. If it's doing its job, it should go about 3/4" and stop with no leaks, and as long as your keeping constant pressure the piston shouldn't move forward at all.

lupo · October 9th, 2007, 19:59

With such a large drop in fps i would also check the mech box for cracks first. Its an armalite so its easy , just look in the mag well for a crack or open the port cover. Next down the line would be a broken nozzle or cylinder.

thatguy2695 · October 9th, 2007, 20:07

thanks guys I checked for cracks and didn't see any. and I don't have the right alan key/wrench to open up the weapon so I haven't been able to check the piston. For the most part I always decompressed my spring I think that it is something with the Seal, alot of sand and crap where I used to go airsofting.

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October 9th, 2007, 14:42	#1
thatguy2695 Trader rating Join Date: Sep 2007	Massive loss in FPS I have a Classic Army M4 and I have assorted internals but mainly i have a m120 spring and it used to fire over 400 with .2 but last time I got it chronod it was 280 which is rediculous. is it a Seal problem or the spring?

October 9th, 2007, 16:33	#3
jerryliang2k Trader rating Join Date: Oct 2004 Location: Mississauga, ON	Maybe its the chrono... Other than that, check to see the seals on your cylinder are nice and tight. Air leaks can cause massive drops in fps as well as inconsistency. Other than that, it may be the spring. Zi-Xiao __________________ Primary - CA M249 - Systema PTW Sidearms - KWA G18c - WE 1911

October 9th, 2007, 16:46	#4
CDN_Stalker Official ASC Bladesmith Trader rating Join Date: Jul 2003 Location: Ottawa, Ont.	Last year my MP5 SD chronied 387fps with 0.20g, this year at two different games it sat between 265fps and 300fps. I figured I didn't properly uncompress the spring (always end of the day fire it in semi a couple times, but now I've found I have to fire it in bursts and check the nozzle to make sure it's full forward to ensure the piston is forward as well). Anyways, I thought my spring had weakened (a PDI 150% that was a good one) since I've been using my more compact MP5 all year, and seems the spring was fine. My Hurricane reinforced mechbox on the other hand, was seriously cracked on both sides. You might have a cracked mechbox, take off the upper receiver and look at the front of the mechbox to see. If not, could just be your piston's O-ring has worn out or just needs some fresh grease applied. It COULD also be your hop up unit, I troubleshot an AK earlier this year that had a large fps drop, I was baffled by it, then though to check the hop up rubber. I cycled it to make it sit with the nozzle full forward, shined a flashlight into the magwell and looked down the barrel to see a sliver of light coming through the bottom. I pulled out the hop up unit to find the plastic C clip had broken in two places. That C clip holds the barrel in the hop up unit, if that clip cracks the barrel will slip forward a little bit every time the gun is cycled, which will end up causing large fps loss. Hope your problem is one of those, good luck! __________________ STKR Knives http://www.facebook.com/#!/pages/STK...17036668380871

October 9th, 2007, 18:59	#6
ThunderCactus Not Eye Safe, Pretty Boy Maximus on the field take his picture! Trader rating Join Date: Feb 2007	could have installed the spring backwards, most likely it's an air loss. In some guns the hopup needs to be pushed backwards via a spring tightly against the mechbox, the hopup rubber could be torn resulting in loss of air, your air nozzle could be damaged, mechbox could be cracked, or it could be as simple as there being too much grease around your piston head! If you take your gun apart, remove the cylinder assembly, remove the air nozzle from the cylinder head and plug the hole with your finger, then ram that piston into the cylinder. If it's doing its job, it should go about 3/4" and stop with no leaks, and as long as your keeping constant pressure the piston shouldn't move forward at all.

October 9th, 2007, 19:59	#7
lupo Trader rating Join Date: Dec 2001 Location: vancouver	With such a large drop in fps i would also check the mech box for cracks first. Its an armalite so its easy , just look in the mag well for a crack or open the port cover. Next down the line would be a broken nozzle or cylinder.

October 9th, 2007, 20:07	#8
thatguy2695 Trader rating Join Date: Sep 2007	thanks guys I checked for cracks and didn't see any. and I don't have the right alan key/wrench to open up the weapon so I haven't been able to check the piston. For the most part I always decompressed my spring I think that it is something with the Seal, alot of sand and crap where I used to go airsofting.

Massive loss in FPS

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