Weird problem : VFC M4, increasing FPS

[Splashx]

I have a pretty wierd problem (I never heard of and never encountered before).

Stock VFC M4 aeg, 2 years old, apart a madbull red bucking. Was shooting 390fps w/ 0.20g last month. Yesterday the chrono was :
(hop-up at zero, 0.20g bbs)
220
235
236
310
345
346
350
350

and around 350 every other shots. 5 min later I tried out again, with the same result! Same as this morning. always increasing slowly from 220 up to 350fps. Very poor range and accuracy (below 100ft and 5ft accuracy at 100ft).

I changed the bucking to a 75deg maple leaf, madbull blue and Lonex hard, without any improvement.
I changed the barrel with a madbull python 6.03mm, again no change.
Nice air seal inside the gearbox (piston/piston head/nozzle). No broken parts inside (gears/piston/tappet...)
The hop-up is looking brand new, tried teflon tape mod, orings added in front of the hop-up to thight it to the gearbox, piston head oring enlargment mod... without any luck.

needing a hand please

[Stealth]

Are you using a fully-loaded or near fully-loaded mag at the start of your test?

[Splashx]

both full and half full, tried king arms plastic mags, metal ACM and magpull pmags

I tought too much bbs would force the spring follower of the mags pushing them too far inside the hop-up, but no change

[Trev140_0]

I have seen a situation where the piston head is the culptrate.

The o ring binding in the slot of the piston and not fully expanding properly for one reason or another.

Do you have another piston to plop in to swap?

.

[Splashx]

Originally Posted by Trev140_0

I have seen a situation where the piston head is the culptrate.

The o ring binding in the slot of the piston and not fully expanding properly for one reason or another.

Do you have another piston to plop in to swap?

.

I will try it out right now. got few in hand

[Trev140_0]

Its messed up, but I saw this before. I call it the "wake up"

Really was pronounced when I played at flag raiders in minus a bazillion.

(don't know what that is in French, but fucking cold)


Anyway, I traced it back to piston head. Not sure if there was a hair line crack it the head, but as soon as I swapped it, was out of that problem

Is your piston head plastic?

Also, get a new o ring on that as well.

[Splashx]

Originally Posted by Trev140_0

Its messed up, but I saw this before. I call it the "wake up"

Really was pronounced when I played at flag raiders in minus a bazillion.

(don't know what that is in French, but fucking cold)


Anyway, I traced it back to piston head. Not sure if there was a hair line crack it the head, but as soon as I swapped it, was out of that problem

Is your piston head plastic?

Also, get a new o ring on that as well.

You were absolutely RIGHT

with a crappy rubber, I get 370-375 pretty consistently!!!

you made my entire week lol thanks

a lonex POM piston head is in place and doin' the job just right

but the old piston head is still looking fine, no cracks or anything, so is the O-ring.

[Trev140_0]

:-)

You over stretch the o-ring it will work against you... there is a fine line. Seen the o ring on the outside of the cylinder body with a lighter. But wft, seriously? These are 40 cents at the hardware store. Buy 20 of them and be done with it.

Just make sure you toss your old stuff in the garbage. Don't be a pack rat.


Over and ---out.


________

End

[SuperHog]

Check the inside edges of the port on the cylinder. Mine had a sharp edge which would catch the oring as it begins to expand to seal the head for the compression.

[Splashx]

Originally Posted by SuperHog

Check the inside edges of the port on the cylinder. Mine had a sharp edge which would catch the oring as it begins to expand to seal the head for the compression.

I swapped the cylinder just to be sure

[Trev140_0]

The overlooked thing of the gun is the ability for the o ring to expand and seal rapidly.

• The longer it takes the least the compression
• The variance time it takes, the wondering FPS
• Not at all no FPS

Remember this one to the rest of the dudes. This it the second time I saw this now. I thought mine was a total freak incident.

As soon as he posted an increased FPS, (and he was not outside in -30) we were going down the same path.

So weird, but clad it worked out for you.

PS, the other one I saw while we are in the topic. OVER GREASE. Yeeep. Too much grease, (badbull piston grease clear) is not a good thing. I over did it once and lost all compression. I opened the gun back up, and cleaned a lot out. Boom. Problem solved. Again, the o ring was being restricted to properly seat during cycling.