Lower than expected FPS: King Arms M4
 

Hey I recently upgraded my KA M4 with these parts:

Modify Enhanced Cylinder (300mm-450mm)
Modify Air Seal Nozzle - M4/M16A2
ARS Hayabusa Bearing Spring Guide - V2
MASK Aluminum Piston Head with Bearing
Modify S110 AEG Upgrade Spring

To my surprise I'm only getting 350fps with a .20g bb which is lower than the 410-ish fps I was hoping to get. Airseals are perfect and my cylinder is matched with my barrel length. I just don't understand what's causing this. Any insight as to how this could be caused would be greatly appreciated.

350 doesn't seem that low for a 110... you probably have a little bit of a leak somewhere in your hop up unit, but that's not crazily low.

Oh haha. I thought it'd be higher because on the website I bought from they were advertising 410fps with that spring.

That is actually low for a modify 110 spring....

I got ~417 FPS out of a Modify S110+ (note that chrono could have been reading about 10 high that day) but after break in it went down to about 394 FPS (used different chrono but same brand/model (Xcortech X3200)).

That's low for S110. You should get atleast 400-420 fps. If you set-up your hop-up properly. S110 never go down to 400 even though you used it after 3000 rd. you need to dial up ur hop-up properly


03ghost79

Your spring is in backwards?

^ +1

I installed a mask piston head in my gun and experienced a similar result. I found the piston head to be too big, it cycled fine but I experienced a drop of 50fps or so. Try swapping out the piston head or the o-ring with the KA one and see if it makes a difference. IMO a modify piston head would work well in your modify cylinder.

Your air seal cannot be perfect if you're only getting 350 on the S110+ spring. You should review how to test each part of the system. My best guesses are either that your nozzle doesn't seal properly against the bucking or -- if you truly feel you've tested everything else and it seems perfectly sealed --you have a volume mismatch between the cylinder and barrel.

Hope that helps to generate some ideas...

Hmmm. I didn't know that springs could be installed backwards! The air seal in my gearbox is good enough that I can clog the nozzle with my finger and try to push the piston through the cylinder without it moving at all once it passes the port window. Anyways, I'll try both of your ideas.

Yeah... make sure the busy end is facing the rear.

I thought I upgraded my Aug when I changed the stock nozzle and put in an upgrade aluminum air seal nozzle and lost a hundred FPS so weird. Try a different nozzle and see if that helps.

I have actually experimentally tested the backwards-forwards non-linear spring theory and have found there to be no appreciable difference in compression from reversing a non-linear spring.

I've heard this opinion echoed elsewhere, along with the theory that most people who have noticed this "effect" in actuality fixed some other compression issue while flipping the polarity of the spring.

I'm willing to accept experimental evidence that contradicts my findings if there's some other explanation (relating to perhaps porting or AoE or something), but for the moment, the reversed non-linear spring theory holds absolutely no water for me.

To nit-pick...and only doing so since you seem to fiddle with a lot of guns...there shouldn't be any difference in compression but rather a difference in resultant FPS of the BB. I know that's what you meant though.

Interesting. What was your test setup, with what power springs? What differences did you get with what springs?

I've definitely seen the results of flipping non-linear springs. What I observed was that the difference will depend on the barrel length and power of the spring primarily. Also the types/makes of springs mattered too. Some have dramatic differences in the spacing/grouping of the coils and others were pretty marginal. Non-linear ones with packs of coils at either end behaved a lot like linear springs so far as flipping them around. Thinking back I definitely notice a very wide range of differences...from as little as "might be just a different airseal this assembly" to 40fps.

(One AKS74U...tiny inner barrel...had some pretty dramatic differences)

I haven't tested this stuff for stats in a long while...just hasn't been worth it.

IIRC...the Modify S110+ has quite a tight pack of coils on it

My test setup was a V3 (G36) with three springs: a stock JG, a Modify S90+ and a Modify S110+.

I'm kinda wishy washy on this one. I don't believe in the reversal effect, but I haven't done enough tests to really convince myself that it isn't happening to other people.

I guess the central question is.. Do non-linear springs really really "know" which way they're pointing? I've always imagined a non-linear spring as a function g(F), where F is the force applied, with no other inputs necessary for the spring to work. When you release the spring at 1/5th the length, does it care which side the force was applied from?

If I were to test this outside of an AEG, I'd build a setup with spring guides on both ends of the spring, with each spring guide able to travel down a long rail once it was unlocked. Then I'd lock the spring guides into place and apply the force from one side, lock the guide, then unlock the same guide, and measure the results. After a few of those, I'd try compressing it from the same side, but unlocking it from the other side, and measure that. Perhaps this wouldn't accurately represent what's going on in our gearboxes, though.

I am pretty lazy when it comes to testing this more rigorously though.. :)

My understanding is this...

As a spring expands it has an energy curve associated to it. Tempering of metal, coil diameter, coil spacing, wire diameter, etc...will all affect that curve.

For an AEG...IMO...springs would serve the system best if they could accelerate as fast as possible and hit it's peak power just as the piston head reaches the cylinder head. The tighter pack of coils is supposed to assist with doing that.

As a spring is compressed going into the mechbox...some quite a bit, others less...the tail end of the power curve isn't as important.

There was a really good article about it somewhere (not on this site) where someone actually graphed the power output of some springs. Can't remember where though.

That cylinder is not mismatched for that length of barrel....but he could have the cylinder in backwards too :) (although the FPS should be much, much lower than what he was getting).

Also..he could be not shooting straight through the chronograph, which would give a false reading (usually higher though)...some guys seem incapable of lining things up
...he could have his muzzle too far away from the chronograph (which would give a lower reading)...one of the easiest ways to cheat a chrony is to just stick a suppressor on your gun
...he could have his hopup cranked on a lot, which shaves off tons of FPS

There's lots of reasons...and too little information to do much more than just toss quick guesses at it :)