Unexpectedly high velocities from "basic" upgrade

[m102404]

WARNING: This is probably going to be wordy, but it'll be as clear as I can make it.

Q - With a stock TM M4A1 New Type, upgraded with only a Systema M100 SS IP spring and Oiless Metal Bushings, I have chronographed my M4 at a fairly consistent 384 fps, which is a good 20 fps more than I expected referencing the different charts, articles and airsoft forums.
1. Has anyone else, who have actually chronographed their upgraded M4's, had similar results? if not, with the same upgrades, what did you get?
2. What may be the cause?
3. Has anyone else seen such fps differences between chronographs?
4. If my "basic" upgrade is unusually high, am I headed for mechbox problems if I don't upgrade anything else?

OR

Is it more likely that the spring was mislabled and is really a Systema M120 Stainless Steel Irregular Pitch (which, after a quick check, should be in the 390 fps range)? If so, is there any way to check what spring it really is?

Background Info:
I've read many posts re. upgrades and refered to a number of sites/charts etc... to settle on a reliable but decent power (i.e. fps) upgrade. Probably too much reading/research, because all the facts/info starts to get jumbled after a while. I wanted to get it into the low to mid-300 range. My stock TM was only chronied once (within first 1000 bbs), where it was running approx 284-286fps.

My stock TM M4A1 New Type has approx 5000-6000 bbs through it. Probably a 60/40 split of full auto/single fire. All internals were stock until a couple of days ago. I've only opened the body (not the mechbox) twice previously. The first time just to check it out and the second because I wiped out in sand and wanted to make sure everything was still clean inside.

Two days ago I installed a Systema M100 stainless steel irregular pitch spring and Systema Oiless Metal Bushings. I had previously read a number of how-to guides and watched the videos from www.mechbox.com. By far the most helpful resources were the posts from this board and the videos from Mechbox.

I didn't run into problems at all throughout the removal of the mechbox, nor did I have any issues come up when I replaced the stock spring and bushings. Replaying the mechbox.com video a couple of times while I had the mechbox open helped a lot. I took a thorough look at the shims, springs, etc... while I had it open, though I didn't change anything else. The actual upgrading part probably took about an hour, from full disassembly to full reassembly.

The stock M4 was originally chronied at a local event with one of those small hand-held chronies that you stick the actual barrel into and then fire a couple of rounds (sorry, didn't know the brand). Yesterday, firing over a Chrony Chronograph (i.e. old fashioned "red" chrony), the M4 is now running 384 fps. It was about 390 fps just after the upgrade and settled down to 384 fps +/- 6 to 8 fps. That result is 20 fps more than expected vs. the most common spring guides/charts/website references.

At any rate the upgraded M4 seems really solid. I've put about 300 more bbs through it (reversed ratio of 60/40 of single fire/full auto). Just wondering if Illusion's defacto standard reference chart may be off a bit. Brian's work seems to be solid and I spoke with him a while back re. velocity differences between a P90 and a M4. Anyways, in the 360-380 fps range, 20 fps is only a 5% difference.

[Skruface]

If you are using a stock TM piston and piston head combo with the piston's spacer/washer/nut thing still intact and holding on to the piston head, it will compress the spring a little more (like a bearing piston head) and give you higher velocity.

[m102404]

Thanks Skruface,

Should have written a shorter post...didn't think of something that obvious. I did retain the spring/nut thing in the piston.

Any thoughts on if there are any reliability issues from the slight increase over the basic M100 upgrade?

[Gryphon]

Stock TM parts aren't meant to go that far, power-wise. At 380+ fps you're getting into FTK territory, especially with a stock TM V2 gearbox which is prone to breakage.

Systema has been having a lot of trouble lately mislabelling their springs. The most likely case here is you did indeed receive an M120.

[m102404]

Just a quick followup to this.

I opened things up again but didn't change anything. It's now shooting 374fps, which is only a little higher than the spring chart.

Also, I was able to chrony everything at TTAC3 last friday and both of my aegs and my gbb were consistently reading 15-20fps slower on Brian's chronograph vs. my own at home. The readings on Brian's chonograph puts the upgrade just about bang on Illusion's spring comparison chart.

[Gryphon]

Bear in mind ILLusion's spring chart does NOT accurately reflect actual spring velocities because the test gun had several bearing-equipped upgrade parts and a tightbore, all of which together will increase velocity at least 20 to 30 fps. Raw velocities will be lower than what the chart says.

[ILLusion]

I've found that generally, basic spring & bushing upgrades output velocities not too far off from my spring chart. The gun platform you use, however, will have a much bigger impact on your velocity outputs.

I'm just surprised m102404 was able to get velocities so high with an M100, but airsoft upgrade precision isn't down to a rocket science yet. The tolerances can wildly vary and so many variables can affect final output. Even something like... if the oil tempering of your spring was off by 1 degree, it could affect how it performs compared to the ones I tested with.

[morsredigo]

you think THATS unexpected?
my G3 has a PDI 120, bushings, shims, cylinder, cylinder head, piston
has a shortened TM stock barrel approx 400mm long
shoots 45fps above the PDI 120 on the chart on .2s (avg 370 fps)
im guessing they accidentally packaged a PDI 130

[m102404]

I had previously spoken with Brian re. what differences to expect for just spring/bushing upgrades between his test P90 and my stock M4. I would not have been surprised if my velocities were lower than his, but when they were much higher (almost into the M120 range) it peaked my interest. My understanding of the spring chart was that it would provide a spring to spring comparison, with a rough gauge on the resultant velocity differences.

I was even surprised to see a 10fps-ish drop (384 to 374, roughly 3%) from my first upgrade to just opening the mechbox a second time, taking out the spring and putting it back together. The only variations that I can think of are the exact position of the half gear (don't have enough experience to know how this may affect the draw of the piston) and the amount of lube (when I wiped off the o-ring on the piston and relubed it and if anything I did put a bit more than the first time).

I suppose that Brian's last post was right on. Very minor variations (tempering, lube, o-ring condition, etc...) with such low velocities will invariably result in significant FPS variations between AEGs. It is what it is...

[bruce]

I've notice that Systema has alot of stainless steel springs now. A few years back, they had the M120 spring in oil tempered and stainless which was labeled as the M120S. They had a difference of 30-40fps. Not sure if Systema now uses stainless for all their springs. Recent experience with a M130 gave a reading of 480fps in my M14.

[ILLusion]

I've noticed the Systema springs M120 and weaker still use oil tempering.