FPS
 

So, my M4 was shooting about 275 FPS with a 0.2g bb with a SHS M110 installed. I took it apart to install a Guarder M120 spring and stretched the o-ring. It is now shooting an AVG of 316 FPS. Keep in mind, my gun is short stroked 2 teeth, but so is my PDW and it is shooting 325 FPS w/ 0.2g bb and a Prometheus M110. So my question is, is why are my 2 guns shooting different FPS and the weaker spring is shooting faster? Air leak somewhere on the M4? Bad piston O ring? The cylinder head is teflon taped and compression check before putting together was great. Just figured I'd ask as more knowledge is great. Thanks!

When you say o-ring did you mean the piston head o-ring? I had a similar problem on my SCAR and replacing that made my FPS hit 360 with a 0.2g bb on a M100. I tried stretching it at one point, but that only lasted for a week and then it was back to garbage compression. I went with the Blackblitz X-Ring, works wonders for me.

My current M4 is running full SHS compression parts with a Blackblitz X-Ring, SHS 12:1 gearset, Modify M100, and gives me 370 +/- 2 FPS on a 0.2g bb. My setup isn't short-stroked though so that may make a difference (I'm not sure about this)

I'd suggest a full run down on every connection point in your compression parts to figure it out. Do it before you try to fix anything.

I have a port on my cylinder, may be this... However, I am unsure if the piston goes back that far to actually USE the ports. Going to try this possibly if it may be it, but I ordered 100 piston head o rings off amazon for $18. Will rule these two out if it changes nothing.

each tooth will take ROUGHLY 10-15fps off.
M110 is generally 380 under ideal conditions, so you should be in the 350fps range.

Things that make a difference:
-piston head bearing
-spring guide bearing
-barrel bore
-whether the hop is on or off
-type of hop
-quality of BBs
-air nozzle O-ring
-overall compression

Do a compression test, make sure there's no air leaking out of the cylinder under compression.

Post says I did a compression test prior to putting back together, with nozzle on, and it did wonderfully. I do have a piston head bearing and bearing spring guide. I have a flat hop and it was on. BB's used were 0.2g Gryphon bb's. Barrel bore is 6.05 I think? Otherwise I'm unsure, like I said could be ported cylinder, but it is only a 3/4 port.

as long as your cylinder size is compatible size to your inner barrel, port location won't be the issues. a few things you can check for a good air compression:
1. check the nozzle air seal while moving it
2. check the length of your air nozzle. if it is too short, it may not be making a good seal. check air coming out of mag part of hop up while you shoot.
3. check your hop up, it would be the next point of leak, specifically the rubber

I went from M100 to M110 and my fps went up from 300 to 340.

Might wanna try chronoing on heavier BBs if you have a shorter barrel as well.
Short barrels tend to joule creep more.

So it seems when doing the paper test on both of my guns, the paper instantly flies off. Anyone know where the problem could probably be traced to? Like I said, compression is great before putting gearbox back together. M4 has SHS rotary hop up and my PDW has a ProWin rotary hop up.

Many things are at play here. The cylinder you’re using in correspondence to your inner barrel. The nozzle length. Your bucking in the Hop Up.

So, the bucking is as far forward in the Hop as it can get, is a G&G green flat hopped. Cylinder is a 3/4 ported with a 275mm inner barrel. Nozzle is original aluminum o ring nozzle that came with the gun, so assume it is correct length.

Different guns will shoot different FPS with the exact same spring.

You tried swapping back to the original piston or tried another piston?

You can over compress too. I did it in my EBR. I had a double o-ring piston head and new piston and for the life of me couldn’t figure out why I was shooting way less than it should have. Until I swapped the piston and piston head. I had serious compression but it was too much for it to handle. I’m guessing most of the air was blowing out the Hopup into the magazine. Almost like I was getting back pressure.

Don’t have another piston/piston head. Figured if a lot of builds use SHS 14 Tooth Swiss cheesed pistons and I use an aluminum piston head, then it should be fine.

Understandable, but as to why tons of air is blowing out Hop Up feeding tube seems to be an issue with an air leak somewhere.

The nozzle isn’t sealing quite right with the hopup rubber lips if it’s blowing lots of air out of the feed tube.

You wrote: “So, the bucking is as far forward in the Hop as it can get, is a G&G green flat hopped.”


As far forward as in towards gearbox or towards muzzle?