I need help!! Cqb m4 build.

[Mr. Fox]

Hey guys. I'm working on a cqb gun at the moment and need a little help.
I bought a JG m4 stubby (cqb full stock, stock jg plastic body, and a 4" metal ris with the barrel just barely coming out from where the rails end) from a friend very cheap. The barrel is so short that it could probably fit in a 5.1 1911.
I opened the gearbox, corrected the angle of engagement, and re-shimed the gears just to help the lifespan of the gearbox. Because of correcting the angle of engagement, my fps dropped by about 50. It was shooting 300 and now its shooting around 250-260.
My local airsoft tech, who is also a friend of mine, told me that the only way to increase the fps is by getting a longer barrel. Is this true? I would like to keep the gun as short as possible, just so it is easier to round corners, etc. Plus, I really don't want to sink more money into parts for this gun since I have already had to put some cash out to fix a few flaws.
If I don't need a longer barrel, what parts should I upgrade to increase the fps? I have already re-shimmed the gears, corrected the angle of engagement, tried to increase compression by putting Teflon tape around the cylinder head by the o-ring, and greased everything up really nicely.
Thanks in advance,
Mr. Fox.

[Mr. Fox]

And by "the only way to increase the fps is to lengthen the barrel," I mean that he said that any parts that I upgraded would be useless without having a longer barrel.

[kagemusha]

What he said is somewhat incorrect. When you corrected your AOE, you altered the volume of air available in the cylinder that piston can push. Adding a longer barrel is not going to help your cause. You may also want to check your compression areas to make sure there isn't a leak somewhere. Also, did you correct AOE by using a spacer in the piston or did you add sorbo to the cylinder head? Just curious, either way you have less air in the cylinder before the modification was performed. I'd say the only way to increase your fps is to throw in a stronger spring. More energy translates to more fps. Anyone more knowledgeable please correct me.

[Mr. Fox]

[Lancer]

If you add the spacer to the piston instead of your cylinder head you will gain your FPS back.

Before someone tells me you lose cylinder volume either way, you are wrong.

If the cylinder is ported at all (which yours most certainly is), then the volume of the cylinder that is actually being used to compress the air will not change.

Also, you should not need teflon tape to achieve a perfect seal between your cylinder head and cylinder, I would recommend lubing it better and/or stretching the o ring for a better seal.
Another good thing to upgrade is your air nozzle to one that has an O ring. If you do this then you should have perfect compression in your entire cylinder assembly pretty easily and that will account for a good increase in FPS/consistency.

Finally, you can always increase FPS by adding a bigger spring, but it won't be a very efficient upgrade without a longer barrel to give the BB more space to accelerate.

If you want 375 fps with your barrel you will need an M120 spring and a good air seal.

Hope this helps.

[Mr. Fox]

[Lancer]

Anything that is lightweight, probably some sort of plastic material. You could even probably just use plastic from packaging, the thin stuff, and cut it in a circle with a hole in the middle (using a hole punch) and then you could very precisely shim your piston to the correct AOE.

[Mr. Fox]

[Lancer]

Sure, anything lightweight is good.

[Mr. Fox]

[kagemusha]

Lancer makes a good point about the ports in the cylinder, when I wrote that either way you lose volume I was thinking of a full cylinder set up. You should really check your seal between your air nozzle and your cylinder head as well as making sure that your air nozzle is making contact with the lips of the hop-up rubber. The latter can be a frustrating issue if you keep having compression issues. I would keep the sorbo over the spacer set up since it relieve some of the stress at the front of the gearbox and version 2 gearboxes are known to crack at that area. In addition, your gun may be quieter because of the shock absorption properties of the sorbo. Just my 2 cents and good luck!

[Mr. Fox]

[Smartkid312]

Yea the sorbo shouldn't have caused that you usually just get a minimal 5-10 fps drop so a compression leak. Also the 120 spring will be shooting way over 350 unless you shortstroke and also I would reccomend a mosfet without active breaking such as a HS5 style or a DE style mosfet, and a shs 7 tooth steel tooth piston and use shs high torque its a good motor for a cheap price.

[Mr. Fox]

[mtmlegit]

m120 may be a little too hot for your cause, id try a 110 or a cut SAW spring, there is something special about a saw spring i've heard. may be stuff but ya know, lookin into wouldnt hurt.