If your gun is shooting too hot and is over the field limit, then you need to install a weaker spring, not increase the bb weight.

I bought a m120 but I'm going to run .25 or .28 and that was the question i was asking. I want my bbs to get good accuracy and still be at 400 fps so which bb weight would lower me to just under 400fps with a m120 spring. But, since i got a new airseal nozzle and a tight bore barrel will i need to use .30s to get under 400?

I still don't understand what you are asking.  

It seems to me that you are asking what weight of bbs will bring you under 400 fps if your gun shoots too hot.  Is that what you are asking?  Or are you just asking what weight of bbs you should use?  If so, I already answered that.  

If your gun shoots 400 fps with .2s, then .25s will bring it down to the 350-360 fps area.  .28s will bring it down to about 330-340 fps.  

.25s typically lower the fps about 40-50 fps.  .28s will lower the fps about 60-70 fps.

I would suggest just getting an Enhanced JG M16 since they are cheaper, but they are out of stock.  (unless of course you like the Stag Arms trades)  

Air leaks between the cylinder and cylinder head can usually be fixed by teflon tape.  I have a JG cylinder head that seals perfectly with an SRC type 0 cylinder once I added teflon tape.

Ehobbyasia.com is great for buying parts.  (just for future reference)

E1s usually come with Madbull blue buckings, so there really isn't a reason to swap it out.  

It depends on the model, but some E1s chrono at like 430, while others are in the 350 fps area.  I would say 350 is probably the minimum for their armalite models.    

I would imagine you could get close to 380 fps out of the stock spring if you fix the compression.  (if the come with an M110)  

I would say keep the stock spring (unless it shoots over 400 fps).  

Systema and Guarder air nozzles seal with well with JG cylinder heads in my experience.  Modify is similar to Guarder (they might even have the same OEM), but I haven't used one.  If you want to replace the cylinder head too, (I don't really see a reason too) then make sure it is the same brand as the air nozzle.  Mixing and matching brands for compression parts isn't always a good idea.  (as tolerances are different)  

Adding a sorbo  pad is a good idea since it will help correct the AOE and can prevent the shell from cracking.  

Spring guide should be fine.  When you get it, check to see if the shaft of the spring guide can be unscrewed.  If it can, I suggest adding super glue to the threads, then screw the shaft back on.  The shaft on my guarder spring guide almost unscrewed all the way one time.  Had I not caught it in time, it could have caused a lot of damage.

I used to be an SCS/Systema bucking fanboy.  However after some experimenting, I found the Madbull X/Fishbone nubs to be better.  

Because of the SCS design and material, it isn't very forgiving, so you get a lot of vertical deviation (because it's made of a hard material) and if it isn't sitting perfectly level, it screws up accuracy big time.  

The SCS would probably work a lot better if the mound inside the bucking was removed.  The SCS is good, but it's a bit too finicky in my experience.  It can be difficult to get it to work right and when it does, it's probably no better than a KWA 2G, PDI W hold bucking, or another "fin" type bucking.

Bioshotbb.com

Most accurate bbs I've used out of KSC .25s, TSD .25s, TSD .26s, Goldenball .2s, Goldenball .25s, BB King .25 bios, and G&G .25s.  

Sometimes they have feeding issues with midcaps though.  Mine feed fairly well (not perfect, but not terrible), but others cannot get them to feed in their mids.  

My next choice of bbs would be G&G or BB King.  BB King bbs have a lawsuit against their name, by B.B. King's (musician) lawyers (lol seriously?) so they might not be available soon.  Their quality has supposedly gone down as well.  G&Gs are pretty solid though.

you don't need  a new cylinder per say as long as you are using a ported cylinder that is near 1/8 of an inch close to a m4 cylinder

I meant to say type 1.  MP5s typically come with Type 2s, so you might see a decrease in fps if you add 363mm+ barrel.  

Either way, really no need to increase the barrel length.

Aren't the externals now nylon fiber?  

I'm really considering getting one.  I really want to get an M4, but the G36 series is so solid.  Only thing I don't like is that I won't be able to use my battle axe magwell converter since the new G36C comes with a 1 piece hop up.

Is the air nozzle moving back and forth when the gun is fired?  If not, the tappet plate spring maybe broken or disconnected.  (or the tappet plate itself is broken)