For those of you that are interested in running external air/CO2 you'll need three things.
Gas operated replica
HPA/CO2 tank
air rig

REPLICAS

Gas operated replicas range from anything between 30 year old classics to a brand new Tokyo Marui pistol, GBB to NBB and the famous Escort internals and the Bullet Valve. Running external air to these types of replicas has some distinct advantages and some obvious drawbacks. For one, you'll be tethered to your replica with a hose. But, you'll have better performance.


TANKS

Tank selection is can be an easy choice. Most use CO2 since its cheap and very effective. Since its is a liquid you can hold a higher volume than most HPA tanks. The draw back is that its liquid and has to boil off before it enters the gun. CO2, in general, gives less consistent PSI and velocities. HPA, is just air under pressure and provides the best consistency although it comes at a much high price by volume. Someone looking to power their sniper rifle would consider HPA while most would option for CO2. HPA tanks range for $60 to >$300 while a common 20oz CO2 tank can be bought at Academy Sports for $30 and comes with a free CO2 fill.

Scuba and most Paintball shops offer HPA and CO2.


THE AIR RIG

There are so many ways to complete this setup but I am going to detail only one. Its simple and cost effective.

Palmer Pursuits female stabilizer. This screws onto your tank. (no, this will not fit a propane bottle)
http://www.palmer-pursuit.com/cart/index.php?main_page=product_info&cPath=75&products_id=824

1/8" NPT 0-200PSI liquid gauge
http://www.mcmaster.com/#atmospheric-pressure-gauges/=4cejg3

1/8" NPT 90 degree swivel elbow to 6mm tube
http://www.mcmaster.com/#5225k722/=4ceefm

6mm nylon tubing (this will be used in multiple places and you'll also need to cut lengths that are comfortable to you)
http://www.mcmaster.com/#5140k234/=4cepho

1/8" NPT to 6mm tube adapter
http://www.mcmaster.com/#5225k712/=4cef1g

1/8" x 1/8" NPT coupling
http://www.mcmaster.com/#4464k211/=4cei8y

Female QD to 1/8" NPT w/ shut-off valve
http://www.mcmaster.com/#9414k19/=4cegz9

Male QD to 1/8" NPT w/o shut-off valve
http://www.mcmaster.com/#9414k62/=4cehlq

1/8" x 1/8" NPT coupling
http://www.mcmaster.com/#4464k211/=4cei8y

1/8" NPT to 6mm tube adapter
http://www.mcmaster.com/#5225k712/=4cef1g

This is where you have some options. If your gun has a 6mm tube (almost all do, if they do) you would just push the male QD unit onto the tube and you're done. If you are planning to convert a green gas operated replica to external air you'll need to first tap the mag to, you guessed it, 1/8" NPT and install one more fitting. You can do this cheaper but its not buy much and in the end you'll really want a swivel type fitting so your threads and seal are not compromised by your hose moving around.

1/8" NPT 90 degree swivel elbow to 6mm tube
http://www.mcmaster.com/#5225k722/=4ceefm

Of course you'll need some Teflon tape, scissors and a pair of pliers. The fittings do not need to be @ss type, just a little bit more than "hand tight". If you want to use some thread lock on the last few threads, it would help keep the fittings tight. I would recommend hand breakable thread lock until you are sure you got it just right. You're also going to need oil to lube you Palmer reg and gun every once in a while. I use 10w silicone shock oil for RC car shocks absorbers. You can get this stuff at any RC hobby shop. Its $3-4 and a little goes a very long way. A few drops, if that, is all your every going to use at a time.

Once you have your air rig complete you need to start at low pressures and slowly increase the PSI until you are at the appropriate PSI/FPS. You need to know that in most classics more is not better and you could actually damage or destroy your replica by using too much PSI. 200PSI is more pressure than you'll ever need. I don't think I can even think of a stock classic that would need anymore than 120PSI although there are some that can use much higher pressure.

If this is for a sniper rifle it is common to use a mini reg to step the pressure down and control the PSI ever more. This increases the consistency of your air rig and inevitably your rifle's accuracy. This is also a convenient addition to your air rig that allows for easy PSI adjustments while at the chronograph.

1/4" NPT to 6mm tube adapter
http://www.mcmaster.com/#5225k713/=4cetnr

1/4" NPT 5-150PSI mini reg
http://www.mcmaster.com/#4956k11/=4cet5w

2" 0-160PSI guage
http://www.mcmaster.com/#3846k89/=4cew3h

1/4" NPT to 6mm tube adapter
http://www.mcmaster.com/#5225k713/=4cetnr

hope this helps,
mateba

Do you have any diagrams or pictures of this...

Also.. any suggestions on just changing out the existing fill-valve... you know the common one you see on every GBB Magazine although I am more interested in the WA M4 GBB Magazine specifically. Or is that what you are suggesting here. I assume its metric but it unscrews to roughly a 1/8" thread. changing this valve out with another one which has a QD attachment would seem the simplest appraach.

I don't think you should do that but if you try it, you should report your results. I would not entrust 100+PSI to just anything.

Well could you diagram how you are doing this? Where exactly are you inserting the new valve in the magazine?

I really don't have the time right now but here is a really nice youtube video on tapping. http://www.youtube.com/watch?v=p7wd2Lenmlo Remeber that 1/8" NPT is tappered so you'll need to remove the tap everyonce in a while to check your progrees and prevent over tapping it by making the threads straight.

hey i have a 40-60 cf nitrogen tank that im going to be using for at the field refills

im not sure how many small hpa tanks it will fill but im pretty sure it would be able to fill several ---------

most hpa tanks can use nitrogen ---so for those who need it i might be able to set up a decent cost for hpa refills at games away from proper facilities

im planning on getting a gas rig setup soon

I have a HPA fill yoke. If you are looking for HPA bottles. Mt. Doom has several in low pressure out put (450PSI) for $55. That is a better price than I have seen on most websites and they refill co2 and nitrogen. I believe its $1 per 1000PSI. Airgas in Pelham on Hwy 31 will too. Maybe cheap in the quantity you are looking at.