Here is a classic Swing Arm launcher as Described first by Scott Chesnut.
With a modified launch head using removable launch tubes, and internal "O"ring sealing system.
Quite similar to the Clark cable launcher head used by Gordon McDonough.

Note for newcomers: 
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This is not the cheapest option as far as launching is concerned.
There is quite a few designs around. Simpler, and cheaper as well.
If you want to find out see my page of links at the end of the general index.

for myself price wasn't a concern. i wanted something "ultra safe" for my kids.
with plenty of room to add and improve.
anyway if building it fancies you. Here is the "recipe" 

Supplies:
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The Frame is all 3/4 inch gal. Reduced to 1/2 inch for the accessories.
From left to right:
1 col: Air intake and pressure gauge to 180 PSI 
2 col:150 PSI Safety valve with manual release and ball valve for an extra 100 PSI safety valve. or accessories.
3 col: "One way valve" (reversed) ,4 inchs gal flange , stainless steel bolts, and 180 lbs swing arm. 
4 col: launch head

Note:
All the components necessary for the construction of the launcher
are available at a reasonable sized Hardware store carrying plumbing supplies.
Apart from 3 or 4 things:
1) the Pressure gauge : For good ones. Look in second hand shops or small antiques shop
   for cheap but still good ones.Scrounge one from a cheap air pump or mini compressor.)
   You may need to take it to a engineering shop 
   to have a proper fitting done due to the small unusual thread)

2) the safety release valve : you could do without i suppose.. 
   But being able to depressurize the bottle if something goes amiss is a plus.
   Now You may be able to find this part at your hardware store. 
   but a wholesale plumbing supplier is more likely to stock it.

3) The 21.5 mm (1/2 in) White PVC tubing: same as above.
 
4) the handle clamp: This one you will have to go to a engineering tools supplier to get it.
   as it is a specialized item not likely to be stocked anywhere else.
   See further down the slide show for an adress.