The launch head uses removable 21.5 mm PVC launch tubes, 
according to the lenght of the bottles being pressurized. 
the tubes slides pressure fit over the copper sleeve and against the "O'ring" 

The combination of the launch head and launch tube create a square groove for the "O'ring".
Sealing perfectly the bottle neck.
(In this picture the launch tube is not fully inserted yet. )

These launch tubes  allow for an easy water filling outside of the launcher. 
Once the bottle filled up i insert the tube in, and turn the bottle upside down... 
The fit is so tight the water stay inside. 
You just have to push down the lot over the copper sleeve(See note). 
Slide the neck over the "O"ring And clamp the handle. 
notice that the "O"ring is about 10 mm above the fitting.
this allows it to be about half way up inside the bottle neck.
 
By the way a deburring tool of sort (to taper the bottle neck a little) is a good idea to buy. 
Jaycar in australia still sell them for about 12 bucks and you use it a lot.

The copper sleeve was made before i knew how to design 
proper internal PVC sleeve for the 21.5 mm PVC tubes.(see the index for the "How To" )
On the other hand it won't wear as fast :-)
It is made of 70 mm of 1 inch copper tube.
Sliced lenghtwise with a hacksaw and then compressed and formed to its new size 
with vice and plastic hammer.(3 times over to get the right diameter)
Silver soldered at the seem 
Polished. to a perfect fit inside the launch tube.
Then finally glued in place in the fitting with PVC cement.