[IKE]

Try using shaving cream as a lubricant. It should help improve your sealing as well.

The longer barrel should also help a lot.

Valve. Is there a reason it must be remotely operated? you could get a much larger pipe valve and a heafty spring. Have a safety lock on this mechanism.

Sabot- Use wadding. It has worked for centuries for cannons and muzzle loaders.

To get an idea of travel here is a little math.

E=F*D
E=1/2MV^2
D=Muzzle length
F=Avg. Pressure times area
(if your chamber volume is large relative to muzzle volume and your valve is not terribly restrictive, you can assume P= reservoir pressure)
F=90psi*7(apporx A of 3" pipe)=630lbs
E=15120 in*lbs
E=1700 J (used converter)
Assume a mass of .5kG (you should wiegh this)
V=82m/s
For a 45 degree launch (assume ideal physics for a minute)
Vx=Vy=58m/s
It will reach apogee when Vy=58-9.8t=0
therefore t=6 seconds
To calculate distance X=Vx*2t=696m

Translation is you are seeing nowhere near this performance. How to diagnose your losses.
First off weight the balloon. Re-do calcs with appropriate weight.
Second, time the ballon as well as measuring distance. From this you can approximate the lateral velocity and can then back calculate your muzzle velocity.
Looking at the volume ratios you have more muzzle volume than chamber volume this means P goes from 90 to about 50. This will give an average pressure of 70psi.

To give you an idea of how sensitive distance is to these numbers let's change the weight to 1kg, Pavg=70,

this give an E=12000 in*lbs
E=1355J
V=52m/s
Vx=Vy=37m/s
t(apogee)=(37m/s)/(9.8m/s^2)=3.75
Therefore Distance=37*2t=277m

My guess is you are getting a lot of blow-by and pressure drop across your valve. Try the shaving cream and that may double your 30m to 60m if you get a good seal. Work on optimizing your balloon fill to find minimum weight, but adequate size to seal well. With some optimizing you should be able to get 100m. 277 is quite unrealistc as it does not take into acount wind resistance and muzzle friction.

Last but certainly not least do your get a lot of kick (backwards motion of the launcher?) If the launcher is moving during the launch, this may dramtically reduce the energy going into the ballon.

Please be safe. 90psi is a lot of pressure. A 50 m/s water ballon is like a 100 mph fastball.

Would you stand in front of a MLB baseball pitch?