SpudFiles

I'm attempting to build a Nerf rocket launcher for my 11 year old brother. I had made a pneumatic launcher with a sprinkler valve and a 1" barrel when i was younger and thought the same sort of thing would work well for this, but it didn't, so now I'm looking for ideas and advice.
Any way, its a 2" chamber, a 1" rainbird sprinkler valve, and a 3/4" barrel that the "Nerf missile"(3.5" pool noodle, with weight added to the end" slides over. well its only going about 20 feet, and that's at a 45 degree angle. id like to get about 150 feet out of it, and at relatively low pressures, maybe 50 psi or so. I've tried different pressures, and don't really get any change in performance, and at above 80 psi, it just rips the side of the dart out.
These are some of the ideas I've had;
-boring out the dart and using a 1.5" barrel, and/or lining the inside of the dart with some kind of thin walled tubing, or wrapped plastic sheet. A.) this should increase flow, change the balance characteristics of the dart, maybe a better flight pattern. and B.) lower the air pressure coming out of the valve and reduce risk of blow-out.
-Making the valve pneumatically triggered(def. wont fix problems, but may help. also then i don't need batteries.)
-using an actual barrel the dart fits in, and shortening the dart.
What are you guys opinions? any advice? i think I'm going to go with an actual barrel, the problem is i cant find 3.5" PVC, or any other kind of 3.5" I.D.. pipe. except exhaust pipe...also I'm trying to come up with a good way to make a head for the dart, i want it to be like a "Elite dart" id like to make a mold and cast it out of some kind of rubber/silicone. Also in the long run id like this to look like a finished product, with a body, and some Nerf gun styling. If anyone has any ideas it's greatly appreciated

For a gentler acceleration, what you want is a longer barrel. If your valve opens faster, note that even at lower pressure it will tend to damage the projectile because of a more violent air blast.

By using a spigot type design with the dart over the barrel, you are definitely limiting performance because the area to which the dart is exposed to is small compared to the overall diameter, and a longer "barrel" in this case means a similarly lengthened projectile that also increases weight.

I'm going to go with an actual barrel

This sounds like the best solution. Try and find an agricultural supply store, 3.5 inch is a relatively common size for irrigation. For that size of pipe though, you'd really want the largest sprinkler valve you can find modded for manual actuation.

Fittings can also get expensive at that size, consider casting your own out of epoxy

id like to make a mold and cast it out of some kind of rubber/silicone

Your one stop shop for this sort of thing