slingingrat made me an excellent blowgun a while back.  It performed beautifully.  He'd have some good advice for you.  Sadly, the blowgun scared the crap out of Odin when I shot it (he's dreadfully gun shy) so I gave it to my young friend Mark.

Quote from Wayfarer on Apr 15^th, 2011, 12:03pm:

 
I use milk jugs- heat over a candle till the plastic turns clear, then push a plumb bob or other conical object into the pipe through the clear spot. when the pastic turns cloudy again, repeat. Trim as necessary. To mount, cut the tip off the cone so the hole is a little smaller than the shaft you want to mount to, then push the shaft on and glue. I usually wrap on some thread ahead of the cone and dab on some super glue to keep it from sliding forward. Trim so theyare just loose enough to slide backwards into the barrel with some resistance. Waterproof, durable and cheap (free). I make a bunch of cones (several milk jugs) and always have them ready to make 6-8 darts at a time.

Quote from perpetualstudent on May 5^th, 2011, 7:47pm:

 i make my blowguns from 3/8 aluminium tubing. its not insanely expensive, and it is so smoth i can get crazy power- like enough to puncture 1/8 steel with tungsten darts(almost a pound.)

Quote from johnny108 on Aug 5^th, 2012, 5:44pm:

 
I've got a cold steel blowgun, and like it a lot. I got the 5' model and bought some extra darts with it. The pipe is great, if overpriced, and the darts work, but not near as well as my homemade ones. Though expensive, these things are a hassle free way to get your hands on a good blowgun. I find I can use the same darts in either emt or the cold steel, but the emt has significant drag against the cones. It got better when I mounted a gun cleaning rod to my drill and polished the bore with polishing compound, but the cold steel is still better. I like pvc better than emt, because it's smoother, and the sag can be taken care of my wrapping some aluminum angle stock on it. Really, the sag doesn't seem to affect how it shoots or aims unless it's really long, but for some reason people get all worked up about it. For a long time (decades) all I ever used was 3/4" grey pvc with no modifications at all. Now I prefer a mouthpiece of some kind and, mainly for aesthetic reasons, a stiff, straight pipe (yeah, I know, "that's what she said," right?)
 
I think 3' is too short but 20mm is about right. I've been hunting game like rats, mice, rabbits and squirrels for 20+ years, off and on, And find a 5' 3/4" pvc blowgun (about .60 caliber, I think) is perfect. The cold steel is stiffer and prettier, but not more effective. Copper works well but it dents easily. I also have one made from A 5' piece of carbon fiber tube that's .46 caliber. I find that the darts have to be a lot lighter for this one, but the speed of the darts is much greater, so that the darts actually penetrate deeper in wood than my cold steel darts. I haven't really explored the hunting potential of this one much, but keep it handy in the barn for mice and rats.
 
If you get the cold steel darts, you'll notice that the cones are often warped. Putting them in hot water usually solves this problem. For hunting, I use broad heads made from xactos or nails hammered flat, mounted with a brass collar and hot glue to a thin dowel or bamboo skewer, about 10" long and weighing about 5-6 grams, with milk jug cones. The center of gravity is well forward of the middle of the shaft (important). These fly straight and hit hard enough to penetrate well and kill efficiently with good shot placement. I've also used simple nail darts made with finish nails, head hammered flat and sharpened with tape and plastic cones. The accuracy and range with these is not as good as the others, though.
 
Another thing to consider is your blowing technique. I use my tongue pressed against the roof of my mouth as a valve and apply some pressure with my lungs. Then I release the pressure and blow hard at the same time, kind of like saying "teee". This literally doubled my effective range, but it takes a little practice to achieve consistency.

Michel is a true master of the blowgun, it seems. I defer to him on any issue where his opinion and mine vary. Wow. Thanks for all that stuff.

a few darts.

This one has diagonal slits cut in the second cone, to give it a rifle-spin. Worked to correct accuracy problems. The auxillary cone keeps the broad head from scoring the barrel. All work well in testing. I haven't hunted much with a blowgun in a few years, so real world tests will follow.

You gotta find a copy of "Blowguns: the breath of death"  by Micahel Janich, from Paladin Press.  Once you own this book- you don't need to buy a blowgun- ever.  I bought this book when it first came out, made a few 1/2"PVC blowguns, then bought a few 0.40" Professionally-made blowguns- homemade PVC tubes with coat-hanger-wire darts did better against real wild game. ("stunner" darts sold in the catalogs, won't do a damn thing!)
Other than friction, there is some math: Lung volume compared to the volume of the blowgun tube.  (if your blowgun tube is bigger than the volume of your lungs, you are not applying more pressure to the dart, and you are wasting your breath.)
Other than that: longer tubes "sag" more, and can be harder to aim.