TNT cannons tips

When building TNT cannons, one wants to ask certain questions, disregard in creative, these are usually for SMP (Survivlal Multiplayer) 

1) Whats the purpose?

a) Bunker busting, undeground bases can be penetrated with slow tnt 
b) Range: good for keeping a distance
4) Damage: good for demolishing many buldings or even people
5) projectile speed: how fast do you need tnt to travel, as faster tnt tends to bounce
6) Height: how high is the base and your cannon

2) Any material constraints?
In survival, there almost always will be some sort of constraint, be it sand, or gunpowder. 

3) How is the base, what is it built with, who owns it?

a) is the base short? is it tall, is it a sky base, if so, tnt cannons will not work unless you build them either equal to or less than (slightly) the height. The lower you build, the farther it must be from the skybase in order to hit it as TNT arcs in a parabolic fashion

b) Is it built with Obisidian (obby), if so, maybe you can damge the things inside it, as tnt will carry its damage if it cant penetrate, up to its maximum range, eg: If block A cant be penetrated, maybe block B can

c) If it's a large clan, its likely to be guarded in some way, possibly even trapped, be careful, but the general rule is: Larger clan=more loot, more danger

4) What design suits the project at hand?

a) for low ground bases, or underground bases, shorter cannons are best
b) for normal bases and houses, any tnt cannon will do
c) for an obsidian base, try to dig around it, depending on size, tnt damage may destroy everything inside, and you can dig in the house and claim it
d) Skybases need special attention, you need to build the cannon to the height equal to the base, any lower warrants a cannon slightly farther away, with a longer cannon.