How can I create a semi automatic TNT cannon in Minecraft which does not need reloading?

Suppose I have a hill or a mountain I wish to demolish. I created a cannon using the classic recipe so I can do this task:

My problem is that reloading is quite cumbersome. I know that I can manufacture cobblestone using lava and water. Is there some way to manufature TNT somehow? If i need a mod for this it is not a problem!

So my goal is to be able to fire a primed TNT by pressing a button and without the need of reloading.

Solution 1:

This is now completely possible if you have access to command blocks. The key difference is that you can use the command blocks to /summon some PrimedTNT all in one spot, and all with very short fuses.

For instance in three command blocks, you could summon the cannon charge at 0, 0:

summon PrimedTNT 0 64 0 {Fuse:5}

Changing the Y value to something appropriate for your environment. In a fourth command block, you would then summon your "cannon ball":

summon PrimedTNT 1 65 0 {Fuse:100}

The Fuse data tag is measured in redstone ticks, i.e. tenths of a second, and have a limit of 127 (signed 8-bit number / Java byte).

Solution 2:

Zisteau created a TNT magazine that could be used for dropping TNT. Multiple magazines, arranged correctly, could be used to create a TNT cannon.

This might not be the best use of resources (or time), unless you're willing to go into creative and/or use MCEdit. In which case, why would you be worried about a mountain?