Find an area you want to shoot at.(http://sphotos.ak.fbcdn.net/hphotos-ak-snc4/hs1393.snc4/164577_192236937469116_100000484339264_675166_7862473_n.jpg)
Now create a 6x3 rectangle on the ground(http://sphotos.ak.fbcdn.net/hphotos-ak-snc6/hs034.snc6/166414_192236957469114_100000484339264_675167_1319205_n.jpg)
Raise everything up 1 block except the first 3(http://sphotos.ak.fbcdn.net/hphotos-ak-ash1/hs773.ash1/166392_192236987469111_100000484339264_675168_2825066_n.jpg)
Fill the inside blocks with water(http://sphotos.ak.fbcdn.net/hphotos-ak-snc6/hs068.snc6/167853_192237024135774_100000484339264_675169_8069633_n.jpg)
Now fill the 2nd layer with TNT including the middle portion of the 1st unraised layer(http://sphotos.ak.fbcdn.net/hphotos-ak-snc6/hs021.snc6/165189_192237067469103_100000484339264_675170_1392379_n.jpg)
Make steps to get to the top. (kinda hard to explain)(http://sphotos.ak.fbcdn.net/hphotos-ak-snc6/hs014.snc6/166452_192237087469101_100000484339264_675171_6909578_n.jpg)
Make lines of redstone coming from the last 4 TNT blocks and connect it to a button(http://sphotos.ak.fbcdn.net/hphotos-ak-snc6/hs050.snc6/168037_192237100802433_100000484339264_675172_700273_n.jpg)(http://sphotos.ak.fbcdn.net/hphotos-ak-snc6/hs248.snc6/179641_192237130802430_100000484339264_675173_6219753_n.jpg)
Now connect the ending TNT piece to a SEPERATE button near the other(http://sphotos.ak.fbcdn.net/hphotos-ak-snc6/hs270.snc6/179886_192237167469093_100000484339264_675174_8241461_n.jpg)
To get the TNT to launch. Click the right button. Wait until the TNT hissing noise stops. and Hit the second. It will send the TNT forwards!(http://sphotos.ak.fbcdn.net/hphotos-ak-ash1/hs740.ash1/163275_192237194135757_100000484339264_675175_347014_n.jpg)