Parts list:

1 - 518, x110c01 or bb39

1 – 2527

2 - 4600, 6157 or 11002

4 - 4624

ok, I'll bite.

Here's a cannon which I intend to be something smallish, like a four-pounder. Some of these pieces are a bit pricey, perhaps similar effects could be accomplished more cheaply. But anyway, hope you like

1x 32530, Technic Pin Connecter Plate, Black

2x 62462, Technic Pin Connecter Round 2L, Black

(and/or 75535 - I used one of these for the front part of the barrel)

1x 6628, Technic Pin with Towball, Black

1x 6558, Technic Pin with Friction Ridges, 3L, Black

2x 4600, Plate Modified 2x2 with Wheels Holder, Black

4x 85861, Plate Round 1x1 with Open Stud, Black

1x 3021, Plate 2x3, Reddish Brown

2x 85984, Slope 30 1x2x2/3, Reddish Brown

Four pounder cannon

Four pounder cannon

Four pounder cannon

Parts list:

5 x 4073, 4 in the colour of the wheels and 1 in the colour of the base

2 x 3623

1 x 62462 or 1 x 75535

1 x 6628

2 x 3062b

1 x 4081

Parts list:

3 x 4073

2 x 3623

2 x 4081

2 x 6636

1 x 3794

1 x 3024

1 x 4697b

1 x 3942c

1 x 30361c

Hello there! 

Here's the cannon design which I usually use - not sure just how original it is (I'm pretty sure it's some kind of combination of a few different designs I'd seen before), but it's not here yet, so! 

Parts list:
2x 3062b (alternately 1 for a shorter barrel)
1x 62462
1x 6628 
1x 4070
1x 3070b
2x 4865b
4x 4265c
2x 2540

Assembled:

And for minifigure size comparison (I hope it isn't too artistic ):

Or if this is better: