Duplo cannon

[Peter Blomberg]

The Duplo cannons are composed of multiple physical pieces screwed together. Looking specifically on the cannon with locking ring and red button 17178c01 or 94886c01.

What is the minimum that need to be modeled?

Some motors, projectile launchers, and springs have been modeled down to each screw and separable part even if that has meant the creation of (sub)parts with u numbers. Obtaining subpart numbers and internal structure requires opening the assembly, possibly destroying its play-ability in the process.

Rubber insets are sometimes modeled only by their visible parts.

Can the firing button be modeled as a red surface or does it need to be a separate subpart?

How accurately does the internal structure (metal spring and projectile guides) need to be modeled?

Do the screws need to be modeled as separate parts?

[Magnus Forsberg]

You are free to create and assume an inside structure, but each part has to be a closed 3D mesh. Use u-numbers.
The red button has to have some sort of structure, even if it is completely assumed. Simplify as mush as possible.
This "part" has to be a Shortcut, made of Parts. No Subpart is allowed to be in a Shortcut.

Since we know the correct assy number it might be possible to find the sub-part numbers without destroying the assy.
Do you want us to look for the numbers?

I don't have the part, but I assume one could look into the slot for the projectile?
Create something behind the slot to make it impossible to peek inside.

Have a look in the library for screws. Maybe there is a suitable screwhead on one of them. 
Ignore the length of the screw.

[Peter Blomberg]

The cannon 94886c01 on BrickLink is composed of three pieces, a metal spring, the red firing button, and two metal screws.
The screw heads are close to u9013.
The bottom piece has the number 94886.
The front piece has number 94884.
The back piece has number 94885.
The firing button has number 94887.
Is there a need for 94886c00 or can you find the assembly number?

The cannon 17178c01 on BrickLink is composed of two pieces, a metal spring, the red firing button, and two metal screws.
The screw heads are close to u9013.
The front piece has number 16703.
The firing button has number 94887.
The back piece has number 16702.
Is 17178 the assembly number?

[Magnus Forsberg]

First one is listed as 94883. And, it's on BS as 94883
https://brickset.com/parts/design-94883

Second I can only find at BS, but since there are many "colours" it must be it. Printed, unprinted and in different colours, all use the same number. 
https://brickset.com/parts/design-17178

[Peter Blomberg]

Are ~parts of the same assembly allowed to collide, i.e. have surfaces that pass thru each other?

E.g. can the mounting of a screw be a flat surface just under the head? This would mean that the threaded part (cylinder) would pass right thru the flat surface.

[Magnus Forsberg]

Are ~parts of the same assembly allowed to collide, i.e. have surfaces that pass thru each other?

E.g. can the mounting of a screw be a flat surface just under the head? This would mean that the threaded part (cylinder) would pass right thru the flat surface.

Since we don't know the correct length, or the depth, I would say yes.
Maybe add an edge prim to indicate there's a hole?