I think you can only use standard objects for any of the 'what object?' inputs in the object editor. At least, I haven't found a way to use custom objects.

I can understand this limitation in general for object sharing to work. How would it otherwise work if you download a custom object that spawns another custom object that ... etc., until a single download depends on 100:s of other custom objects.

But what I don't understand is why the draw surface for a gun doesn't both include a gun and an ammo mode, just like it works for costumes, i.e., costume object, and worn costume are drawn separately. This seems a huge oversight, given how much fun people would have building their own spectacular guns.

Similarly I'm missing an "on projectile hit" event in the scripting, which I think is possibly an even larger oversight.

Maybe "Collides with an object" would work. Thats the one i wanted to use for the gun.

If it was possible to make your own gun, bullet and explosion and save those 3 items in a collection. When someone downloads said gun all 3 items you would need are shown.
A limit could be made so that you cant put 100's of different objects in it. since for a gun you use the same bullets over and over again.

I wanted to make a Custom crossbow with multiple Custom arrows which would bounce of walls, with an item limit of 2, I would have my own customized gun.

It would be a big inprovement towards Unlimited creativity if they had such a feature.

I think collide with object would only work if you were scripting the bullet, but without a way to script both the gun and ammo - i cannot see how that can work for a gun design.

Unless the multiple arrows you are after exist as a standard object, a gun like the one you describe is going to be tricky. I'm trying to work out some workarounds for a similar problem, so I may get back to you if I have any clever ideas...

Just say anything for ammo in the 'weapon' category, i made a guy shoot exploding bananas, and making your own object as ammo doesn't work and i have no idea why