I am making a Home-brew weapon and am trying to make it deal 2d8 damage, but I cannot because of the base weapon. Is this a bug or do I just not know how to do it? Ive tried replace damage type and damage. Nothing seems to work.

So what you could do is create a magic item from scratch, pick something that does 1d8, and then add another d8 damage to it.

another thing you could do is you could add a melee attack. to it, (it would have 2 attacks, like a warhammer, as it would be versitile...)

I ran into a similar problem and continue to be frustrated with weapons creation. I was working on a firearm, ended up using the blunderbus template HEAVILY modded.

The real problem is that there is no way currently to make an item from scratch (note-publishing non-magical items in the magic item category will yield a rejection slip.) there is a plan on the roadmap to institute a scratch item creation homebrew, but when we'll see it is anybody's guess.

For melee weapons the only 2d weapons are maul & greatsword. Both are d6. Even although there's a slot that says set die type, it does nothing. 

You can add a bonus die to make your weapon fit your idea, but it will still display as a 1d weapon on the character sheet actions page- you need to click on the weapon to see the extra die listed in the description. 

Make a copy of the dwarven thrower & you can see how it's done.

thanks!

np, glad to help.