I was reading the other day about mithral armor and it got me to thinking. Say someone had mithral ore and wanted to make a weapon with it. What effect would using the different material have on said weapon?

The best I could come up with would be that since armor is made lighter that it would make the weapon lighter as well. However the armor is also said to be more flexible, should that have an effect on say bludgeoning weapons? The rules also do not have any info on how much the armor weighs in comparison to the iron version. 3/4 or 1/2 seems a reasonable metric but I would like input. My thought is that any weapons with the "Heavy" tag would loose that tag, would it make sense for the weapons which were not heavy to gain the finesse property?

I welcome the input of others on this.

It's getting into Homebrew moreso than Rules & Game Mechanics, but yeah... that makes sense to have Mithral change Heavy weapons to lose the Heavy property, and non-heavy weapons become finesse weapons. I think pushing it further (like making finesse weapons considered light or monkeying around with throw distances), would probably take it too far and be too easy to abuse.

There's a decent article on homebrewing this sort of thing here:  https://blackcitadelrpg.com/mithral-armor-weapons-5e/

Note that in RAW, mithral armor is not actually lighter than the standard version, but it's a common house rule that it weighs half as much.

This is homebrew territory so you might want to read thru the thread posted above for ideas pro and con. I would not change the heavy and light properties as they don’t actually have much to do with weight but are more about balance and maneuverability- just as a big empty box is light by weight but unwieldy because of its size and shape. Mithril weapons aren’t a listed thing because (IMNSHO) the weight change etc is relatively minor for the item. Keep in mind that the two biggest things Mithril armor does are to allow the stealth to not have disadvantage and to allow the wearer to ignore strength limitations on wearing the armor - neither having a normal impact on weapons. The kinds of things that Mithril vs steel/iron/bronze would have an impact on are no longer parts of the game (weapon speed values etc.). Because Mithril is supposed to be rarer and harder to work with  (like it’s real world analog titanium) I use it for masterwork and magic weapons - especially fire/acid/corrosion resistant ones like dragonslayers, flametongues etc. but that is just my personal flavoring of it.

Using a longsword or battle axe doesn't actually break sneak attack; the bonus damage is independent of weapon damage die, so practically speaking you're maybe moving the needle by 2 points of average damage per hit to use a d12 over a d8. Now, thematically it doesn't make sense, yeah, but they've honestly been a bit inconsistent on that point considering you can Sneak Attack while Raging.

Again, the terms heavy and light , as used in 5e, are misnomers - they actually have nothing to do with weight and far more to do with size( length) and weildiness. Heavy actually represents long 2 handed weapons that need space to use effectively. Light actually means short and easy to use in confined spaces. So one bugbear wielding a  halberd complexly fills a 10’ hall but it takes 2 ( maybe 3) dwarves with short swords to block the same hall effectively. “Heavy” Weapons need leverage to use properly and small characters (gnomes, halfling, kobolds, etc) simply can’t generate it because they are not big enough to have the reach needed. (A longbow at full draw calls for a 3’+ long arrow, if your arms are only 2’ long …) this leverage and unwieldiness is not changed by making the weapon somewhat lighter, only by making it somewhat shorter (and then you won’t get the same damage - a halfling sized halberd should only generate a D8 damage). The most making a weapon lighter should do is make it slightly faster so if you have to give something to Mithril weapons ( you shouldn’t but …) give them a +1 to initiative.