This is my attempt to make a more balanced version of "the shield".

A few of things:

• Adamantine doesn’t inherently reduce Crits to regular hits, that’s a function of a specific enchantment worked into a set of adamantine armor. If you just make a suit of armor out of that metal it will function no differently than a standard suit of armor unless and until it is made magical.
• Every shield can technically be an improvised weapon that does 1d4 bludgeoning damage on a hit and has a range of 20/60, those are the standard rules for improvised weapons. And every improvised weapon already requires the Tavern Brawler feat to add the wielder’s Proficiency bonus to the attack roll, that is also already standard RAW for all improvised weapons.
• Sharpshooter only works on specifically “ranged weapons,” thrown melee weapons do not count.

1. This would be a magic item and fall under the adamantine armor rules. https://www.dndbeyond.com/magic-items/5370-adamantine-armor

2. Yes. I merely specified that in the description.

3. According to Jeremy Crawford and Sage Advice bullets 1 and 2 apply to all ranged weapon attacks while bullet 3 only applies to attacks with a ranged weapon. Ranged weapon attacks and ranged weapons are different things. https://www.google.com/amp/s/www.sageadvice.eu/does-sharpshooter-feat-should-work-with-daggers/amp/

Thanks for the input. I'll reword to specify that this is a magic shield/weapon and maybe move the damage to notes. I also need to add the crits break objects part of adamantine.

Quote from KidThunderbolt >>

1. The fact that the armor is made of adamantine is what reduces criticals.  https://www.dndbeyond.com/magic-items/5370-adamantine-armor

 
That is incorrect. While the armor is reinforced with Adamantine, it is also a Magic Item. There are no inherently magical metals in this edition. Therefore in order to be a “Magic Item,” it must also be enchanted in some way. If it was just a matter of slapping Adamantine onto the armor it would be listed as a regular Item, not a Magic Item. The Adamantine is the rare component required by the formula to make adamantine armor.

Crafting a Magic Item (DMG)

Magic items are the DM’s purview, so you decide how they fall into the party’s possession. As an option, you can allow player characters to craft magic items.

The creation of a magic item is a lengthy, expensive task. To start, a character must have a formula that describes the construction of the item. The character must also be a spellcaster with spell slots and must be able to cast any spells that the item can produce. Moreover, the character must meet a level minimum determined by the item’s rarity, as shown in the Crafting Magic Items table. For example, a 3rd-level character could create a wand of magic missiles (an uncommon item), as long as the character has spell slots and can cast magic missile. That same character could make a +1 weapon (another uncommon item), no particular spell required.

You can decide that certain items also require special materials or locations to be created. For example, a character might need alchemist’s supplies to brew a particular potion, or the formula for a flame tongue might require that the weapon be forged with lava.

Crafting an Item (Xanathar’s Guide)

Crafting Magic Items. Creating a magic item requires more than just time, effort, and materials. It is a long-term process that involves one or more adventures to track down rare materials and the lore needed to create the item.

Potions of healing and spell scrolls are exceptions to the following rules. For more information, see “Brewing Potions of Healing” later in this section and the “Scribing a Spell Scroll” section, below.

To start with, a character needs a formula for a magic item in order to create it. The formula is like a recipe. It lists the materials needed and steps required to make the item.

An item invariably requires an exotic material to complete it. This material can range from the skin of a yeti to a vial of water taken from a whirlpool on the Elemental Plane of Water. Finding that material should take place as part of an adventure.

Just like how dragon scale mail requires dragon scales/hide, which is also not inherently magical. The same applies to mithral armor, the formula to make that suite of magic armor requires the mithral, but the mithral is in-and-of itself not inherently magical. Since your shield is also a Magic Item and not regular equipment, this is merely an intellectual exercise. I was just pointing out that the Adamantine itself doesn’t provide the benefit without also being enchanted to.

2. Yes. I merely specified that in the description.

 
As long as you’re aware of the redundancy. However, if you want to make this shield special why not make it do 1d6 damage when used as an improvised weapon? And why not also apply the benefits of Adamantine Weaponry (which is regular equipment and nonmagical in nature just from the Adamantine alone):

Adamantine Weapons

Adamantine is an ultrahard metal found in meteorites and extraordinary mineral veins. In addition to being used to craft adamantine armor, the metal is also used for weapons.

Melee weapons and ammunition made of or coated with adamantine are unusually effective when used to break objects. Whenever an adamantine weapon or piece of ammunition hits an object, the hit is a critical hit.

The adamantine version of a melee weapon or of ten pieces of ammunition costs 500 gp more than the normal version, whether the weapon or ammunition is made of the metal or coated with it.

3. According to Jeremy Crawford and Sage Advice bullets 1 and 2 apply to all ranged weapon attacks while bullet 3 only applies to attacks with a ranged weapon. Ranged weapon attacks and ranged weapons are different things. https://www.google.com/amp/s/www.sageadvice.eu/does-sharpshooter-feat-should-work-with-daggers/amp/

 
True, the first two bulletpoints would apply, but most people focus on the third, so that’s the one I was referring to. I should have specified.

 Thanks for the input. I'll reword to specify that this is a magic shield/weapon and maybe move the damage to notes. I also need to add the crits break objects part of adamantine.

Happy to help?

I had edited my reply about the same time that you replied which agrees with most of what you stated.

I didn't make it a 1d6 to keep the rules/ruling for it a simple as possible. I also see this as being used by a ranger which would allow them to add hunters mark to the damage.

Quote from KidThunderbolt >>

I had edited my reply about the same time that you replied which agrees with most of what you stated.

 
lol. Well, we got there in the end.

I didn't make it a 1d6 to keep the rules/ruling for it a simple as possible. I also see this as being used by a ranger which would allow them to add hunters mark to the damage.

If you omitted all of the redundancies in the description then it would simplify things enough to allow for that little change. Also, since it includes both Mithral and Adamantine you could also give it the Light so its wielder could take advantage of the Two-Weapon Fighting rules while wielding this and, say, a shortsword. Something kinda like this:

Shooting Star Shield

Armor (shield), very rare (requires attunement)

 

While holding this shield, you have a +1 bonus to AC. This bonus is in addition to the shield's normal bonus to AC.

You can don or doff this shield as a bonus action, or as if interacting with an object. While you're holding it, any critical hit against you becomes a normal hit.

In addition, this magic shield can also be used as a weapon, proficiency with a shield allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Name	Damage	Weight	Properties
Shooting Star Shield	1d6 bludgeoning	3 lbs	Light, Special, Thrown (range 20/60)

This magic weapon grants a +1 bonus to attack and damage rolls made with it, and it returns to the wielder’s hand immediately after it is used to make a ranged attack.

Notes: Combat, Damage, Utility, Warding

Now that👆is a “very rare” magic item.

You've turned it into a higher level item that can only be used for ranged attacks once per round. I'm trying to keep it a lower level item that can be used for multiple attacks per round (with the thrown weapon fighting style). Though you did list both a bonus and free action in the same line. Interacting with an object is a free action not bonus.

So yes that is what I'm going for without the +1s. Your format is better but doesn't include the adamantine benefits. And you've set it as shield proficiency when it should still be an improvised weapon proficiency.

Quote from KidThunderbolt >>

You've turned it into a higher level item that can only be used for ranged attacks once per round. I'm trying to keep it a lower level item that can be used for multiple attacks per round (with the thrown weapon fighting style). Though you did list both a bonus and free action in the same line. Interacting with an object is a free action not bonus.

So yes that is what I'm going for without the +1s. Your format is better but doesn't include the adamantine benefits. And you've set it as shield proficiency when it should still be an improvised weapon proficiency.

 
I turned it into nothing other than what you wrote.

You originally listed it as a “very rare” item, I just went based off of what you wrote. (You have apparently already edited that out since then, but you did have that in your post.)

And I listed that it could be donned/doffed as either a bonus action or as if interacting with an item. (You can tell because of the word “or” I put in there.) If you have not used your 1ce/turn free item interaction then you can don/doff it for free. If you have used it you can don/doff the shield as either an action or a bonus action. So you could doff it for free, throw it a bunch of times with your action, and then don it again as a bonus action to use for defense.

I wrote up exactly what you had, only written differently. If you want the item to be weaker then drop the +1 to defense and the +1 to attack and damage rolls. Whatever, I’m just tryin’a help you out. Here:

Shooting Star Shield

Armor (shield), very rare (requires attunement)

 

This enchanted, disc shaped metal shield is made of adamantine and mithral. This makes the shield lighter than a standard steel shield, while gaining the strength of adamantine. it is extremely well balanced, and also aerodynamic.

While holding this shield, you have a +1 bonus to AC. This bonus is in addition to the shield's normal bonus to AC.

You can don or doff this shield as either a bonus action, or as if interacting with an object. While you're wielding it, any critical hit against you becomes a normal hit.

In addition, this magic shield can also be used as a weapon, proficiency with a shield allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Name	Damage	Weight	Properties
Shooting Star Shield	1d6 bludgeoning	3 lbs	Light, Special, Thrown (range 20/60)

This magic weapon grants a +1 bonus to attack and damage rolls made with it, and it returns to the wielder’s hand immediately after it is used to make a ranged attack.

Notes: Combat, Damage, Utility, Warding

 
Better? I also added in some of your flavor text, and even took the liberty of fixing your punctuation and typos.

Hahaha. Not trying make this antagonistic. We are just working with different baselines. The shield is really more about the player/character wielding it than just the shield.

Like i said your writeup/format was better than mine but details were/are different.

For example

"In addition, this magic shield can also be used as a weapon, proficiency with a shield allows you to add your proficiency bonus to the attack roll for any attack you make with it."

However I would (will in my final draft) say that proficiency with improvised weapons not proficiency with shields.

Thanks for helping me BOUNCE this around.

Yeah, I tossed in shield proficiency counted for attacks because that’s more fitting for your concept of them wielding the shield both offensively and defensively. I even once wrote a feat expressly for the purpose of letting any character do it with any shield: (https://www.dndbeyond.com/feats/215786-all-a-hero-needs). Take my suggestions or don’t, modify it to your liking. It’s your baby after all.

I would however recommend that you skip the redundant text about using it as an improvised weapon since any character can use any shield as an improvised weapon. It’s like saying “you can defend yourself with this shield, granting you a +2 bonus to your AC.” That’s already a rule, it doesn’t need to be stated. It’ll save space and avoid confusion. But again, you do you.

Happy to help.