Silvered weapons counts as magical, so they ignore "immunity from all nonmagical damage" and other resistances
Silvered also bypass resistance and immunities when saying Bludgeoning, Piercing, and Slashing From Nonmagical Attacks Not Made With Silvered Weapons, but otherwise is not inherently magical.
So if you would roll a natural twenty on an object while using an adamantine weapon, would you get a double crit, since they crit on objects
I don't know whether this is specifically addressed anywhere because it's an edge case, but there's no allowance in the rules to "double crit". Adamantine weapons simply treat all hits against objects, including natural 20s, as critical hits.
One could argue that adamantine weapons are magical. Harken back to 1E when magical weapons were based on constructed material. As an example a +5 Longsword was made of Adamantine. Bruenor Battlehammer admittedly not a caster forged Aegis Fang with a block of Mithral for the head and rod of adamantine for the handle. Using a ritual calling upon the dwarven gods he forged the weapon imbuing it with the extra powers throwing and returning onto the mighty maul, to which he would gift to his adopted son Wulfgar.
One could argue that adamantine weapons are magical.
In 5E adamantite weapons are not magic weapons despite exceptional material but a DM could always instead say they are.
Perhaps I should say, one could use older edition material in the place of a low magic campaign so that damage resistant creatures wouldn’t be a slow whittling away of HP. I am well aware of the 5E RAW.
If your goal is to hit a creature's armor for the purposes of damaging the armor, it may work in principle, but not very well in practice. Armor is an object, so an adamantine weapon would do additional damage. But we don't really get into how many structural hit points a piece of armor has, nor is there really a general rule that talk about when your armor breaks in combat. Actually, there might be, but I can't find it. I'm sure someone will post it if there is. We have specific rules for things like the rust monster or the black pudding that can damage armor and weapons, but RAW those are specific to those circumstances and not applicable in your example.
if you hit armour would that not count as a crit it's not a monster, it's his Armour
5e has changed almost everything regarding damage to items and whether they are being held/worn by a creature.
Previous editions had rules for damaging items on another creature. Currently, many spells and effect say something like, "items not being worn" that will be damaged, meaning if somebody is wearing or wielding it, it cannot be broken. The exception being rust monsters and such.
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"Sooner or later, your Players are going to smash your railroad into a sandbox."
-Vedexent
"real life is a super high CR."
-OboeLauren
"............anybody got any potatoes? We could drop a potato in each hole an' see which ones get viciously mauled by horrible monsters?"
Actually, there are quite a few: See here: https://www.dndbeyond.com/monsters?filter-type=0&filter-search=&filter-cr-min=&filter-cr-max=&filter-armor-class-min=&filter-armor-class-max=&filter-average-hp-min=&filter-average-hp-max=&filter-is-legendary=&filter-is-mythic=&filter-has-lair=&filter-damage-immunity=29&filter-partnered-content=f these are all monsters that are immune to nonmagical bludgeoning, slashing or piercing, the lowest CR of which is the Coatl at CR 4, then skips to the Alhoon at CR 10.
Subclass: Dwarven Defender - Dragonborn Paragon
Feats: Artificer Apprentice
Monsters: Sheep - Spellbreaker Warforged Titan
Magic Items: Whipier - Ring of Secret Storage - Collar of the Guardian
Monster template: Skeletal Creature
Can adamantine weapons like become an automatically critical hit
Read the thread, my friend. Your question is specifically answered a few times.
"Not all those who wander are lost"
Silvered weapons counts as magical, so they ignore "immunity from all nonmagical damage" and other resistances
No, they don't.
Silvered also bypass resistance and immunities when saying Bludgeoning, Piercing, and Slashing From Nonmagical Attacks Not Made With Silvered Weapons, but otherwise is not inherently magical.
So An artificer cannot infuse Adamantine suit of plate???
Adamantine Armor is categorized under Magic Items (uncommon), so by RAW, you wouldn't be able to apply an infusion to it.
Helpful rewriter of Japanese->English translation and delver into software codebases (she/e/they)
So if you would roll a natural twenty on an object while using an adamantine weapon, would you get a double crit, since they crit on objects
I don't know whether this is specifically addressed anywhere because it's an edge case, but there's no allowance in the rules to "double crit". Adamantine weapons simply treat all hits against objects, including natural 20s, as critical hits.
Even a blind squirrel finds a nut once in awhile.
One could argue that adamantine weapons are magical. Harken back to 1E when magical weapons were based on constructed material. As an example a +5 Longsword was made of Adamantine. Bruenor Battlehammer admittedly not a caster forged Aegis Fang with a block of Mithral for the head and rod of adamantine for the handle. Using a ritual calling upon the dwarven gods he forged the weapon imbuing it with the extra powers throwing and returning onto the mighty maul, to which he would gift to his adopted son Wulfgar.
In 5E adamantite weapons are not magic weapons despite exceptional material but a DM could always instead say they are.
Perhaps I should say, one could use older edition material in the place of a low magic campaign so that damage resistant creatures wouldn’t be a slow whittling away of HP. I am well aware of the 5E RAW.
You sure can use older edition meterial or any other houserule that suites your campaign indeed!
if you hit armour would that not count as a crit it's not a monster, it's his Armour
If your goal is to hit a creature's armor for the purposes of damaging the armor, it may work in principle, but not very well in practice. Armor is an object, so an adamantine weapon would do additional damage. But we don't really get into how many structural hit points a piece of armor has, nor is there really a general rule that talk about when your armor breaks in combat. Actually, there might be, but I can't find it. I'm sure someone will post it if there is. We have specific rules for things like the rust monster or the black pudding that can damage armor and weapons, but RAW those are specific to those circumstances and not applicable in your example.
"Not all those who wander are lost"
5e has changed almost everything regarding damage to items and whether they are being held/worn by a creature.
Previous editions had rules for damaging items on another creature. Currently, many spells and effect say something like, "items not being worn" that will be damaged, meaning if somebody is wearing or wielding it, it cannot be broken. The exception being rust monsters and such.
"Sooner or later, your Players are going to smash your railroad into a sandbox."
-Vedexent
"real life is a super high CR."
-OboeLauren
"............anybody got any potatoes? We could drop a potato in each hole an' see which ones get viciously mauled by horrible monsters?"
-Ilyara Thundertale
Adamantine weapons are like silver weapons in terms of obtaining and magical rules, they just also score well against objects.