The spell states that if your physical body is reduced to 0 HP, the spell ends, and if your cord was intact, then your soul returns to its body. 

As a monk you can kinda use this to become a statue.

Quote from Xix_of_Waterdeep >>

Quote from the Dungeon Master's Guide, Chapter 2: "Creating a Multiverse," Section 3: "Astral Plane" (p. 47)

Astral Projection

---

Traveling through the Astral Plane by means of the astral projection spell involves projecting one's consciousness there, usually in search of a gateway to an Outer Plane to visit. Since the Outer Planes are as much spiritual states of being as they are physical places, this allows a character to manifest in an Outer Plane as if he or she had physically traveled there, but as in a dream. A character's death—either in the Astral Plane or on the destination plane—causes no actual harm. Only the severing of a character's silver cord while on the Astral Plane (or the death of his or her helpless physical body on the Material Plane) can result in the character's true death. Thus, high-level characters sometimes travel to the Outer Planes by way of astral projection rather than seek out a portal or use a more direct spell.

(emphasis mine) 

So the DMG states that Astral Projection allows you to go through portals to any other Outer Planes in your astral body, leaving your physical body at home; but the spell description states that when you "enter a new plane or return to the plane you were on", your physical body is transported to the new location...

Since the spell makes a fairly general statement about entering "a new plane", while the Dungeon Master's Guide makes specific mention of the Outer Planes, I think we can draw an inference of general rule & specific exception, here: 

If your astral body exits the Astral Plane through a portal to another material plane, your physical body will be pulled there by your silver thread; but in the specific case that your astral body exits the Astral Plane through a portal to another Outer Plane, your physical body remains in place & the astral body effect is maintained. 

This means that characters adventuring in the Outer Planes by means of Astral Projection, will neither die (unless their silver thread is cut), nor bring anything back. On the other hand, your astral body stepping through a portal from the Astral Plane to any given location on a Material Plane, would bring your physical body along for the ride.

So Astral Projection is a slightly risky way to get your physical body (& by extension, loot) to a distant location on any Material Plane, and a relatively safe way of leaving your physical body at home while visiting any Outer Planes.

So can you cast demiplane... hole up in that plane, then astral project to the material world?

Your astral body replicates your possessions... so what happens if you use them? Drink your astral potion of healing? Use your astral diamond dust to cast stoneskin? Shoot your astral arrows at a githyanki warrior? Do the originals still exist? Do they mysteriously vanish?

Quote from ProphetZarquon >>

Quote from Xix_of_Waterdeep >>

Quote from the Dungeon Master's Guide, Chapter 2: "Creating a Multiverse," Section 3: "Astral Plane" (p. 47)

Astral Projection

---

Traveling through the Astral Plane by means of the astral projection spell involves projecting one's consciousness there, usually in search of a gateway to an Outer Plane to visit. Since the Outer Planes are as much spiritual states of being as they are physical places, this allows a character to manifest in an Outer Plane as if he or she had physically traveled there, but as in a dream. A character's death—either in the Astral Plane or on the destination plane—causes no actual harm. Only the severing of a character's silver cord while on the Astral Plane (or the death of his or her helpless physical body on the Material Plane) can result in the character's true death. Thus, high-level characters sometimes travel to the Outer Planes by way of astral projection rather than seek out a portal or use a more direct spell.

(emphasis mine) 

So the DMG states that Astral Projection allows you to go through portals to any other Outer Planes in your astral body, leaving your physical body at home; but the spell description states that when you "enter a new plane or return to the plane you were on", your physical body is transported to the new location...

Since the spell makes a fairly general statement about entering "a new plane", while the Dungeon Master's Guide makes specific mention of the Outer Planes, I think we can draw an inference of general rule & specific exception, here: 

If your astral body exits the Astral Plane through a portal to another material plane, your physical body will be pulled there by your silver thread; but in the specific case that your astral body exits the Astral Plane through a portal to another Outer Plane, your physical body remains in place & the astral body effect is maintained. 

This means that characters adventuring in the Outer Planes by means of Astral Projection, will neither die (unless their silver thread is cut), nor bring anything back. On the other hand, your astral body stepping through a portal from the Astral Plane to any given location on a Material Plane, would bring your physical body along for the ride.

So Astral Projection is a slightly risky way to get your physical body (& by extension, loot) to a distant location on any Material Plane, and a relatively safe way of leaving your physical body at home while visiting any Outer Planes.

 Ok, going off of this logic, let's say that I use Astral Projection to travel to an Outer Plane (we'll go with Gehenna, but it could be any), and then I cast Gate and walk through back to the Material Plane. What happens? Because now my astral form and my physical body are on the same plane. Does my astral self have to go find my physical body and rejoin it? Would that let me bring stuff back from that plane? 

Quote from ProphetZarquon >>

Quote from Xix_of_Waterdeep >>

Quote from the Dungeon Master's Guide, Chapter 2: "Creating a Multiverse," Section 3: "Astral Plane" (p. 47)

Astral Projection

---

Traveling through the Astral Plane by means of the astral projection spell involves projecting one's consciousness there, usually in search of a gateway to an Outer Plane to visit. Since the Outer Planes are as much spiritual states of being as they are physical places, this allows a character to manifest in an Outer Plane as if he or she had physically traveled there, but as in a dream. A character's death—either in the Astral Plane or on the destination plane—causes no actual harm. Only the severing of a character's silver cord while on the Astral Plane (or the death of his or her helpless physical body on the Material Plane) can result in the character's true death. Thus, high-level characters sometimes travel to the Outer Planes by way of astral projection rather than seek out a portal or use a more direct spell.

(emphasis mine) 

So the DMG states that Astral Projection allows you to go through portals to any other Outer Planes in your astral body, leaving your physical body at home; but the spell description states that when you "enter a new plane or return to the plane you were on", your physical body is transported to the new location...

Since the spell makes a fairly general statement about entering "a new plane", while the Dungeon Master's Guide makes specific mention of the Outer Planes, I think we can draw an inference of general rule & specific exception, here: 

If your astral body exits the Astral Plane through a portal to another material plane, your physical body will be pulled there by your silver thread; but in the specific case that your astral body exits the Astral Plane through a portal to another Outer Plane, your physical body remains in place & the astral body effect is maintained. 

This means that characters adventuring in the Outer Planes by means of Astral Projection, will neither die (unless their silver thread is cut), nor bring anything back. On the other hand, your astral body stepping through a portal from the Astral Plane to any given location on a Material Plane, would bring your physical body along for the ride.

So Astral Projection is a slightly risky way to get your physical body (& by extension, loot) to a distant location on any Material Plane, and a relatively safe way of leaving your physical body at home while visiting any Outer Planes.

 so what what happens if you use astral projection to go to somewhere other than the outer planes? Because from I know there are color pools that also go to all inner planes, the ethereal plane, the feywild and the shadowfell. also, let's say you are in , for example, mechanus, and you cast astral projection and go to the material plane. Would your physical body be teleported to you, or would the material plane create a temporary body for your astral form to inhabit

How is this spell necromancy and not divination? 

When you want to return to the physical world, do you just return to where your body was or does your body get transported to where your projected self is on the physical realm?  So would you be able to use this ability to get out of a prison for example.  Otherwise, this spell is just to walk around as a ghost and explore, but thats about it.

Gate anyone?

Quote from YanElrad >>

Gate anyone?

For getting to the Astral Plane, yeah, Gate is way better than this due to being cheaper in material costs, more precise if you know where you're going, and more versatile in what it can potentially do.

Here in the spell, we read: "If you enter a new plane or return to the plane you were on when casting this spell, your body and possessions are transported along the silver cord, allowing you to re-enter your body as you enter the new plane. Your astral form is a separate incarnation." (emphasis added).

This seems to imply (strongly) that upon entering an Outer Plane the character's body vanishes from the Prime Material, travels along the silver cord, to where the spirit/soul of the character is, and both are truly and fully re-united on the Outer Plane, suggesting that to die there, in that state, would result in actual death. 

However, in the section about the Astral Plane, in the DMG, (and specifically about Astral Projection) we read: "Traveling through the Astral Plane by means of the astral projection spell involves projecting one’s consciousness there, usually in search of a gateway to an Outer Plane to visit. Since the Outer Planes are as much spiritual states of being as they are physical places, this allows a character to manifest in an Outer Plane as if he or she had physically traveled there, but as in a dream. A character’s death — either in the Astral Plane or on the destination plane — causes no actual harm. Only the severing of a character’s silver cord while on the Astral Plane (or the death of his or her helpless physical body on the Material Plane) can result in the character’s true death. Thus, high-level characters sometimes travel to the Outer Planes by way of astral projection rather than seek out a portal or use a more direct spell." (emphasis added)

Contrary to the spell's entry, this seems to imply (again, strongly) that the Character's body remains on the original plane, and only a dreamlike version of their body travels along the cord and manifests in the outer plane, and that to die in this state does not result in the Character's actual death.

Given the fact that this is a 9th level spell, and can be undone with a simple Dispel Magic spell, it would seem that the latter understanding, given in the DMG, ought to be the correct reading, but it really does seem to contradict the reading in the PHB. I am wondering how others have handled this, and whether there are any clarifications given elsewhere?

Thought I'd look into this further, and found this, from the 3rd Edition Manual of the Planes:

Traveling Elsewhere: If an astral form passes through a color pool or otherwise manifests on another plane, it forms a new body from the building blocks of the plane itself. That body is identical with its natural form, except it is immune to the natural hazards of that particular plane. An astral body that travels to the Elemental Plane of Fire is immune to damage from the fire-dominant trait, for example. If the astral form is slain, the soul returns to the unharmed original body and location.

I think I'm going to go with this, as the way to reconcile the 5e difference.

Quote from Sgil >>

Here in the spell, we read: "If you enter a new plane or return to the plane you were on when casting this spell, your body and possessions are transported along the silver cord, allowing you to re-enter your body as you enter the new plane. Your astral form is a separate incarnation." (emphasis added).

This seems to imply (strongly) that upon entering an Outer Plane the character's body vanishes from the Prime Material, travels along the silver cord, to where the spirit/soul of the character is, and both are truly and fully re-united on the Outer Plane, suggesting that to die there, in that state, would result in actual death. 

However, in the section about the Astral Plane, in the DMG, (and specifically about Astral Projection) we read: "Traveling through the Astral Plane by means of the astral projection spell involves projecting one’s consciousness there, usually in search of a gateway to an Outer Plane to visit. Since the Outer Planes are as much spiritual states of being as they are physical places, this allows a character to manifest in an Outer Plane as if he or she had physically traveled there, but as in a dream. A character’s death — either in the Astral Plane or on the destination plane — causes no actual harm. Only the severing of a character’s silver cord while on the Astral Plane (or the death of his or her helpless physical body on the Material Plane) can result in the character’s true death. Thus, high-level characters sometimes travel to the Outer Planes by way of astral projection rather than seek out a portal or use a more direct spell." (emphasis added)

Contrary to the spell's entry, this seems to imply (again, strongly) that the Character's body remains on the original plane, and only a dreamlike version of their body travels along the cord and manifests in the outer plane, and that to die in this state does not result in the Character's actual death.

Given the fact that this is a 9th level spell, and can be undone with a simple Dispel Magic spell, it would seem that the latter understanding, given in the DMG, ought to be the correct reading, but it really does seem to contradict the reading in the PHB. I am wondering how others have handled this, and whether there are any clarifications given elsewhere?

^this right here. DnD Beyond staff, which is correct? Your body follows you through to the next plane when using Astral Projection, or your body stays on the native plane and you can move freely because you're using Astral Projection, as opposed to Gate or something?

Can your projection cast spells

I feel like the fact that this spell requires an absolute fortune of material components, is ninth level, and allows up to 8 characters implies that the Astral Plane is ridiculously dangerous. (8 level 17 characters?)

With this in mind, does that means that the whole bag of holding inside another thing is an almost guaranteed death sentence for lower level PC’s and lower CR NPC’s?

Quote from DragonRiderHN >>

I feel like the fact that this spell requires an absolute fortune of material components, is ninth level, and allows up to 8 characters implies that the Astral Plane is ridiculously dangerous. (8 level 17 characters?)

With this in mind, does that means that the whole bag of holding inside another thing is an almost guaranteed death sentence for lower level PC’s and lower CR NPC’s?

It depends on the DM. The Astral Plane does have terrors like the astral dreadnought (which can one-hit kill someone who's astral projected with a critical hit), the githyanki generally aren't friendly (and the knights have magic silver swords that can also one-hit kill someone who's astral projected when they crit with those swords), and you can have all sorts of fiends and celestials passing through since the Astral Plane is directly linked to the Outer Planes. The psychic winds are also an environmental problem since they can throw the party off course.

However, because the Astral Plane is a big empty space for the most part, the DM could have the party encounter few or none of these things and get to where they need to go without much trouble. The main reason to be in the Astral Plane (assuming the campaign isn't set there like in Spelljammer) is to get to the Outer Planes without needing a tuning fork for plane shift or needing permission to get into these planes from their rulers like with gate.

Here's how I adjudicated astral projection for my game, to reconcile the spell description with the DMG and answer lingering questions. The key for me is that, being 9th level, the spell should have a benefit over lower level planar travel spells like plane shift and gate.

Everyone affected by the spell remains in their astral form if they enter an outer plane. This makes characters effectively immune to death while visiting the outer planes.

• As a benefit, this is highly dependent on the DM respecting the notion that it's rare to encounter creatures than can sever the silver cord. If, as a DM, you're going to have your PCs encounter githyanki knights every single time they use astral projection (out of spite rather than story), this is no longer a meaningful benefit.

Entering any plane other than an outer plane brings the creature's physical body along to the new plane, but does not end the spell. Creatures under the spell's effect can re-enter the astral plane whenever they wish, leaving their physical bodies on the new plane.

• The spell description says that creatures return to their bodies when the spell ends, but not vice versa.
• The spell ends when the caster uses an action to dismiss it, or when dispel magic is successfully used on a creature affected by the spell. Ending it for the caster ends it for all affected creatures.
• If a creature using astral projection encounters and takes a ride on a spelljamming ship in the astral plane or an outer plane, they are considered to have entered the material plane once they enter any wildspace system, and thus their physical body will manifest.

The astral form replicates a creature's possessions. Using the astral versions of any mundane consumables (arrows, spell components) does not deplete their physical counterparts. Similarly, losing or destroying an item's astral replication has no effect on the physical item. When a creature re-enters its physical body, any astral replications left on an outer plane vanish.

• Any lost or stolen items are recovered when the creature returns to its body.

Astral replications of magic items draw on the magic of their physical counterparts through the silver cord. Using consumable magic items or magic item charges also depletes their magic in the physical item. Charges and other limited use abilities are recovered normally. 

• I decided that this was a reasonable restriction for purposes of balance. Characters should not be able to replicate powerful single-use items, or bypass long recharge times, simply by returning to their bodies and projecting again.
• Expensive consumable spell components can be replicated, but that's okay. It's a 9th level spell, after all.

Any objects in the possession of your astral form are transported back along the silver cord when you return to your physical body, including new items you claimed on an outer plane.

• Not being able to claim treasure while using astral projection is too heavy a price for the benefit.
• The spell establishes that physical items can travel along the silver cord from one plane to another, so why not?
• If characters in your campaign encounter other astral projections, take their stuff without killing them, then return to their physical bodies... figure that one out yourself. Or don't design encounters like that.

If you cast this while in an area protected by Forbiddance, does it fail?

Quote from DJP_15 >>

If you cast this while in an area protected by Forbiddance, does it fail?

For sure you can't enter the astral plane while within the area, so creatures can't leave their bodies. At the DMs discretion, the spell may be active, but creatures must leave the protected area in order to leave their bodies. Alternatively, it might simply fail. At my table, I'd rule it fails, just to keep it simple.