If I have a bag of Holding with approx 3000 arrows (rough math low balled) how quickly would the arrows be expelled, and how much damage would the target infront take when the bag is flipped inside out.

Bbeg final move absolute last resort players want a challenge 

The bag is on the head of a modified arrow so they'd have some velocity

If you flip the bag inside out it expels it contents sorry I wasn't clear at first

It's also on a modified arrow i apologize I didn't and that initially 

So, let me see if I understand the concept... the bag itself is affixed to an arrow that's been modified to be able to fire and then invert the bag of holding? I still don't think there's anything in RAW that says the contents of the bag would be expelled at the same speed as a traditional arrow. It's kind of a cool way to dump 500 pounds of arrows on someone from a distance, but I don't feel this would work the way you seem to intend without your DM just letting you get away with that. Either way, they're not precisely aimed, it would be quite a feat to be able to load all the arrows in the bag so that they all spill out arrow-point out. Your plan seems to be based on the idea of treating it as firing 3000 arrows simultaneously at a single target, which aside from being horribly unbalanced conceptually... even ignoring real-world physics and just going off in-game rules... there's nothing in-game that implies the arrows would spill out in that manner. Really the main benefit is just the sheer weight of the bag's contents.

That said, I looked up some recommendations on improvising damage for something like this. A site I found recommends an object of 200 pounds or more falling 10 feet would deal 20d6 damage. I think that's how I would cap that... since it's being fired from a modified arrow I'd just make it an attack roll... if you miss, the bag opens up behind them and just spills a big mess of arrows on the floor.

Umm ... don't the arrows just fall out on the ground?

They aren't directed. There is no directed energy towards the points of the arrows. They are just sticks with pointy tips released into the air where they just fall to the ground. There is no force of release.

"If the bag is turned inside out, its contents spill forth, unharmed, but the bag must be put right before it can be used again."

I would tend to interpret "spill" to mean they just drop out as you invert the bag. 20 arrows weigh 1lb ... so you are looking at about 150lbs of arrows with 3000 of them. However, each is a separate item so dropping 3000 arrows on a person isn't the same as dropping a 150lb weight on them.

Anyway, it is entirely up the the DM if there is any damage at all. Personally, I would tend to rule it as doing little or no damage (maybe 4d6 and most of that would be scratches) and being very difficult to target. It might surprise someone though when a pile of arrows spill out of this airborne bag and pile on top of them.

Umm ... don't the arrows just fall out on the ground?

That is how I would rule. There is nothing about a bag of holding that suggests a high velocity expulsion. They would fall at the speed imposed by gravitational acceleration, and I'm not sure there's anything that would guarantee they all fall pointing the same way.

Ah, looks like there was some duplication, so here's my response to the other thread:

 Though, with respect to the physics of 3000 arrows. I believe their velocity is constantly 0, the extradimensional space just gets smaller, so the most realistic scenario is that the arrows would attempt to occupy the same space on the material plane, building up pressure and slowly "bursting" outward in an "aimed" hemisphere. More like expanding foam than an explosion.

The overall pressure would be relatively low, and they would all interfere with one another, so the majority of them would be flying sideways or spinning before landing on the ground.

Assuming the bag is able to open at height, it would require the shot to be made to at least a couple hundred feet in the air. This will allow the arrows to align point down and accumulate velocity from falling. 

Figuring out the effects of that is too hard for me right now. How many arrows can come out of the bag per round? 

I would either give it a large AOE or do a pick a spot, roll 1d6 for direction, 1d6 for distance from that spot for shot placement of multiple 5' effects.

I realize your DM already said ok, but I wouldn’t even let you do it. A bag of holding weighs 15 pounds. There is no way to modify a 1pound arrow to carry that load with any sort of velocity. Seems like you’d need a ballista to fire something that heavy. 
For reference, an Olympic shot put weighs 16 pounds, the same as the combined weight of the bag and arrow. 

If the arrows don't have coverings over their points, then the bag of holding has already exploded from being punctured.

"How" to puncture a Bag of Holding is kind of interesting. Externally, it's easy, you just stab it. However, internally, it's an extra-dimensional space that has volume, but no dimensions, so the bag should conform perfectly to the items within it. No pressure means no puncture.

If overloaded, the bag ruptures regardless of sharpness.

Rangers have access to a level 5 spell called Conjure Volley that involves hundreds of arrows being fired against a group of enemies at once. So you could always just replicate the effects of that spell for this attack.

Try it like this.

1. Familiar carries a bag of holding full of Flasks of Alchemist Fire and/or Acid (switch to holy water where appropriate)
2. Familiar flys over bad guy.
3. Familiar turns it inside out.
4. Alchemist fire/Acid/holy water fall to the ground and break.

I feel like this whole topic has been going Monty Python. Its not a question of where the familiar grips it. a 5 oz familiar cant carry a 15lb bag of holding. lol