On a side note, here what we do in my game when a player whant to throw a large quantity of alchemical components bound in a single bag.
RULE CLARIFICATION :
Player1 - I throw my Alchemical bag at the bad guy !
DM- All of It ?
Player1 - Yes, and Player2 will then throw a fireball at it !
Player2 - Right on ! There must be about twenty alchemist fire in that bag ! Its gonna blow big time !
DM - Ok, let me figure the various save and rules to resolve this. (Dam, forgot they had so much stuff )
Player1- Don't forget to take in account the five acid flask, three thunderstone, two tanglefoot bag, fifty tintertwig and eight smokestick also in there.
DM- Man, this is going to take some time?
What happen when whole bag of explosive substances are throw at the face of the enemy ? Pure game mathematics suggest you resolve each attack independently without consideration of the whole. Although game wise this may sound accurate, it is time consuming and somewhat unrealistic, as a larger quantity of dangerous material usually make a greater effect than a single once of it. Here is some clarification and house rule to simulate the circumstances when the player ?Throw the Whole bag? at a target.
1- Distance : Bag and container usually don?t have great aerodynamics qualities, otherwise arrow would be bag shaped. Usual bag or container (Backpack, sack, jug, chest, saddlebag, etc) will have a reach increment of 5 ft. Bag with a long strap that can be whirled for greater momentum (haversack, belt along with belt pouch, bandolier, satchel charge) have a 10 ft reach increment.
2- What?s in the bag : Try to eliminate any component of the bag that will be destroyed by explosion or mixing of content. Acid will be destroyed by almost any other liquid, thunderstone won?t explode if not throw by themselves against a hard surface, holy water will be spoiled by acid, etc. The winner of any explosion are usually gunpowder and alchemist fire. If you want to use real chemistry notion and precise physic in your game that?s fine by us, but for a quick rule, simply assume that the substance present in the largest amount in the bag will decide of the final effect of the impact, all other substance in the bag being lost in the blast
3- Effect ? For Damaging item (Acid, Holy water, Alchemist fire, ect) each time you double the amount of item present, add one time its damage value and 5 ft to the spread of the area it affect
Example ?
1 Alchemist fire - 1d6 5 ft area
2 Alchemist fire - 2d6 10 ft area
3 Alchemist fire - 2d6 10 ft area
4 Alchemist fire - 3d6 15 ft area
5 Alchemist fire - 3d6 15 ft area
6 Alchemist fire - 3d6 15 ft area
7 Alchemist fire - 3d6 15 ft area
8 Alchemist fire - 4d6 20 ft area
16 Alchemist fire - 5d6 25 ft area
32 Alchemist fire - 6d6 30 ft area
Status inducing Items (Thunderstone, Tanglefoot Bag)
Use a progression similar to Damaging item, Increasing save DC and duration accordingly (each time the amount present is doubled, add one to the save DC and one unit of time to duration (either being it round, minute, hour or day)
DM - The bag explode with a mighty bang, sending fire and various alchemical substance all around, throw 5d6 additional fire damage in that fireball ! 25 ft of smoke also fill the area
Player1 - What about the other stuff I had in the bag ?
DM- The Thunderstone did not break, as you must throw them hard against a surface and the bag cushioned their landing, the acid was to thinly spread by the explosion to cause any effects and the tanglefoot bag where incinerated by the fire once they burst. Sorry.
Player 2- Nah, its cool, Those 5d6 where totally worth it and we can collect the stone afterward. Teach us for throwing all our stuff without consideration.
While the above may not help with the specific of a barrel of alchemist fire, it may be usefull for your average alchemical bag.