Mundane Detail: Barrels

Barrels is licensed under CC-BY 4.0. 

Barrels are the most versatile containers, they can be used for liquids or dry goods. They come in many sizes, from humongous to very small. Both the term barrel and cask are used to refer to these containers in general, irrespective of actual volume.

The 5e PHB has a barrel that holds 40 gallons, has an empty weight of 70 pounds and costs 2 gp in the equipment list, the DMG lists the same as a cask. This is a good all-round size to use. I call this a barrel.  

The following list of barrel names and sizes is is relatively close to the historical standardized volumes (a more complex matter when you look into it), but follows a simple mathematical ladder in which each size is twice as large or small as the next and contains a round number of gallons volume. In a medieval fantasy setting, barrels are made manually so they obviously could be of any size. 

Name         Volume         Weight              Relative   Price (empty)

Barrel         40 gallons     70 (390)  lbs                       2 gp

Tun            256 gallons     448 (2,496) lbs  1 tun        

Butt           128 gallons     224 (1,248) lbs  1/2 tun     a.k.a. Pipe

Hogshead    64 gallons     112 (624) lbs     1/4 tun     

Cask            32 gallons     56 (312) lbs       1/8 tun     

Keg              16 gallons     28  (158) lbs      1/16 tun   a.k.a. Kilderkin

Firkin            8 gallons     14 (78) lbs         1/32 tun   

Pin                 4 gallons     7 (39) lbs           1/64 tun   

There are also these, which do not fit into the doubling/halfing ladder:

Puncheon    85 gallons     150 (830) lbs    1/3 tun     a.k.a. Pon, Tertian, 

Tierce          43 gallons     72 (416) lbs      1/6 tun      very close to Barrel

Roundlet     18 gallons     32 (176) lbs      1/14         a.k.a. Kilderin (similar size)

Weight is shown empty, in prarens if full of liquid. The weight of the empty barrel in pounds is about twice the number of gallons minus 10%. Each gallon weighs about 8 pounds if the barrel is filled. 

The DMG has a gunpowder keg that weighs 20 lbs, filled with gunpowder. The density of gunpowder varies but it generally is about one and three quarters times as heavy per gallon as water, so a keg of that weight would need to be smaller than a pin, holding only about one to two gallons of powder, a small fraction of the volume of an actual keg.

Prices for full barrels

The bulk prices for typical alcoholic drinks, as inferred or interpolated from the PHB are, per gallon:

Beer/Ale             16 copper

Wine                   32 copper

Mead                  64 copper

Brandy/Spirit: 128 copper

Good Wine:    1600 copper (bulk price of 1/3 single bottle in a inn price).

Wine has a huge price range -- even the lowered price for good wine is 50 times that of normal wine. You essentially can just make up whatever price you like for wine.

Beer and normal wine are cheap and therefore mostly come in larger barrels. I would treat alcohol as trade goods, fetching 80% of the value if sold.

A full barrel of beer would cost 8 gp and 4 sp. 

A full barrel of wine would cost about 14 gp and 8 sp.

Brandy is expensive and thus typically comes in smaller barrels. A firkin of brandy would cost about 12 gp (depending on what cost you use for the firkin, I use 1 gp as it is a lot less material, but there is a minimum effort to build it that is not just the material). 

Good wine is very expensive, a tun of good wine would be worth over 4,000 gp. It might pose an interesting challenge for transporting this treasure at the eponymous weight of about a ton.