There are a lot of methods of making barrels for Necromunda. The easiest, but also most expensive, solution is to buy the accessories sprue from Games Workshop. Then there are the alternatives....

Coke-lid barrels

The general idea is to take two lids of a more-or-less appropriate diameter and glue them together, preferably with a bit of sand inside one of the lids. Kevin Jones (Table Top Terrain) has embellished on the idea. He suggests using a bit of electrical tape around the barrels, thereby creating a border between the lid and the "body" of the barrel. Great tip, Kevin!

Balsa-stick barrels

For this idea, I have used lids made of the metal inside the candles (where the wick is fastened) used for warming tea. (Danish: Fyrfadslys). Then I have bought a balsa stick of the same diameter and sawed off about 2 cm balsastick for each barrel. The barrels are very straight, but about the right size and one balsastick yields lots of barrels.

Plumbing barrels

This idea originates at Terra Genesis. In general, the idea is to look for a piece of plumbing pipe that is the appropriate size for a barrel and then create a lid. Gary James suggests using shields and pipe of approximately 1,5 cm diameter. Personally, I happened to have some shields from my Skeleton Warriors - and they had the same diameter as a fitting for an antenna...

Coin barrels

I don't remember where I saw this idea first, but in general, the smallest coin of the currency is almost worthless - and almost the same diameter as a barrel should be. So a stack of small coins will make a nice barrel if covered on the sides with electrical tape.