Barrels of any sort / size are NOT good for making a pond. They are however good for making water features from, or filling with water and adding a few plants and wait and see what moves in, but pond, NO!
The main problem with using a barrel is its size or more to the point, lack of size. During the hot times all of the water can be heated which is no good for any wildlife, this also causes water to evaporate, when water in a barrel evaporates the level drops, so the timber that the barrel is made from dries out and so can leak, hence the need to keep them topped up, but if you have any wildlife in the pond you can't top up with neat tap water as the chlorine in tap water may kill any wild life.
If you really want a barrel water feature, the best option is to line it with a pond liner, or only put a few plants in it, never any fish.
I should also like to add, that the afore mentioned barrel website also offer barrels for water features with lining, I wonder why...........