I decided to see what R’ Falk from Gateshead had to say on the subject. I was quite surprised with the psak. This is taken from his sefer Zachor V’Shamor (almost verbatim).
Opening Beer Cans
To open a beer can (that has a pull ring on the top, which when pulled cuts a neat oval shaped opening into the top of the can) is not Koreya (ripping) since this melacha does not apply to stiff items. Mechatech (cutting into shape) is also not involved because Mechatech only applies when one is changing the overall size or shape of the item, whereas to cut a small hole into an item which does not change its overall shape, is not considered Mechatech.
However, large beer cans are not emptied in one go, as one usually stores part of the drink inside them for later use. To open them is therefore considered as making a container, according to the poskim who maintain that if an opened packet or can is used to store part of the contents it is considered a kli (a container).
The poskim who maintain that this is not considered as making a container would not forbid the opening of such a can, although this appears to involve the making of a Pesach (opening). This is because the beer can has a comparatively small hole, and it is therefore unlikely that a person will pour any of the liquid back into the can. Due to this some people are lenient, and open these cans in the normal manner on Shabbos.
Small beer cans are in a somewhat more favorable position than large ones because they contain no more than one glassful of drink and it is common to pour out the complete contents of the can immediately after opening it. When this is the case, the can serves no function of storage whatsoever and according to all opinions, Maake B’Patish would not be involved. However, it is also common to drink from such cans with a straw, which means that the opened can stores the beer until it is drunk. When this is the case, there is no advantage in the smaller cans over the larger ones.
***JayMatt19 note – at a different point, R’ Falk mentioned that the (according to the ones who hold the large cans would be assur), this is due to the “normal usage”. Therefore even if you intend on pouring out the entire contents immediately, it would nevertheless still be forbidden by these authorities. ***