The Poskim have disputed the halacha regarding opening bottles with a metal cap, such as is common on bottles of Kiddush wine or grape juice. Many Poskim forbid opening such a cap for the first time, since it only served as a lid before it was opened, whereas afterwards, once it’s been opened and detached from the thin metal strip that holds it in place, it has become a useable kli – it is now a stopper which can be used to open or close the bottle.
However, many Poskim permit opening the metal bottle cap, claiming that it was considered a useable stopper which could be opened and closed even before it is actually opened for the first time. Therefore, by opening it one does not make it into a new kli. These opinions claim that detaching the cap from the metal strip is akin to breaking a nutshell in order to get the nut out. Additionally, since nobody intends to make a new stopper when opening the bottle – they are merely opening the bottle – there is no prohibition to open the bottle even though this will certainly result in a newly useable screwable stopper.
Even though many have the custom to rely on the lenient opinions, it is ideally correct to follow the stringent opinions and open these types of bottles before Shabbos. If one forgot to open such a bottle before Shabbos, one may open it on Shabbos if he will throw it out immediately or if he will use a different cap (from an old bottle) instead. Another permissible method of opening it is to pierce the cap before opening it, making a big hole. By doing this one has not made a useable kli – since the cap has a big hole in it one would not consider it a good useable utensil. However, if one follows the lenient opinions mentioned above, it would actually be better to open the bottle without piercing it first.
Most of those who forbid opening metal bottle caps would still agree that plastic bottle caps may be opened on Shabbos, since they are considered useable stoppers with which to open or close a bottle even before they are ever attached to the bottle. Therefore, opening the bottle for the first time does not change the caps’ status at all. The minhag is to allow opening these types of bottles on Shabbos. One may also open beer bottles, even using the opener specifically meant for opening them. One should be careful not to erase any lettering on the cap.
Notes and Sources
 This is either the melacha of metaken mana (making a kli) or makeh bepatish (finishing off a kli – literally, the ‘final hammer blow’). Rav Shlomo Zalman Auerbach in Shulchan Shlomo 91,12, Shemiras Shabbos 9:18, Rav Moshe Feinstein in Rivevos Efraim Orach Chaim IV 96. Rav Elyashiv held it was also a transgression of the melacha of mechatech (cutting to size).
 Even Yisrael. Be’er Moshe III, 90, Devar Yehoshua 2:45, Tztitz Eliezer 14:45,1, Rav Ovadia Yosef in Yechave Daas II 42, Ohr Letziyon II 27:8
 Shemiras Shabbos 9, note 66. Devar Yehoshua concludes that Rav Shlomo Zalman Auerbach agrees that if one intends to throw out the cap then it is permitted to open it on Shabbos.
 Shemiras Shabbos 9:18
 Since there is no need to pierce it one could question if it is allowed to make a hole in the cap. If the cap is considered a kli already (as the lenient opinions understand), then to destroy it (by making a big hole in it) would be rabbinically forbidden.
 Shemiras Shabbos 9:18
 Mishnah Berurah 314:2, 17
 Mishnahr Berurah 340:41