Poskim disagree about whether one may open a wine bottle that has a metal screw cap. Some prohibit opening it, maintaining that before the bottle is opened, the cap is simply a cover, but after it is opened and separated from the little metal rim that is left behind on the bottle, it becomes an implement, because it has become a screw cap that aids in opening and closing the bottle (R. Shlomo Zalman Auerbach).
However, most poskim are not concerned about opening such a cap. They maintain that even before it is opened it is already considered a bottle cap. Opening it does not create a new entity. Separating the metal rim from the cap is comparable to cracking open a nutshell in order to eat the nut. Additionally, the one who opens the bottle has no intention of making an implement; he just wants to open the bottle. Even though it happens to be that a screw cap is created, there is no prohibition.
Although one who wishes may be lenient may do so, and there are many who are lenient in practice, le-khatĥila it is preferable to defer to the stringent opinion and open such a bottle before Shabbat. An additional solution is to keep around old bottle caps and then immediately throw out the new cap after opening a bottle on Shabbat. Since in this case there is no intention to use the bottle cap that was opened on Shabbat, even those who are stringent would concede that one may open it.
If no old bottle cap is available and a bottle was not opened before Shabbat, another solution for those who are stringent is to puncture the bottle cap before opening it. This effectively ruins the bottle cap so that after it is opened it is not considered an implement (SSK 9:18). According to those who are lenient, it is preferable to open the cap without puncturing it.
Even most of those who are stringent regarding a metal bottle cap maintain that a plastic cap may be opened, as this is considered a bottle cap even before it is opened. Thus, opening a plastic bottle cap does not make it an implement (SSK 9:21). This is the custom. One may also use a corkscrew to remove a cork on Shabbat.
. The stringent opinion of R. Shlomo Zalman Auerbach regarding the removal of metal bottle caps is quoted in Shulĥan Shlomo (314:9:4-5). It is also recorded in SSK 9:18 and Orĥot Shabbat 12:17. Responsa Devar Yehoshua 2:45 disagrees with R. Shlomo Zalman Auerbach, based on Magid Mishneh 12:2, which explains that as long as one does not intend to create an implement, we do not consider it a psik reisha. Therefore, one may pour a large quantity of cold water into a very hot kettle. Since he does not intend to temper the kettle and thus apply one of the final stages of the metal forging process, it is not prohibited even though in fact he is doing so. MA 318:36, Bi’ur Ha-Gra 314:11, and MB 318:80 follow this position. Here too, one’s intention is to open the bottle, not to produce a bottle cap. R. Shlomo Zalman Auerbach disagrees with this position, maintaining that even though one’s primary intention is to open the bottle, he also wishes to produce a bottle cap. However, many believe that the bottle cap was considered functional even before the bottle is opened, but could not be used in practice because it was attached to the bottle by the metal rim. This is the approach of Tzitz Eliezer 14:45:1, Yeĥaveh Da’at 2:42, and Or Le-Tziyon 2:27:8; R. Mordechai Eliyahu ruled this way as well. If one intends to throw away the bottle cap, R. Shlomo Zalman Auerbach agrees that one may open the bottle (as explained by Devar Yehoshua ad loc.). If one needs the cap, R. Shlomo Zalman Auerbach allows him to puncture it and then open it, because by doing so he renders it less functional as a bottle cap. However, according to those who are lenient, since it is not necessary to puncture the cap, it is questionable whether one may do so. If it is considered an implement even before it is opened, it is rabbinically prohibited to destroy it. If it is comparable to a mustekei, it might be permitted to destroy it (even if it is not necessary). Those who take into account the opinion of R. Auerbach and his followers do so in an attempt to avoid a possible Torah prohibition.According to R. Shlomo Zalman Auerbach, one may remove the ring that remains on a metal or plastic bottle cap, since the cap is already functional without it. Others forbid this on account of Makeh Be-fatish. R. Yosef Shalom Elyashiv maintains that this is forbidden because of Meĥatekh (Orĥot Shabbat 12:19-20). It would seem that even those who prohibit this would agree that the prohibition is rabbinic.