We know that Setirah (demolishing) for the sake of building is a Melachah
d'Oraisa, and Setirah not for the sake of building is forbidden
mid'Rabanan. Why, then, should it be permitted l'Chat'chilah to break open
a barrel on Shabbos?
(a) The RASHBA explains that Rashi follows his opinion elsewhere (47a, DH
Chayav Chatas), where he says that the Melachos of Binyan and Setirah
(building and demolishing) do not apply to utensils. They apply onto to
structures that are attached to the ground. Therefore, if a person makes a
utensil on Shabbos, he is not Chayav for Boneh; rather, he is Chayav for
Makeh b'Patish ("the final hammer-blow"). *Breaking* a utensil, therefore,
is not forbidden, because Makeh b'Patish has no destructive counterpart.
(b) TOSFOS (DH Shover) argues and maintains that the Melachos of Binyan and
Setirah apply to utensils. The only time that it is permitted to make or
break a utensil, according to Tosfos (102b, DH Hai), is when that utensil
is *made to be* assembled and disassembled. Why, then, is it permissible to
break the barrel of our Mishnah?
Tosfos explains, based on a Gemara in Beitzah (33b), that the Gemara here
is referring to a barrel that was broken and later fixed by gluing together
the broken parts. Since the barrel is no longer considered a complete
utensil, it is permissible to break it open to obtain its contents. Any
other barrel, though, may not be broken on Shabbos.
(c) The RITVA explains that breaking a barrel should actually be prohibited
mid'Rabanan, because it is an act of Setirah without intention to build.
The only reason it is permitted in our Mishnah is because the Rabanan
permitted it for the sake of Oneg Shabbos (since one wants to use the food
inside the barrel to enhance his Shabbos pleasure).
(d) The YERUSHALMI here, cited in SHITAH L'RAN, explains that opening a
barrel to get at the food inside is comparable to opening the shell of a
nut to remove the nut inside. Since the barrel is the normal container for
the food inside, opening it is not considered Setirah. Rather, it is
considered as though one is merely taking food out of its container.
However, it is permitted to open plastic bags and containers that contain
food because those containers are made for temporary use and thus they are
comparable to a broken barrel that was repaired with glue, which is not
considered a real utensil.