Gemara

(Beraisa): "(Ba'Chalal) Cherev" teaches that a Cherev (sword) that killed a person is Tamei just like a Chalal (corpse).

54a (Mishnah): A Nazir does not shave for the following Tum'os:

Schachos (Safek Tum'as Ohel due to branches)... Chutz La'aretz; Golel (a coffin cover); a Revi'is of blood; a quarter Kav of bones; Kelim that touched a Mes, the days a Metzora counts between the two times he shaves...

We sprinkle (water with ashes of the red heifer) on him on days three and seven.

54b - Inference: Tum'ah was decreed on Chutz la'Aretz for towering above the soil. Haza'ah (sprinkling) would not apply to a decree for entering the air!

Rejection: They decree was due to the air. Not all these Tum'os obligate Haza'ah.

Support: The Mishnah lists Kelim that touched a Mes, which do not obligate Haza'ah!

Shabbos 101b (Mishnah): If a ship was tied with something that can hold it (and one end was in Ohel ha'Mes), it brings Tum'ah to the whole boat.

(Shmuel): This is if it was tied with an iron chain.

Shmuel requires an iron chain only regarding Tum'ah, for it says "Ba'Chalal Cherev" - a sword is like a corpse. Regarding Eruvin, even a drawstring suffices (to make them one Reshus).

Chulin 123a (Beraisa): If a troop on the way to war entered a house, the contents are Tamei, because a troop always carries skins of heads (for witchcraft).

Bava Basra 19b (Mishnah): If a box full of figs is in a window, and the figs could stand there even if the box was removed, the figs block Tum'ah from passing through the widow.

20a - Question: If the barrel's opening faces away from the Tum'ah, the barrel itself blocks Tum'ah! Cheres (earthenware) acquires Tum'ah only from its interior!

Answer: The case is, the barrel is made of metal.

Ohalos 1:3 (Mishnah - R. Akiva): If (a Mes is in an Ohel and) a spit is stuck in the Ohel, they and one who touches the spit become Tamei for seven days...

Rishonim

Rambam (Hilchos Tum'as Mes 5:3): If a Keli became Tamei by touching a Mes or being in Ohel ha'Mes, a person or Keli that touches it is Tamei for seven days, like one who touched a Mes. "Ba'Chalal Cherev Oh v'Mes" teaches that a sword is like a Mes. The same applies to all Kelim, of metal, Klei Shetef (that can become Tahor, i.e. anything except for Cheres) or garments. ("Kol Horeg Nefesh v'Chol Noge'a be'Chalal (becomes Tamei Shivah)." Surely, one who shot an arrow and killed is not Tamei! Rather, it discusses one who killed with a sword or something similar. He touched a Keli that killed and touched a Mes.

Ra'avad: Only a sword (is Metamei others for seven days), or b'Chiburin (one touches a Keli while it touches the Mes). All the verses discuss b'Chiburin.

Kesef Mishneh: The Rambam holds that even if a Keli (did not touch a Mes, but) was in Ohel ha'Mes, it is Metamei people for seven days through touching, but not through Ohel.

Rambam (13): Even though clothes that touched a Mes are like a Mes to be Metamei those who touch them for seven days, they are not like a Mes to be Metamei through Ohel and Masa.

Ramban (Bava Basra 20a): R. Tam asked how a metal barrel can block Tum'ah. A sword becomes like a corpse even (without touching,) through Ohel ha'Mes (Ohalos 1:3). The barrel itself should bring Tum'ah to the other room! Perhaps there is not a Tefach above the barrel in the window, so the Tum'ah goes straight up. Alternatively, the figs are pressed over the entire opening of the barrel and block the Tum'ah. Some say that a sword is not Metamei b'Ohel. Bamidbar 19:16 mentions Tum'os, including a sword. We expound (53b) this verse to discuss things with Tum'as Ohel. Verse 18 mentions the same Tum'os, except for a sword. We expound (54a) verse 18 to discusses (Haza'ah for) things with only Tum'as Maga. This is like R. Tam.

Ramban (ibid.): Some reject the proof. Verse 18 omits a sword, lest we expound that Haza'ah does not help (it has no Taharah). All Kelim become like a Mes. "Sword" excludes only Cheres, which cannot be an Av ha'Tum'ah.

Question (Mishneh l'Melech 3 DH v'Da): We already know that a Mes cannot make Cheres an Av ha'Tum'ah! "Veha'Nefesh ha'Noga'as Titma" applies only when "v'Chit'o" (Taharah) applies (Rashi Eruvin 104b DH Lo). "V'Oso Sishboru" (Vayikra 11:33) teaches that Cheres cannot become Tahor.

Ramban (ibid.): The Sifri expounds about a sword, and also "Bigdeichem." This connotes that all Kelim are the same. However, perhaps it refers to metal garments, e.g. armor. The Tzitz is called a Beged. We are concerned for Tum'as Ohel from a troop due to a head (Chulin 123a), but not due to swords. Perhaps this is when we see that they have no swords. However, the Mishnah (Ohalos 1:2,3) discusses Stam Kelim, implying that all Kelim are the same.

Tosfos ha'Rosh (Shabbos 151b DH Koshrah): Rashi says that the other end of the iron chain was in Ohel ha'Mes; it brought Tum'ah to the ship. This has nothing to do with Eruvin! Why did we need to say that the ship was tied? The Rashbam says that Tum'as Mes was under one side of the ship. The chain held the ship rigid, so underneath the whole ship became Tamei. (A moving Ohel doesn't bring Tum'ah.) He deletes from the text "Ba'Chalal Cherev." Why is Eruvin different?! R. Tam says that the chain was tied to the ship wall (on bottom). The chain needed to tie the boat down (and to be of metal) to be Metamei the entire bottom.

Question (Tosfos Nazir 54b DH Ta): How did the Gemara prove from Kelim that Haza'ah does not apply to all of the Tum'os? Perhaps it refers to metal Kelim, which obligate Haza'ah!

Answer #1 (R. Tam): It cannot refer to metal Kelim. A Nazir must shave for them, for we expound that 'a sword (that killed someone) is like a corpse.'

Objection (R. Chaim Kohen): Kohanim are commanded about any Tum'ah for which a Nazir must shave. A Kohen cannot ensure that his house not contain a nail or metal Keli that was in Ohel ha'Mes! A Tosefta (Ohalos 1) says that Kelim Tamei through a Mes impart Tum'as Shivah for Terumah and Kodshim, but a Nazir shaves only for a Mes, i.e. but not for Kelim. The Tosefta discusses metal Kelim, for only they impart Tum'as Shivah. The Gemara did not say that the Mishnah discusses metal Kelim, for in any case not everything obligates Haza'ah, e.g. days that a Metzora counts. Our Sugya refutes Rabbeinu Yitzchak me'Simpont, who says that any Keli that touched a Mes is like a corpse!

Defense (of R. Yitzchak - Tosfos): When the Gemara said 'Kelim do not obligate Haza'ah', it meant 'what follows Kelim in the Mishnah', i.e. days of a Metzora...

Poskim

Rema (YD 369:1): Some forbid a Kohen to become Tamei to a sword that became Tamei through a Mes, and some are lenient. The custom is to be lenient.