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it fires a 5.56 by 51 milimeter standard NATO round.
no... the m-16 and m4 are 5.56 x 45 NATO. 5.56 x 51 doesnt exist. its a 7.62 x 51 NATO round that an m-14 fires.
25.4 mm = 1 in
→ 62 mm = 62 ÷ 25.4 in ≈ 2.44 in
The greatest common factor of 62 and 51 is 1.
The standard NATO cartridge is the 62 grain 5.56x45 SS109.
Looks like 51 and 62 are missing...