As far as I know, MG ammo is still measured by rounds, not length. You might carry a 250 round "belly bag" of 7.62 mm ammo, or a belt of 100 rounds of .30 caliber. I never heard of someone asking for, say, 3 meters of 7.62. Someone might have done so, jokingly, in the same way we once went to the store and ordered a "yard of rope sausauge."

Speaking as a former grunt (and I'm sure airmen felt and feel the same way), I'd much rather know that I had 200 rounds than 5 yards of bullets remaining.

The expression could have come from the ammunition industry, back when cloth belts instead of "disappearing links" were used to MG ammo, I suppose.