While M855 uses a 62gr bullet, it is a longer bullet due to the steel penetrator in the front of the bullet core. Steel is less dense than lead, so more volume of steel is needed to end up with the same weight (mass). There is also a small air cavity in front of the penetrator, unlike a bullet with a solid lead core. Any non-M855/SS-109 62 grain ammo (such as Wolf and Federal's American Eagle 62gr FMJ offerings) will have a solid lead core, and the resulting bullet will be significantly shorter than an SS-109 bullet. That means you can expect trajectory and penetration performance to differ as well.
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