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Yep as djw mentioned.  Stands for "Open Tip Match" bullet.  OTM for short.  Not to be confused with Hollow Point although I guess you could say they look like hollow points.

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What's the difference between HP & OTM?

As previously described, the MatchKing is a boat tail, ogival spitzer tip bullet with open tip. The "open tip" is a shallow aperture (approximately the diameter of the wire in a standard size straight pin or paper clip) in the nose of the bullet. While sometimes described as a "hollow point," this is a mischaracterization in law of war terms. Generally a "hollow point" bullet is thought of in terms of its ability to expand on impact with soft tissue. Physical examination of the MatchKing "open tip" bullet reveals that its opening is extremely small in comparison to the aperture in comparable hollow point hunting bullets; for example, the 165-grain GameKing is a true hollow point boat tail bullet with an aperture substantially greater than the MatchKing, and skiving (serrations cut into the jacket) to insure expansion. In the MatchKing, the open tip is closed as much as possible to provide better aerodynamics, and contains no skiving. The lead core of the MatchKing bullet is entirely covered by the bullet jacket. While the GameKing bullet is designed to bring the ballistic advantages of a match bullet to long range hunting, the manufacturer expressly recommends against the use of the MatchKing for hunting game of any size because it does not have the expansion characteristics of a hunting bullet.

The purpose of the small, shallow aperture in the MatchKing is to provide a bullet design offering maximum accuracy at very long ranges, rolling the jacket of the bullet around its core from base to tip; standard military bullets and other match bullets roll the jacket around its core from tip to base, leaving an exposed lead core at its base. Design purpose of the MatchKing was not to produce a bullet that would expand or flatten easily on impact with the human body, or otherwise cause wounds greater than those caused by standard military small arms ammunition.