What is behind the brick? Is it a modern house with a brick veneer over a studded wall, or is it old-school, and the interior of the fireplace is the same stack of brick you see from outside? A good clue is if the firebox is metal, even metal with a thin layer of firebrick, and especially if (like on my 1978 fireplace, and most recent fireplaces), the frigging chimney stack is wood.
Not sure what would work on veneer, maybe lag bolts all the way into the studs. Unless I knew there were tie straps holding the brick veneer solidly to the wall, I'd be reluctant to put a lot of lateral load on what is basically a stack of bricks. If a traditional fireplace, a DEEP hole (at least 1/3 as deep as the part sticking out), and rebar epoxied into the hole, and allowed to cure before pushing the mantel over it. Back in stone age, my father got them to pick the mantel before fireplace was laid, and his mason spotted rebar J-hooks into the structure. This was mainly 'rustic' mantels back then, not the Norm Abrams fussy cabinetry style ones. Those old barn beam sections were not light.
The OP only mentioned brick behind the cue stick holder, he/she didn't say anything about the wall around and above the fireplace. That's why I asked what it was made of. Waiting for the OP to reply-------------
I'd use the regular lead inserts into the mortar. Use more for the mantle if it's really that heavy. I would not drill into the brick because if anyone's plans ever change it is a lot easier to repair the holes in the mortar.
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