I've never heard of a 300 grain bullet or load in 10mm caliber; the heaviest I've heard of was a 230 grain bullet. Based on my experience and a little research, I don't think it's possible to safely load a 300 grain jacketed bullet in this caliber to any decent/useful velocity. Even if a safe load powder charge could be found to drive such a heavy bullet, the bullet itself would have to be specially designed with a tapered base to prevent the case from bulging when the case web begins to thicken, which would cause difficulties in chambering.
For an idea of some of the variables involved in making/loading such a heavy bullet, I'd recommend reviewing this discussion thread over at Glock Talk:
anyone intrested in heavy 10mm bullets, read this and give me some input. - Glock Talk