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

Very interesting reading, thanks for the input.

Well, a 300 grain bullet would be awfully long (since it can't be bigger around). That puts the base a lot farther into the case, which ups the pressure quite a bit. Now, I hadn't heard of a 230 grain 10mm bullet before. The largest I'm aware of is a 200 grainer. You would definately be on the cutting edgewith that load. Of course, most folks on that cutting edge get cut, so be careful. <g>

Don't feel left out; until I did some research for this thread, I hadn't heard of a 10mm 230 grain load either.
It must be real, though, because MidwayUSA is selling it in two different flavors:

JHP:
DoubleTap Ammunition 10mm Auto 230 Grain Equalizer Jacketed Hollow Point Box of 50 - MidwayUSA

Gas-checked lead WFN:
DoubleTap Ammunition 10mm Auto 230 Grain Lead Wide Flat Nose Gas Check Box of 50 - MidwayUSA

Wow DJ--Thanks!I didn't know they existed and was ready to throw the BS towel in on this one. That's gotta be a long bullet. The 200 grain scares me a little.

As far as Glocks "exploding" with unjacketed lead rounds, Glocks rifiling is such that lead builds up in the grooves. You can buy an aftermarket barrel from LoneWolf or one of the other brands and shoot lead rounds with no problem. I think I will keep the 180 gr as my top load with my G20. It should handle a Zombie attack just fine.

That's the popular myth Tex. If ya read Chapter 4 of "The GLOCK in Competition" he does an outstanding analasys of the issue. It's not the polygonal rifling that causes the problem!