Tasca/Asuka makes perhaps the best injection molded .50 Caliber MG, they sell them as a set of two. Academy also has a nice set of American MG, including both .50 Cal and .30 cal.
I agree, I use the Academy set a lot and is the most cost effective. Tasca/Asuka is more expensive and difficult to find, although it will get more easy to get them since they changed over to Asuka and have more sellers. But the Tasca/Asuka sets are by far the best, giving you the same effect you would get with a turned brass barrel.