Well, that's what I consider a problem. What if instead of AR-15s Texas issued shoulder fired rocket propelled grenades? Are you saying there couldn't be an amendment that limits that particular state right?

Oh, there could be an Amendment that did that. But there isn't presently. And unless there's a federal prohibition on possessing rocket propelled grenades (I assume there is, but I don't know for sure), then each state is free to decide for themselves whether to allow their citizens to own rocket-propelled grenades.

Again, our country is a bit of a kludge, as far as nations go. We're a hybrid between a collection of sovereign countries that are bound by a common treaty area (like the EU) and a single nation. The States are sovereigns - they are not political subdivisions of the national government, and they do not derive their power from the national government. They have transferred some of their sovereign power to the federal government and have subordinated themselves via the Supremacy Clause - but in areas where the federal government has not exercised its power (or where it doesn't have power), the States are the final and supreme sovereign.