Texas doesn't have a minimum caliber for muzzle loaders. We also do not have a minimum caliber for modern guns either, as long as it is centerfire. It's one of those things of, yes, you can legally hunt deer with a .25 ACP, but that doesn't make it a wise choice, so you have to use your head and choose carefully like with anything else. The smallest I have seen used was a .45, but have heard of .40s being good too if you pick your shots and place them exactly where needed. Havinf said that, I use .50s and .54s, because the deer deserves respect and to be taken as humanely as we can possibly do it. I have always been told to consider the .45 as a .243 with a 75yd effective range, and a .40 as a .223 with a 50yd range. Not that these calibers won't do it at longer ranges, but I will stick with my .54 as my most used ML rifle and my .50 as second most used.