Right size, but the copper might be a problem. Stick with pure soft lead for air.
NSA makes great pellets/slugs and are more than consistent enough to make for great accuracy. Don't forget the round balls for plinking. They allow quite a few more shots than the heavier slugs.
Enjoy the Texan. Probably the most adequate air rifle ever. Think 45 Colt from a carbine ballistics. Awesome.