I've been looking for a range too. There are some around but I don't know anything about them because I haven't had time off from work to check them out yet. Red's Indoor Range was mentioned, go south on I-35 and take the exit for 1825 then go east for a few miles, it's on the left. You said you want >100 yd rifle range so indoor might not be good enough.
If you go south on 195 between Killeen and Florence is a range but I don't know anything about it. From Florence go west to 183, on 183 is Tony's gun range, I don't know if he has long distance or not. I heard there's a place in Copperas Cove but haven't been able to find it.
There's also a place north of Killeen that has several outdoor ranges including long-range rifle. It's about 60 miles up 317, but if Tony's or the range on 195 don't have what you need it might be your best choice. There's a website, I think it's carlchandler.com
If anybody else knows about the range in Copperas Cove please post the info here.
Thanks