I can only think of three ranges off the top of my head that are anywhere near Dallas and allow you to shoot rifles up to 300yds...
TDSA - They have a 100, 200, and 300yd. berm and 12" steel gongs at each berm. It is a private club; but the rates are very reasonable. Unfortunately, it is geared primarily towards pistol and action shooting and the rifle range is more of an afterthought. There are only a few positions available for rifle shooting; but there is rarely anybody using them anyway - bring everything you need with you.
The range itself is in a flood plain and while the pistol bays have a deep drainage trench dug through them, the rifle range regularly floods when it rains. You must also pass a test to be qualified to use the rifle range. the range is about 30 minutes south of downtown Dallas.
L.T. Dillon - This range is in Waco and is two hours away from the 635 and Tollway junction. It is a beautiful range with positions available from 200yd to 1,000yd. Not much in the way of facilities; but nice ranges. Up to 45 firing positions.
Tac-Pro Shooting Center - This range is a bit south of Mingus (on the west side of Ft. Worth) and is also two hours away from 635 and the Tollway. It has a 1,000yd berm and all shooters must move the firing line forward or backward to adjust ranges from 300-1,000yds. It has a separate rifle range with 100yd and 200yd berms in addition to that. This is my favorite of the three; but it is a long drive. It also has better facilities than the other two.