In sanctioned matches they limit steel shooting with rifle calibers to a minimum of 100 yards, shotgun 50 yards minimum. Reason is splash-back. Pistol caliber firearms can be as close as 10 yards but you will get some jacket pieces coming back. Best thing is to use either polymer coated lead or plated bullets but most places don't require that so its up to you. Up to you about how far you want to shoot but they arrived at those numbers after testing.
Home built Texas Stars are not easy. Not only do you need a channel that the base of the plate need to fit exactly into you have to have a spring on it that has the power based on the kind of ammo to be used, light spring for rimfire, heavy for centerfire to keep it in place as the star spins. Easier with a CNC machine to cut the plates and channels. Best thing to do it find one and take pictures and measurements. Our club, MPC, has both centerfire and rimfire Texas Stars plus both types of dueling trees also. Also have one thats like a single dueling tree plate that drops down a step and switches sides every time you hit it until it falls out the bottom.
Most dueling trees can be had with AR500 plates but the center post is usually soft angle iron. So using a rifle at any distance will result in craters in the center post and spall flying in all directions. Frangible ammo will still crater soft steel.
Here is one that I use as a reactive rifle target but not as a "dueling tree"
Its FULLY ARMORED and very heavy duty.
Front armor is 3/8 AR500
Mounts on a T Post so it can go anywhere....even on a hillside and it won't fall over.
Hang Fast Targets sells them.
