Pat,
If you look closely, you will notice that receiver tubes are swaged for a certain distance from the end.
You can also feel where it steps up and down with your fingers.
This allows the really “square” (solid) hitches to fit into the receiver.
(Without, having to grind the corners off of the hitch.)
Now, if you just HAVE to use plain old square tube …
One “gentleman” that I worked for, made a foot long “sled” out of a piece of square bar.
The sled had seat for holding 2 square carbide tool bits, staggered and slightly overlapping. (He also staggered some!)
With a little grease on the solid sides, he used his log splitter to push it through the square tube to remove the weld seam.
Quicker than grinding!
Cheers,
jb