Take a 4"x4"x8' (or longer) and have them "knock the corners off." They'll give you a funny look, but if you explain that they are for jumping horses (and the person is male), their eyes will glaze over. Men! :roll:
You "could" leave them square. We've had square rails before.
Steppin Not Dragon "Bella"
Top Shelf "Charlie" Check out the Military + Horses fb page!
What the real rails are called is "machined round", which means they are put into a machine that spins them and shaves around until it is exactly round and uniform in diameter. The average diameter is 3-1/2" to 3-3/4". Hope this helps.
Do you have a Lowe's store near you? In their garden department they have really nice 8' round rails that are PERFECT for jump rails at a fraction of the cost. The last time I bought them, they were about $6 - $7 each. Not all Lowe's stores have them though. Of the three that I have checked in a 30 mile radius to me, only one of them has it.