Machine rounded poles

[GAB]

PVC would be too light. The rail should stay put if a horse just brushed it. The rails typically sit in a shallow curved cup on the standards.

Yes, round will be more kind when hit. Also, IMO, removing the corners reduces the weight significantly. Both for the horses sake and the grounds crew. I've seen plenty of octagonal rails.

I would make a jig to sit on the log bunks to hold one or more 4x4s at 45°. Make the Vs deep enough to touch the full 4" face of the 4x4. Clip the point, rotate and repeat without having to adjust the head height. If need be, a screw through the jig into each 4x4 to hold steady for the cut.

Thanks for the reply, since my last horse day was 70 years ago I did not know that.
To saw octagons from square cant it requires "V" blocks that are all cut at the same time.
For a 4" cant I would recommend one "V" block for each bed rail.  You only need to use them on two sides then lay the piece flat on the bed rails and saw the last two corners.
For ease in use make sure your "V" blocks have exactly 1" between the bottom of the V and the flat side.
GAB

[Southside]

The other problem with PVC is that unless it's specifically UV treated it shatters into shards when it gets older or colder.  That could be ugly to a horse or rider if something goes wrong.  

PVC horse fencing was a thing for a bit what back in the '90s or '2000s, then those "boards" began to fail in the most horrible ways and that was the end of that. 

[Ljohnsaw]

Yeah, a guy I handy man for put in 3 rail PVC arena fence. He likes to let his horses wander the arena. The push on the fence to get to the grass on the other side. They keep snapping rails.🙄