Post Rail Fence..help!!!

Driveway 80m with a few awkward angles on sloping hilly ground ... In building P&R fence using 1.8m 150-174mm Half Rounds, with 2 [6m] Rails at 150 x 50, and 6 strands of wire .. and if there's a few corners..

QUESTION.. Is it OK to use the ½ Rounds at the ends, angles and corners? Will they be strong enough to take the tension? On say 25 - 40m lengths?

With the amount of shrinkage and expansion of the rails when they are wet and dry, I would not consider nailing the rails to the posts, even at 3 meters apart. The timber movement loosens the nails. And even at 3 meters post distance the rails will warp, again causing the nails to loosen.
Our driveway had no fencing when we arrived, so I put up 7 wire fencing. The sheep immediately got out of these fences so I added an extra low wire. They still got out so I put battens (droppers) 900 mm apart. They could still get out, even with regular wire tightening and batten staple hammering. Now most of my great 7 wire fences have sheep netting over them.
Without an electric wire on the paddock side, your fence won't be strong enough for cattle and horses. For sheep and goats the rails and wire will not be close enough together, and not be able to be kept tight enough.
Personally, I think your plan won't work.

did you mean to write half rounds at the ends etc?

You will need strainers at the ends etc

What are you trying to contain? Is this basically aesthetics? You may not need five wires.
If you were to add another rail (instead of just having two) you could easily make it stockproof by using three electric wires. I have extensive fencing like this. But, it does depend upon the type of stocking you envisage.
The bonus is; if you are using electric fencing between the rails the fence will be quite strong enough in the way you are planning it (because electric fencing only requires light straining).
If you don't use electric at all even the strongest post and rail fence is going to come under attack from stock, over a period of time.

wrong thread?
Sheep netting is good and easy to install. However, nothing will keep a hungry animal in except electric. If you have cattle/horses, a solar electric fence is ideal but needs to be monitored regularly. Even 7 wire fences are almost useless if animals are hungry and see grass on the other side.
My fences are sheep netting with a barb at bottom. If I was crazy enough to go horses or deer, I would run a hot wire around the top. Old vineyard posts are cheap but need to be put in with a P.H. borer rather than a driver due to their brittle nature.