I have a Shaver HD-14 driver. I am not sure they make it anymore but it is the largest they made at the time. I routinely drive railroad ties without doing anything to them. Square end right down into the ground.
On large round post like yours I have found that if you sharpen them to a point they will drive crooked. You never get the angles exactly the same on each side. So I have found that if you leave a spot 3-4 inches flat in the center of the post and taper the rest down to that flat spot they will drive easier and go straighter.
They make an attachment to to newer Shaver drivers that swings a small hydraulic drive post hole auger right under the driver. So you drill a small 4 inch hole and then drive the post down into it. This works great.
I usually use a hand twist digger that digs a 6 inch hole on corner post. Then drive the corner post down into the hole. They drive straight that way.