Using nail gun on deck fence?
 

I am repairing a section of my deck fence. I need to install 8 decorative fence posts but I can't nail them straight in from the top because the fence is too complex. I'm thinking about renting a nail gun and nailing the top of the posts to the horizontal board from underneath at a 45 degree angle. My question is will this knock the post out of position when the nail goes in? Or does the nail gun shoot so fast the post won't move?

Offset the post slightly so that when you nail it will jump back into position.

I am talking 1/4"- no more.