I'd like to know as well. No smith at any of the outlets seems to have any ideas. They all reccomend the P500 but none can say if it will size up. What's your experience been like? Have you tried it on your A1?
Babj has tried most of them. They didn't work. The best thing to do is to get a round piece of brass rod and file/grind it down to where it fits the slot. Then you can gently drive the sight into position - use tapping strokes with the hammer. Be careful, as the sights are sintered metal and can break. I've never had a problem with the sights, but I didn't buy a used weapon, so no one had had an itch to "improve" things. Put some Kroil on the sight before trying to move it, it will help.
I've used brass punches, segments of brass rod, and aluminum punches from Dawson to remove and insert front and rear sights on every one of my 6 Steyr uppers. I have never ground them to fit tho it is smart to do so. If you make sure the new sight goes in about halfway with hand pressure before trying to drift it home, no problem. If you are impatient, you will dent/peen the edges of the dovetail.
I have evidence of my impatience on a few sights I installed in my Steyrs. I don't much care, as plastic guns have limited visual appeal to me anyway, but I admit that I should have been more patient when filing/fitting the dovetails of new sights. I don't recall ever marring the Steyr tri-trap sights while removing them. I also use Kroil before removing a sight to help lubricate the dovetails.