G21 front sight won't come out

I have a used G21.3 that I believe is out of a law enforcement armory. The original sights have been replaced with Trijicon H3 06 sights which have lost their glow, so I want to replace them with Truglos. Here's the problem: I can't get the front sight loose. Strangely, the little hex head screw that's supposed to hold the sight on was missing. I have tried to punch the sight out, even with significant heat applied, and the sight won't budge, even a little. It bent my punch, but wouldn't move. I've performed this operation on other Glocks with no problem, but this has me stymied. Any ideas?