I have a 16 inch heavy barreled A1 upper which I installed on a old Sendra lower I have had for years. I installed it and noted some play in the front hinge. When fully installed I noticed that the upper was very snug with little or no play. The problem occured while at the range today. I noticed the front hinge pin would start to come out, sometimes quite a bit. I think the front hinge pin is either undersized or the pin hole is getting oversized due to use.

What can I do to fix this issue?

Replace the pivot pin, spring, and detent. And while you have things apart, clean out  the hole in the lower where the spring and detent go with some brake cleaner. Blow out the hole with a shot of compressed air. There may be some fossilized oil/dirt in the hole which is gumming things up.

thanks