I also broke my lower hand guard tension pin removing it.
For some reason it broke into two pieces when I removed it to install my nea lower hand guard.
I looked around and went to lowes low and behold found a replacement for it that fits into the hole.
The pin is 5/32 x 1 1/4 long pn#0823671804
Made by Hilman.
The best part .89 cents for two of them.