5/32 I think
Gonna say the Americans can be so annoying. Just like the barrel bolt. I got lots of 6mm but no 1/4". Even Home Depot didn't have 1/4" T-handle hex. Had to order from eBay off American seller. SAE can be such a PITA.
Dont blame the Americans for that, blame us for going metric.
Do not put the Magpul screw into your XCR. The provided screw is too large threaded. It'll go in a couple of threads and make a mess of them. Use the XCR's screw with lockwasher, despite what the magpul instructions say.



























