https://midstatefirearms.com/product/replacement-knurled-ar15-gas-block-set-screws-set-of-10/

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10-32 thread x 3/16″ long. Standard for most set screw type low profile gas blocks. Uses 3/32 hex or allen wrench drive. Sold in multples of 10. 

These are knurled for a seure grip. You can upgrade your regular screws to these.

How often have you lost one of these when building and crawled around on the floor looking for it? it’s bound to happen. Grab a couple just in case.