This is one of the better gas blocks on the market for military correct "clone" rifles. If you are building a Mk12, Mk18 or a M4A1 Block 2, NSWC RECCE or URGi, this is of the military correct, gas block for your clone build. The nice part about DD gas blocks is the kit includes red Loctite.
This gas block requires a 0.75" gas journal barrel, which is what most military correct barrels are currently. Some barrels could be larger or smaller, so be sure to check. Most all Colt, FN, Daniel Defense, Mk12 SPR or Holland profiled, NSWC RECCE, etc. are 0.75". Older M16A1 barrels are more narrow, and some target barrels come in larger diameters, so check your gas port journal size.
Charlie's also features the same gas block from two other suppliers. The gas blocks are identical. Daniel Defense gives you a complete kit. But, take a look at: