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This is product code 1799. On the Huxwrks site it is listed as "Flash Hider-QD 762 5/8x24". There is no reference to x39 vs x51. There is no other suppressor for 762 Caliber so length of projectile should not matter and it looks like this is the only QD flash hider for the Flow 762.
According to Huxwrx website this will extend a 13.7 inch barrel to 16 inches when pinned and welded.
Yes it will. The 5.56 muzzle devices will fit interchangeably with the 5.56 and larger cans. However, a larger muzzle device (6mm or 30 cal) will not fit into smaller cans (due to the overall length of the device) to prevent a larger round from going through a smaller diameter can.
Most common 5.56-223 barrels are 1/2x28. Typically 5/8x 24 are for larger calibers such as 30 cal or 762. I guess there could be a manufacturer out there making 5/8x24 thread pitch for 5.56 cal but it is very highly unlikely.
5/8x24 for your AR platform. 300 blk is equivalent to 7.62.