Home Forums Hunting & Shooting AR and Tactical Rifles Tac 20 build (pic heavy)

Final assembly and test firing happened this weekend.

For final permanent assembly, I decided to use a BCM upper since this'll be more of a precision rifle. (This was originally a left-over parts build, but has morphed into two complete uppers with the extra parts I picked up along the way.) I like the BCM uppers because they are a tighter fit to the barrel extension, and are usually a couple thousandths press fit rather than the typical loose slip fit of most uppers. Heating the upper with a heat gun expands it enough to install the barrel:


I stamped the cartridge and twist rate on the barrel as well. This isn't a fancy way to mark the barrel, but is quick and expedient, and I'm more interested in function than looks on this one anyhow. I do mark a line on the barrel to index the stamps against, and always test each stamp on a piece of scrap first (note the aluminum block near the top of the pic). Stamping a letter upside down is embarassing, testing on scrap helps avoid that. Stamped letters always look a little bit drunk, but IMO this one looks worse in the pic than it does in person.


Inital test firing went pretty well, other than the chrono not picking up any readings. Cycling was just a little sluggish though, with the bolt not locking back on most loads, so I stepped up the gas port size to .089" (from .082").


Indexing the gas port hole is pretty simple - I use the previous .082" drill bit in the chuck backwards, and use the drill bit shank to locate the hole. When the drill bit can enter the hole without any deflection, it's centered. This is accurate to within a thousandth or so if a guy is careful.


Final assembly is complete. Blue #243 Loctite was used for all scope, handguard, and gas block screws, as well as between the gas block and barrel for sealing. Green #620 was used between the handguard and barrel nut per Midwest's instructions. The rifle is topped with a 3-9x HD SS on a 20 moa riser. Still couldn't get reliable velocity readings, but the rifle cycles well with both 32 and 40 gr V-max over TAC, 2460, 8208, and Benchmark.


Just for fun I fired a 32gr V-max @ ~3900 fps (if the 2 chrono readings are correct) into this 5/16" steel plate. Shown for comparison is an impact crater from a 75gr 5.56. It's not a completely fair comparison, 24" vs 16" barrel, but the 75gr 5.56 won't punch through from a 24" barrel either. Max loads with that 32 gr V-max should add another ~200 fps or so.


I'll follow this up with some velocity and accuracy results when I can, but for now this build is done. Thanks for reading.