I've put together many, many small frame uppers and the only time I've had an issue is when I pulled apart my dad's DPMS upper to change the handguard. I have the Brownell's tool, but the barrel would twist while tightening the barrel nut and cause enough misalignment that the BCG wouldn't go into battery. I got it lined up after a few tries. Not sure why, the pin looked a little buggered, but not too bad (DPMS OEM upper) and the pin slot looked fine too. There was just enough tolerance stacking that the barrel would twist. That M.I. wrench is better, but not sure it would solve my issue. I have the M.I. small frame tool and it does seem to hold things tighter.