I wanted to convert my DI gas-tube upper with Midwest Industries Gen3 free float handguard to piston, with the Adams Arms P-series non-adjustable piston gas block (AA piston kit). I used the edge of a cut-off wheel to slightly profile the sides of the top of the gas block so that it would fit inside the HG upper "channel" with some clearance, how much clearance is a judgement call on your part, but I wouldn't use a lightweight profile barrel (think barrel whip). I think the minimum width of the gas block top would be the same width of the fat end of the op rod. I also tried using a end mill but that didn't work as good as the cut-off wheel following by abrasive stone file. The AA GB is hardened steel.
I assembled the barrel to the upper receiver, and torqued the barrel nut to 43 ft-lbs, the spec is 40. I marked where the op rod passes over the barrel nut with a Sharpie, and then disassembled. The barrel nut needs to have a notch cut out to clear the spring cup for the op rod, and the bottom of the notch needs to be about .850" from the breech-end of the barrel nut. The bottom of the notch needs to be perfectly flat for the AA spring cup to rest against. Before cutting the notch, I deposited a large booger weld on top of the barrel nut, to build up additional support for the spring cup. Then I proceeded to cut, file, measure, repeat. Cut & file the sides of the weld to fit inside the HG channel. Cut a round-bottomed groove in the weld for the op rod to pass through without rubbing, requires removing the weld material all the way down to the original barrel nut surface. Cut the front notch in the barrel nut using a cut-off wheel, finish using a file. Make sure the face of the notch is perfectly perpendicular to the barrel bore axis, to support the spring cup.
Obviously, this heavily modified barrel nut is now permanently dedicated to this specific upper receiver, and this specific barrel (actually, the specific barrel extension flange). If you change the barrel, or change the upper, you will need to mod a new barrel nut ($20 from MI). When you reassemble, you want to make sure you have equal clearance on both sides of the gas block in the HG gas tube channel, and make sure you leave about .025" clearance between the gas block, and the barrel gas block shoulder (thickness of the absent sheetmetal handguard cap). As the AA instructions say, there should be about a "business card" clearance between the op rod and the bolt carrier.
The reason for the earlier over-torque is because as you reassemble the barreled upper, the barrel nut goes just a little bit further each time. Just for consideration, the spec for the notched MI Gen2 barrel nut is 30#-60#, and the USGI barrel nut is 35#-80#. So IMHO, just as long as you're in the safe range.
On the shorter one, when the barrel was still 16" (before shortening to 14.5"), I put a couple hundred rounds, couple of mag dumps, through it for function check, etc, I had painted the sides of the gas block, whew, no paint was rubbed or marked, so I'm pleased. Same with the 20", except for the mag dumps. The paint does get rubbed as I'm pulling the HG on-and-off, so now I slide a paper tube-sleeve into the HG over the GB, before disassembly, reassembly. really should get that Cerakoted or something more durable.
misc build details: lapped front face of receiver, smoothed sharp edges on barrel extension lugs & feed ramps, then polished feed ramps. AIM Surplus nitride bolt, POF 6-pos M4 receiver extension (buffer tube) because they have that extended lower lip (carrier tilt), POF bolt roller cam. KNS Gen G pins. PA M4-72 brake, Rainier Avalanche CH.
7.62x39 carbine barrel, Geissele SSP, Troy front, KAC rear BUIS.
Dead Shot Barrel 20" mid-gas 7.62x39, SSA-E, Spikes T2 buffer.
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