I've been taling about building a 9mm direct impingement upper since this past april and have been gathering parts since early summer. Over the past two weeks I've managed to make some progress on the build.
I started with: a 9mm blank; a $10 barrel extension from CDNN; a spare carrier and upper I've had for years; A set of used USGI bolts off of gunbroker; and a Caterpilliar diesel engine nut 13/16"-16tpi.
The barrel blank I started with was one of the .800" blanks that have been floating around for the past few years. This barrel is too thin to turn a shoulder into for the barrel extension to rest against. I decided to used a savage style locknut to solve this problem. An internet search turned up Catapilliar diesel engine grade 8 nuts for about $1.50 each so I got several.
After short chambering the barrel I threaded the last 1.150" of the barrel to 13/16"-16tpi so that the barrel extension was a little difficult to thread on. Then I screwed on the barrel extension so that the back end of it was 1/2" from the back end of the barrel and drilled the index pin hole to a depth of just past the bottem of the barrel threads. I also drilled a gas port about 1.5" in front of the barrel extension since it was already squared up. Then I screwed on the locknut as far as it'd go, and then screwed on the barrel extension with red locktite and secured it with the index pin made from 1/8" drill rod. After letting it set for a day I torqued down the lock nut and secured it with red locktite. Before the loctite set up I turned down the locknut to just under 1" with the barrel extension towards the tail stock so that any cutting force would be tightening the lock nut and not loosening it. Then I finished chambering the barrel after opening up the 556 bolt to .398" so the 9mm case would fit.
I then made a gas block for it out of steel and cut down a used rifle length gas tube to fit. The first few rounds of test fire showed that with a .090" gas port the bolt comes back enough to extract the case from the chamber and stop with the hammer half cocked so that the hammer catches the firing pin as it tries to go forward.
Thats where I'm at right now. The wonder 9 part of it is I wonder what it'll take to get it to run. More work and current photos to come!