Bolt sounds like it broke where most break, seems this only happens w/ "cheap non-USGI spec bolts" or one that's seen alot of FA fire.
As to the rest.....
Wish I could help ya, but there's too many odd things here:
1. [red]this upper was from DPMS that [b]wouldn't headspace[/b][/red]
Never heard of this from a factory DPMS upper, I'd have sent it back...
2. [red]my smith did some machining of the chamber to allow it to headspace[/red]
As the "barrel extension" mating w/ "barrel" determines the proper headspacing, why would gunsmith be modifying the "chamber"? (see chamber question below)
3. [red]Also, this upper has never quite worked properly, i.e. serious extraction problems that were somewhat remedied by enlarging the gas port[/red]
Sounds like a parts rifles, ya know part from here, a part from there, oh... this bolt cheaper, I'll take it.....
I'd be willing to bet money there was nuthin wrong w/ the gas port to begin with.... as I have heard of no problems w/ DPMS M4 barrels and there have been quite a few sold by MSTN/CMMG and others on the Equipment Exchange, (ya think we'd have heard something by now), suspect problem w/ bolt carrier, bolt, extractor and ejector related to begin w/ or ya've got a non-spec upper/FN reject.
As to theses problems:
[red]The malf's I'm getting are a) leaving a spent round in the chamber b) pulling the spent round out far enough to cause a double feed with the next live round c) the spent round gets crunched by the bolt, and d) what I call a "phase-9" malf, the spent round gets caught between the bolt and/or key and the charging handle.[/red]
I'd replace the bolt w/ a mil-spec one from Bushmaster or DPMS (I use their parts and have had no problems, w/ their lower kits or complete b/bc's), maybe your problems will be gone, if their not I'd suspect my gunsmith already screwed up my rifle by altering the gas port to begin with.
I am understand your post correctly, in that you started this project w/ a new DPMS M4 1/9, 16" barrel, correct?
and this barrel was chambered in 223 or 5.56?
Please take this w/ a grain of salt, but I trust the gunsmith who does work for me now, he's tops, but until I met him, I wouldn't trust a gunsmith further than I could pitch him/her..... as I had been screw'd, by more than a few who didn't have a clue, some who messed up trigger jobs, a relatively simply chore for most. And I still run into supposed "AR" smiths who when question'd reveal they know nuthin 'bout how an AR operates.
While your's may not be in the "bad" category, suspect too many ???s above mean accurate diagnosis of your AR cycling problems will be an exercise in patience on your part and require detail explainations on how this rifle was put together.
BTT for other advise....
Mike
BTW - please don't consider me "supremely wise" as sometimes, I have trouble findin my butt with either hand........