Great thread, very interesting stuff.

I think the consensus of those that have tried DI 9mm is that it requires reloads tuned for it. Might be the same for piston

Yeah that's what I was pretty much expecting going into this project.  I thought hand loads would be the key but after experiencing the same malfunctions I saw with factory ammo I began to think the issue was somewhere else.  

Next step is to work the bugs on the extractor/ejector.  I need to get the gun to cycle without the brass falling off the bolt face or hanging loosely on the extractor hooks.  Once that's complete I can revisit the ammo issue.  I still expect hand loads will be required but we'll see.

Have you considered modifying the ejector for more outward travel? Lengthen the slot so that it protrudes from the bolt face more, giving a longer "push" to the cartridge case, hopefully causing it to spin free?
As for the extractor not snapping over the case rim; what's the distance from the bolt face to the extractor hook on the bolt?
Spec for a rim on .223 is .045, vs .050 on 9mm, if I recall

That will cause it to bind up on the case lip after stripping from the mag and before chambering.
Anything sticking out past the face of the bolt will do this, and on short pistol cartridges that tend to feed at a steep angle, you sometimes have to chamfer the bottom of the ejector a bit to prevent it from binding on the case lip.

I was actually thinking about doing that exact thing to help with ejection but RDTCU brings up a good point about the case binding up.  I wonder though...if it was just a little bit longer (maybe 1/16") with a chamfered bottom do you think it would still bind?  I'm trying to visualize without it in front of me but it seems like after the bottom lugs strip a round, a properly placed chamfer on a slightly elongated ejector would push the round in front of the bolt face vs binding up.  Then again, the juice is probably not worth the squeeze for all the potential issues that might come from it.

I'll try to measure the distances tonight to compare.

I had not considered the feeding issue with pistol length cartridges, but we did it sometimes with short .223 blanks that were running spotty on ejection.
It could also depend on how far out the ejector sits currently. I've seen them well below flush at full extension.

Whoops I misspoke earlier when I said there was a 0.009" difference in extractor gaps; I was referring to the difference in case rim thickness (9mm is 0.050", 5.45 is 0.059")

I measured the gaps and came up with this:

5.45:  0.070"

5.56:  0.055"

The back of the 5.45 gap sits flush with the bolt face so the face-to-hook distance is right around 0.070".  The 5.56 is slightly recessed so it's distance is closer to 0.050".  I tried another 5.56 extractor and the distance was about the same.

The tight tolerance between rim thickness and face-to-hook distance might be just enough to where it's not allowing the hook to snap over the rim.  I'll keep gradually filling down the forward edge of the hook until it does snap over the rim.  Hopefully that doesn't make it too loose like the 5.45.  I'm also looking for aftermarket extractors that might have a little more space but not as much as the 5.45.

Be careful if you're filing on the actual face of the extractor hook, If you don't keep it perpendicular or negative rake, it will likely slip off of the case during extraction. A mill with the right slot cutter would be a better tool, but obviously not everyone has a mill...

I've been going nice & slow and making sure I file at a shallow angle so if anything it has a slight negative rake.  

I looked at the extractor on my blowback and it's small like a handgun extractor.  I thought about narrowing the 556 one to make it similar.  Something like this:
Attached File


Maybe the smaller contact area will allow it snap over the case rim since the tolerance is so tight?  Worth a shot I suppose.

If you were able to push the round back into the bolt, using a rod down the barrel, so the extractor could snap over the rim, I'm guessing your headspace is too long.
You might be messing with the extractor when the chamber is the real problem.  This doesn't really fix your ejection issue, of course.

Agreed.  I was JUST talking to a friend about that very thing and we came to the same conclusion.  As I'm filing away at this 5.56 extractor, I realize I'm basically just turning it into the 5.45 one with excessive gap.  Better to start fresh with a proper headspaced barrel and correct extractor to eliminate them from the pool of mysterious reasons why my beloved project won't work.  

My friend and I were discussing that too!  A suggestion came up of trying reloads without trimming the cases so that they're a little long with the hope they'd make up the difference in headspace.  If it works, cool.  But seems like a hassle to limit the gun to just reloads that happen to have elongated cases.  Ideally I'd like this to work with everything but I'll settle for hand loads with slow powder.

Rudy you have PM...

Update - IT WORKS!!!

Turns out all those extraction/ejection problems had nothing to do with the gas system.  It came down to one thing:  the 5.45x39 extractor was too big for a 9mm case rim.  That in turn threw off the headspace when the barrel was built so a simple changing of the extractor did nothing.  With the 5.45 extractor, the brass was simply 'floating' in the bolt face and not being held securely, hence all the ejection issues.  With the 5.56 one, the excessive headspace prevented the hook from snapping over the case rim.

So with the headspace fixed and using a 5.56 extractor, everything is perfect.  I didn't even need to redo the dowel pin mod.  A light polish on the front of the barrel extension lugs was all that was needed to prevent empty cases from binding when coming out of the chamber.  All eject well clear.  Had I caught the extractor problem initially, it would've worked from the get go.  Lesson learned.

I went to the range tonight and put almost 300 rounds through it.  I did a repeat of my first test, using factory ammo of various grains and brands.  Shot both suppressed and unsuppressed.  Most functioned flawlessly.  The one brand that had issues was Freedom Munitions. Their 147g RN seemed very under powered and short stroked several times with a few stovepipes. 147g HP's didn't feed well at all.  Most came out of the mag nose high and jammed into the top of the barrel extension.  Of note, Speer Lawman also has a truncated bullet but they all fed great.

This is what it looked like afterward:




Nice little ring of carbon/powder in the piston assembly:



This thing is definitely dirty but no more so that a blowback.  Toward the end it seemed to have a few more feeding issues, specifically the bolt wasn't closing all the way.  I think it was from carbon buildup in the chamber.  I have no doubt it will continue functioning with an occasional squirt of lube on the bolt and piston.

Next time I want to retry hand loads to see if it makes a difference in the amount of carbon buildup.  I'll also try giving it a steady diet of Speer Lawman Cleanfire ammo; spent cases come out looking like they came out of a tumbler.  Pretty impressive.

I will officially call this project complete and move on to Phase II...45 ACP!  

Sweet!

Well done, OP.  You truly have gone where no man has gone before.  Congratulations.

Where's the videos?

Thanks, it's been a fun learning experience.  I now know way more than I expected about fun stuff like math & fluid dynamics.    Unfortunately it's only made me want to tweak with more stuff.  I already have plans for a standalone billet receiver that uses Sterling mags.  I talked to my machinist friend and he can wire cut a profile and broach the mag well for me.  Basically a 50% receiver.  This "hobby" is quickly getting out of control...



 
Haha I was just thinking about that!  What good are pics alone right?  I'm planning on going back to the range tonight for some more testing.  I'll see if I can take some video.

It doesn't get better, it's terminal...

My good sir, the Sterling magazine may be one of the finest pistol caliber magazines ever created, but it was never intended to fight gravity.  Might I suggest a side feeding UPPER receiver as a project for your machinist?



http://www.thefirearmblog.com/blog/2016/04/13/frankenstem-16-p5



Attached File


A 9x19mm Lyndon would be something to rejoice over...

Now THAT is awesome!

That's what I was afraid of......  

Ok I cant get the video to load.  I need to research more how to embed it.  It was coming up when I hit the preview button but not when I posted it.  

Ok I've given up on trying to embed this video.  There's a link which hopefully works.

Anyway, I didn't get a chance to go to the range tonight but had just enough daylight left when I got home to try a few rounds in my sand bucket.  Luckily the Revo 9 can is pretty quiet.  At least more quiet than the kid across the wash who practices his trumpet at 9pm.  But I digress:

<iframe src="https://player.vimeo.com/video/212861097?title=0&byline=0&portrait=0" width="425" height="756" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>

Video Link

The green tape on the magazine is to make it nice & snug in the well.  It was a little loose so I wanted to eliminate it as a cause for all those malfunctions.

Ammo was Freedom Munitions HUSH 147g RN.  Unlike their regular 147g, these did work.  Not sure if it's a different powder they use.

I've pretty much found the gun's limit.  I've gone through 500 rounds and it's starting to show signs of 'failure'.  The last 50 or so began to have a lot of malfunctions; stovepipes, double feeds, etc.  It came about rather quickly too.  Upon closer inspection, I found the bolt wasn't closing completely.  It was chambering and locking, but back just far enough to not look quite right.  I opened the piston chamber and found ALOT of carbon.  I unfortunately didn't take a picture but it was more than double what's pictured here (last pic after 300 rnds):



That carbon "ring" had built up so much it was starting to prevent the piston from coming full forward.  This in turn was preventing the bolt from completely closing.  I'm considering opening the vent hole in the piston housing to allow more junk to (hopefully) pass right through.  
This guy here:


Think this would help?

Also, here're questions for any Sig MPX owners - Does the MPX have issues with it's piston getting gunk'd up?  How is it vented?

Thanks Steve.  Anxiously waiting to see what you guys have planned for the summer!  But I'll settle for an updated gas regulator in the mean time.  

The MPX has about twice the vent area, with a hole straight out each side.  The gas entering the MPX gas block is going to be at a higher pressure than what you have entering the PDS gas block as well, due to the restriction of the gas tube setup, so that may cause it to blow more of the unburned powder out of the vents.
Another difference is that the piston/tappet uses gas rings and the gas valve has tapered clearance around it so any crud left in front of he piston has somewhere to go until you can clean it.

Interesting.  I've never seen that drawing of the MPX barrel.  I did a cursory google search for 'MPX gas valve' and found some more useful info & pics.  The port is only .045-.050 on those things. That might explain why they don't get as much junk in the piston chamber!  

Are these the vent holes you speak of?
Attached File


If I'm understanding the MPX design, it looks like the PDS piston and plug both have similar tapered areas like the MPX:
Attached File


That entire cavity was packed full of crap.  Now I realize I may have contributed to the mess by previously squirting some lube into the piston chamber.  That's apparently a no-no with piston systems; whatever doesn't burn off acts like glue for carbon & debris.

I was going to open the vent hole last night but figure I'd try some more Cleanfire ammo first.  I shot 200 last night all suppressed and the housing looked ok.  My goal is to get through another 300 and see what happens.  If it's not as bad as last time, I will restrict the ammo to Cleanfire.  Not ideal but no different than a DI9 requiring special hand loads.

Shot 150 more yesterday for a total of 350 since clearing out the piston housing.  All suppressed Speer Cleanfire 147g.  Not too bad:



The gun was still jamming however, with lots of FTE's.  I assume from the blowback that had built up in the action & upper receiver (super dirty; haven't cleaned since initial testing).  I oiled the bolt carrier and it seemed to help a little but it still jammed way more than expected.  A good dip in the sonic cleaner should help things out.  I'm happy overall and will feed this thing cleaner ammo from now on.