Ditto with Circuits,   but post a photo of your hammer, and the bottom of the 9mm bolt so we can weed this out  quickly.

Hence 9mm hammer looks like this,


And has to be used with a non ramped 9mm bolt that will look like this,


This is a ramp cut 9mm bolt,


Hence if you are using standard  tailed hammer with a unramped bolt then the carrier locks up on the hammer wedging  before the bolt can come all the way back.
If you are using a tailless 9mm hammer with a ramped bolt, then its a crap shoot if the hammer is lowered enough for the disco to grab the hammer instead.


Also, are you sure that you have the disconnector spring installed in the back trigger slot under the disco tail, with large coil side down into the trigger slot?


Lastly, don't touch the trigger cock the hammer back, and tell me the distances of the rear hammer sear hook as it passes the front of the disco sear hook.  The correct free gap between the two should be in the .001" to .003" range (thickness of a sheet of paper) and if the free gap is way too large isntead, then could be the disco is way out of release timing.

As for the springs, should look like this,




Lastly, since this may be a 9mm short stroking problem, the 9mm mag block should be installed so the ejector as high and as close to the center of the bolt, as it can get in the bolt slot for it , with out binding on the boltslot as the bolt is moving full stroke in the upper receiver.


As for the ARC stock, you have to use the buffer that comes with it, due to the much shorter receiver extension is has.
To check to make sure that you have the correct buffer for it in play, pull all the way back on the charging handle, and confirm that the face of the bolt is retracting about 1/4" behind the bolt catch.

Wrong hammer in play for your none ramped carrier.  
Hence without the ramp on the carrier for a tailed hammer, the non ramped carrier is going to bind on the hammer on the way back every time, and in fact over stroke the hammer to bounce the disco open as the bolt is coming back, and the disco still open as the carrier moves forward to allow the hammer to follow the carrier back home instead.  And yes, I can tell that you have a wedging problem, since I seeing the denting on both the hammer and the disco tail as the tell tail marks.

So, either change the hammer out to the above Colt 9mm hammer (as I posted above), or have the bolt ramped instead.
http://www.brownells.com/rifle-parts/triggers-parts/hammer-parts/hammers/car-15-m4-9mm-hammer-prod44737.aspx

Or

M60 Joe, Adco, or even maybe Circuits can ramp the carrier for you.

Fact is, the blow back action will snap hammer pins in short order (even the KNS reinforced pins) and the reason that the bolt is ramped is so the blow back action is more gently on  the hammer pins instead (go longer before the pin is snapped).  The down side, the flat back low edge of the none ramped carrier help to hold the bolt forward during ignition, so if you go to a ramped carrier, you have to add either hammer increased tension spring  (will increase trigger pull weight) or increase the buffer spring tension instead.  But with the Arc butt stock system having a  buffer and spring, the only quick/cheaper solution to the problem (without increasing trigger pull weight) is just to run the correct hammer for the non ramped bolt isntead.




Also, loss the grease on the rig.  It's going to cause you nothing but problems on a blow back rig; with the grease just collecting all the fouling debris.

CLP works great for the platforms, including the blow back AR rigs as well.  Hence CLP had a cleaning agent in it, and not only will it help to combat the fouling when used as a lubing agent, but will help to remove fouling that you missed when it was being used as a cleaning agent as well.  If you need a source for CLP, the large spray cans of BreakfreeCLP goes for  $10 at walmart.
http://www.walmart.com/ip/30626861?wmlspartner=wlpa&selectedSellerId=0&adid=22222222227020395024&wmlspartner=wmtlabs&wl0=&wl1=g&wl2=c&wl3=62882835489&wl4=pla-99386458089&wl5=9028719&wl6=&wl7=&wl8=&wl9=pla&wl10=8175035&wl11=online&wl12=30626861&wl13=&veh=sem

As for the bore to clean copper out of it (if you are not just running Powder coated bullets isntead), then something like Sweets copper solvent makes quick work of removing the copper without the need of a lot of scrubbing, and does not leave behind a residue like hoppes that when mixed with CLP becomes a sticky mess isntead.  If you are just running powder coated bullets, then clean and lube the entire rig with CLP only instead (think of the cleaning as just a oil change).

To add, I need one more photo since your disco top tail looks like shite , and that is cock the hammer back with out touching the trigger and get the rear hammer sear as close to the disco sear as you can.

I'm looking for the gap distance between the two, and hopefully it not a country mile like in the below photo.




As for once we get the rifle semi running correctly, then I need to walk you through tuning the rifle correctly next.

Off the top of my head, that would be getting the ejector tweaked so the rig makes a nice pile of the spent brass, the bolt limited to travel about 1/8" behind the bolt catch so it not snapping the bolt catch in short order, and a few other tweaks as well.

MeanGun, you have two contact points of the hammer when it being lowered back down.  

In the case of a tailed hammer in a Gas rig type rig, you have the tail to the disco and this is kind of normal since the tail of the disco is spring loaded up, and it can act like a buffer to slow the hammer down at full cock.

Now with a blow back action like a 9mm, the bolt is never locked up, so its the recoil spring, the bolt mass, and in the case of a un-ramped bolt, the bottom edge against the hammer unlocked holding the bolt forward.  Since the bottom edge of the carrier will cock the hammer faster, and more rear downward, it really the top edge of the hammer contact pad that is lowered down, and regarding limiting the hammer being over cocked, it the top of the disco hook itself.

Hence its the hammer that has to be over cocked enough for the disco to be able to catch the back sear hook, but not enough for the hammer to end up wedged  between the top of the disco hook, and the bottom of the carrier.  So in colts case, since they want a top of hammer contact pad to have the L cut, which will catch the back of the bolt/carrier if the disco fails to catch the hammer and stop the bolt/carrier from coming all the way forward, they have the mod'd non tail 9mm hammer.

Now if you want to forgo this safety option, then a M-16 type hammer with it auto sear ground off (none top of hammer contact pad L cut, but rounded instead) can be used, but you have to round/lower the top of hammer contact pad to keep the hammer from wedging as the bolt is coming all the way back.

So as Circuits pointed out, if you want to test a hammer to bolt combo, just hold the trigger back, pull the charging handle back very slowly, then ride the charging handle forward slowly as well.   If the top of hammer contact pad is not high enough against the bottom of the bolt, then the hammer is not  cocked down far enough for the rear sear to engage the disco, and the hammer will follow the carrier back forward.
Note, in the case of the colt 9mm bolt with it lower  catch cut, and a ramped carrier, the hammer is not lowered enough for the disco to catch the hammer, and the bolt will lock up about half way back forward isntead when you have the trigger held back.

On the other hand, if the top of hammer contact pads is too high, then as you are pulling back on the charging handle slowly, then the hammer is going to wedge between the bottom of carrier and top of trigger/disco hook, and your going to feel this in the trigger instead (trigger trying to move back forward in the case of tailed hammer, or if not tailed hammer, then wicked binding since the back of the hammer is just binding up against the disco hook instead.

So non ramped bolt and no mods to the hammer, non ramped bolt uses the colt 9mm hammer (top of hammer contact pad height).
Also note that on the 9mm colt hammer, instead of having a top L cut, it has a lower cut below the hammer contact pad to catch the bolt moving foward if the disco does not catch the hammer: to keep the bolt out of battery.

If you are using a ramped hammer, then since the height of the carrier is raised, then you need to use a hammer that the top of it hammer contact pad is higher instead (think uncocked).

As for which of your hammers will work with which bolts, without having them in hand to see there radial heights of the fully cocked to kiss the top of the disco hook with the trigger held back and there contact pads heights over the top of the lower receiver deck height, nor the bolt in the upper receiver to see it's needed clearance depths  to he bottom of the upper receiver (much less that lower to upper receiver mating lash), its hard for me to give you a definite answer which is going to work with which.  So, short of measuring, Circuits has given you the easy check to see which combo's will work with each other isntead instead. Hence either the hammer will not be lowered enough for the disco to catch the hammer due to it top of contact pad not being tall enough, or the contact pad being too tall, and the hammer is going to end up wedging between the disco/trigger and bottom of bolt when the bolt is pulled all the way back isntead (trigger slap isntead).
Note, when the hammer goes to engage the disco sear, your going to have a slight trigger feel of the disco spring being compressed, but it should not end up with the trigger cam'd forward once the bolt is all the way back isntead.

As for hammer/disco free play (hairs width is fine, since it puts the gap about .003"), that needs to be checked with the hammer that works with the carrier in play.

So non ramped carrier, then you need a hammer with top of hammer contact pad that of the colt 9mm hammer.

To add, only the colt 9mm bolt you posted has the needed bolt lock feature cut on it to keep a 9mm bolt out of battery if the disco fails to catch the hammer on the set.