Check the firing channel for debris and binding, and if new check pin spring, should be holding the pin back, not pushing it forward. Also, are you positive it is slam fire, or is the hammer moving forward un-commanded? That would be a FCG issue, not a bolt issue. Check for a broken hammer or trigger pin.

 9mm's don't "headspace" like a rotating bolt, it works just like a pistol. The bolt does not "lock" meaning there is no "timing" so when the bullet heads down the barrel, the bolt heads rearward. Your issue will most likely be firing pin binding or FCG.

And hey, Welcome to the Site

Quoted: I've spoken to a couple gunsmiths but they're obligated by law to dispose of an "unsafe" weapon in the event that they cannot fix it.

What a bunch of Asshats!!!!!!!  Point blank, if they cannot fix this simple problem with an AR rifle, then they have no bussiness  taking one in to shop for repair, nor even offer any advice on the subject.

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As stated above, check the firing pin channel for burs or debris, and the return spring as well.  You are confirming that the FP is not binding up/out forward out the bolt face as the round is feed (real slam fire).


Onto the FCG,
Hold the trigger back, with a third hand, cock the bolt back on the catch.  Now while still holding the trigger back (hammer retained on the disco), hit the bolt release button and let the carrier crash forward.  Now release the trigger and pull it again.  The question is if the disco retained the hammer during the crash, or allowed the hammer to slip and it caused the fire on closing.

To push on after you have made the quick test, pull the FCG and clean them off with CLP.  Once cleaned/re-lubed (with the parts checked for wear or breakage/ confirmed that you have the right size FCG pin sized parts in the rifle*), and after checking the receiver cavity for piece of debris (blown primers) that may have been hidden under the FCG, reinstall the parts.   The disco spring goes large coil side down into the back of the trigger slot, and the hammer spring legs rest on top of the trigger pin. From here (upper still shot gunned open), hold the trigger back, and cock the hammer onto the disco.  With your hand in front of the hammer, very slowly release your finger tension on the trigger.  The hammer should slip from the disco, and be retained by the trigger (primary sear).  If the upper fails this test (or the crash test), then we need to check free gap between the hammer and the disco.

To test free gap, with the trigger at rest, cock the hammer back until you can get the disco sear and the hammer sear (the rear one) as close as possible.  The ideal free gap is in the .002 range, with gaps larger than this allowing the disco to release the hammer too soon and missing the trigger primary sear for a clean reset). If this is the problem, then I can write you the novel on how to reset the disco.


* The old style colts took .171 pins and had the same FCG hole sizes.  The new colts take .154 pins/FCG parts and if you have .171 parts on .154 pins, you are going to have problems from the slop (read pins and FCG parts need to be the correct size for the holes in the receiver).

To pull and reassemble the lower,
www.ar15.com/content/guides/assembly/lower/

P.S. Feel free to name the shops that you spoke to.  This will save someone else the trouble of having their rifle fucked up by these clown at a later date.

Oh, almost forget,  Welcome to the site!!!