I have the same gun as the OP, a 13" blued factory folder. I think I bought mine in the early 2000's for $2500 give or take. This was the era when Ruger still provided factory support for the gun.
They are not "bad" machineguns by any stretch. However, there are some challenges to the AC556 that keep pricing down compared to its peers.
One of the biggest contributing factors is the fact that there are just "better guns" out there and a M16 conversion isn't huge financial leap from AC556. If somebody can afford $10 to $12K for an AC556, they can probably with a touch more saving afford a $17K to $18K M16.
In terms of quantifying the "better guns" statement.
- The original gun the AC556 is based off of (the Mini-14) which is basically the Rodney Dangerfield of the semi-auto rifle world and just gets no respect. So its more challenged from a pricing perspective to get folks to pay a premium price for an auto version of a semi-auto gun that has some reputational challenges.
- As a shooter the AC556 is not as smooth a shooter as its primary competitor the M16. The AC556 also is not as configuration friendly so its more difficult to upgrade (rails, lights, optics, suppressors, etc) and there is the risk of the AC556 looking "Tapco-ed" if it is upgraded with bolt on additions. Clyde Armory was the only shop that made a good looking upgraded modern stock system for the AC556, but its now out of production. Other than the Ciener 22lr kit there are really no other caliber options unless you wanted to try and turn one into a 300blk.
- The AC556 (especially the 18" full stocked variety) to be honest just don't aesthetically look the part. If somebody is spending $10K+ on a machinegun....they generally want it to look like a EBR machinegun. Probably 99% of the world will think a fixed stock AC556 is a hunting rifle of some sort. Even gun people will probably say "hey is that an older Mini-14" or if its a folder tell you about the "neat A-Team gun" you got there. Conversely more than half of the general public thinks an AR15 is a machinegun even though its not.
Personally, I think the 13" guns have a very unique, cool, and distinct look to them but even the 13" version doesn't command a pricetag comparable to other guns that look like machineguns. (M16, HK33, or FNC)
- As mentioned already parts are not as prolific as other platforms and many folks seem to believe that now that Ruger no longer provides factory service that parts are not available.
I think the parts situation is pretty overblown. Are AC556 parts as ubiquitous as AR/M16 parts....of course not. However, a large number of the primary wear parts are the same as older generation Mini-14s. Barrels, gas blocks, spring, ejectors, bolts, stocks with minor mods, LRBHO, etc. could all be harvested from an older gen Mini-14.
The majority of the AC specific parts are in the fire control group assembly and if you are really concerned there is pretty much always an AC556 kit available on sturm, gunbroker, etc. for the $1000 range. Ruger made a lot of post sample AC556s for a decade+ after the ban and there will probably be a long runway of cut up PD guns as they get traded out for M16s or something else more modern.
- There is an impression that the higher capacity magazines are unreliable. This is probably a reputation earned when Ruger 20rd and 30rd factory mags were more rare and the world was awash in cheap and shitty USA brand Mini-14 mags. Today its easy to grab some Ruger factory 30rd mags. However, if you want drum-esque capacity levels your options are a bit more limited.
- There is also virtually no military service for the AC556 in the US and elsewhere its pretty limited. Machinegun buyers if given a choice will gravitate towards guns with a military pedigree that the AC556 just doesnt have.
Overall I like my AC556 and enjoy it for what it is. For some odd reason I have always been compelled buy a peer stainless 13" folder as well as I think they are just really cool looking guns. However, if forced to give up one of my transferable 5.56 machineguns the AC556 would probably be toward the top of the list to go.
Congrats on the rifle caliber machinegun and enjoy shooting it as they really are (imho) much better machineguns than folks generally give them credit for.