I'd say around $350-$550, in my opinion. Used SL8 barrels go for around $250 or more, and people like Chopstix or Tbostic charge around $300 to cut down a normal SL8 barrel to different specs. Do you have any pictures?
I would put it up in the for sale section, (if you're trying to get rid of it). Someone will more than likely buy it, as there are a few people working on builds right now. You'll have to state it it's threaded and what measurement, as I'm sure people will ask.
If you're going to post it here for a price reference, you might as well offer it up to the HKPRO's first! The fact that it has no markings from HK is because it was turned down, and all markings would have been turned into shavings. Looks to be in good shape, though - and has the piston and op rod in your picture, those are in addition to the value of the barrel, of course.
Pistons (if HK factory) are worth about $40-50, and the op rod is about $50-60. Factor that into your sale price, and good luck,
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