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Finally got to run some rounds through the new blaster. Was just doing functions checks and ran about 20 rounds total. At first I ran .223 but it was short stroking on the second rounds of a quick double tap(usually.11 to .13 splits) you could see it short stroke on the slow mo video we took, first shot would get a full reciprocation, as soon as the bolt would close I fired off another round, and the bolt would short stroke and just rechamber the empty cartridge. Even on the last round it would fail to lock the bolt back.
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I don't think 20rds is going to do much more than begin break-in...seems like folks here have needed ~100rds to get them to run smoothly. I don't recall what I shot in my 7.5 (same upper but Tac Elite handguard) so maybe that's the key, but I've torn down, cleaned and lubed (with my favorite CLP product) all my AA uppers before first shooting and I've not needed any break-in at all. YMMV of course. I only tried one other buffer/spring combo and it wouldn't lock back on empty, so I left in the stock carbine spring and buffer. I need to experiment some more to see what will run.
Richard