Apparently most agree the potential exists, there are businesses who do this legitimately with a good track record, but nobody will point out who they think it is?
We've got 23 posts and nobody has searched and recovered names of the OP's potential victimizers. That means everyone agrees - they don't want to be the one to out the company, either.
Since no illegal act has yet occurred, it's not chancy at all. Unfortunately, it takes the actual commission, and then conviction, to say somebody has been wronged. And since these things do happen correctly with some frequency, saying "these guys are planning to rip you off" has no credibility.
Not that they can't - there's just no possible way to tell. What we are dealing with is an interpretation of the circumstances and filtering it thru our experience to forecast a possible result. We all do this, all day long.
Like, if I sit here in my jammies typing much longer this morning, I AM going to hear about if from my wife.
Just for other side of the coin, we've got an year long window, if not more, of unanticipated demand, and plenty of incentive to make a profit. Any CNC shop of repute can machine uppers and lowers to spec - most of the receivers sold come from shops that don't forge. Even Colt buys finished receivers and doesn't do the work inhouse ( all you Colt fans need to wise up. ) If the platter forgers add a shift and get busy, there's plenty to go around for the next year, nobody is going to ignore the profit.
Just in time for all those pensions and banks to bail out, too. Oh well, their bad.
CNC isn't the hard part, the blueprints and tolerances are practically public knowledge. The programming isn't difficult, those folks capable are already hired. The shops have an incentive to fill the schedule and reduce down time. There's plenty of room to open up another line and make more receivers - which are going to sell. The QC is the critical issue, I don't see aerospace or turbine makers having any major difficulties. Me, I bought an AGP lower two years ago - same guys who make turbo kits for tuners.
Should you be cautious, yes. Are they going to rip you off? If nothing materializes, the credit card company stands the risk - duh - don't pay the bill until after delivery.
BTW, check and see if you get popped for the extra 4%. That is much more likely.