because it was cheaper to re-weld an existing receiver. That was back before the days that CNC machines were affordable for smaller shops, so all the machining on a new receiver had to be done manually, and it was expensive to do. So it was cheaper to buy a barrel of "scrap aluminum" and re-weld them.
Normal everyday use shouldn't hurt it any.ETA, I agree that most, if not all Hydra-matics on the registry are re-welds. IMHO, they're still useable guns, but the "hardness" of the metal has been impacted by the rewelding, and therefore it may not withstand being run over by a tank.
This could be a fun test, can I drive? I didn't realize it was off-track. In your original post you said you wanted any information about them. We're providing it. IMHO, this is a very educational discussion for somebody that isn't familiar with the hydras.
According to the post on arfcom Tony_K says there are less than a half-dozen non-welded hydras. It would be very unlikely for a shop to have 2 of them at once. What you likely saw were some very good re-welds (likely done by Norrel). He did such a good job at rewelding them that ATF confiscated a bunch of lowers because they didn't believe they were re-welded, until they x-rayed them. Norrel had them all promptly returned.
I believe you're wrong on this. Once a MG, always a MG only applies until it is no longer a firearm. Once a firearm has been properly demilled, it is no longer a firearm but is now scrap metal. The MG parts can then be modified to not be MG compatible (weld the sear-hole shut etc.) then it can be re-welded into a complete firearm that is considered brand new, as if it was just made that day and never existed before, no history attached.
Who's got a tank?
Why would you want to do this to a perfectly good registered receiver 
Who's got a tank?
knock over my M4 when we were unloading a CONEX once, which put it under the rear tire of a humvee we were loading up. The guy drove off, it rolled over my weapon, and all it broke was the plastic snaps on the sling.
