I have several Bushmaster upper receivers.  But, I recently got several DPMS stripped A1 upper receivers.

I noticed that when I thump (hit) the DPMS uppers with my finger, it will have a "ting" or "ping" sound to it, and I can feel the vibration travel thru the piece.  It is very similar to a tuning-fork type of sound and vibration.

When I do the same thing to my Bushmaster uppers, they produce a dull, dampened "thud".

Both Bushmaster and DPMS claim that the uppers are Forged from 7075 aluminum that has been T6 heat treated.

The DPMS upper will ping (act like a tuning fork) the most if thumped along the carry handle railway.  I have several DPMS A1 uppers that I just purchased from a very reliable vendor.  It does Not seem to matter if the upper is stripped or completed assembled and attached to a lower (in condition 3).  The "ping" sounds the same with the stripped and assembled DPMS upper.

WHY DOES THE DPMS ACT LIKE A TUNING FORK ???

The recievers have less metal [thinner walls ,handle area ect.] and they are made from forgings the same place as the A2and A3 upper receivers come from. I have several old and new A1 and A1E3  receivers that do the same thing and these come from 3 different forging company's.