This is the reply i received from PTR:
As for the magazine fitment, there is a manufacturing tolerance for the width of the magwell. We maintain a magwell width that is on the lower side/tight side of the manufacturing tolerance to ensure tight mag fitment, and proper feeding. Due to this, it's not surprising that the magazine fit on your PTR is tighter than that of your friend's HK. It is definitely not recommended to do any filing on the receiver. You can spread the width a bit with a pair of pliers (insert into the magwell then pull the handles apart from each other to spread the sides of the magwell), but I would recommend leaving the magwell as is. If you do choose to go this route, please keep in mind that any modification to the receiver can potentially void the warranty.
Best Regards,
xxxxx