I have FTF(eed)s every now and then where the bolt slams shut (partially) on a halfway fed round, usually disforming the case. It happens every now and then on the surplus aluminum 20s I have, while it happens all the time multiple times per magazine) on some steel 20s I just bought. The steel 20s were still in wrap and some in plastic, but all dried out with some corrosion/rust to varying degrees.
It is possible that these issues are magazine related - I haven't really paid much attention to trying to track whether it is tied to specific mags yet but will label them to do so going forward. That said, I know it has occurred using multiple mags, so I am not sure that it is.
That said, the steels for sure need some kind of remedy, they are rough and stick and don't feed as stated above. Is sand/bead blasting a good way to go here? Dremel? What else?
Edit: 4/6/2020 - Updated assumptions given conversations
Edit: 4/6/2020 - Contact made with PTR - sending rifle in for inspection.