Can't do a picture now, but it will APPEAR To be oval because it's drilled on a curved surface. In reality, they don't cut an oval slot and they don't make any oval drill bits I'm aware of. Look at the hole from directly above and/or stick something you know is round in the hole. It should be round. At any rate, it's VERY easy with a standard buffer tube to start to back it out and let the spring and retainer fly. It's a different story with carbine tubes. These tubes must be indexed and often they are indexed too far out to sufficiently stop the buffer retainer from flying out. If your buffer smacks the retainer enough times it could, I suppose, wobble the hole larger and let spit out the retainer.
I've learned that carbine stocks need to be 'tuned' and cut the the exact right length if they fall in that 'sour spot' where they are tight on the buffer retainer JUST before they are in the proper place for you to tighten the nut. A file and a few minutes of careful modification can solve that problem. Me, I like my Carbine buffer to be as short as possible so that the stock retracts to nearly flush with the nut. That's just me. I'm a stickler for those kind of things.
BTW, Scavanged pen-springs work passably for buffer retainer springs. Shhhh, don't tell anybody.
