Cartridge case blew out at the rim.
I've seen it infrequently. Usually ammo of questionable quality. I had a lot# of Hansen ammo imported in late '80s. (I believe it was SKG, of then E.Germany. Same a Wolf Match Target today.)
The ammo was exceptionally accurate. However, in some Savage/Stevens single-shot rifles, it would frequently blow out, typically where the firing pin would pierce the case.
When it did blow out, the report was loud (to the person firing, and bystanders). It would also cause some gas (smoke) to escape around the bolt face. Annoying, but not a significant issue. However, if sufficient gas escapes, it could cause the semi-auto to not cycle properly, and perhaps jam as a result. Also a good reason to always wear ear and eye protection, even if shooting a .22rf.
I have two Rem. 597's; a LR and a WRM. Neither has had the blow-out's you described. If it happens again with a different lot#/make of ammo, it could be the firing pin. An excessive sharp or mis-shapen firing pin can cause this. Extensive dry firing a rimfire can cause a burr on the edge of the chamber where the firing pin impacts it. This too can cause slam-fires, mis-fires, and case rim blow-outs.
If it continues, have the gun professionally inspected/repaired.
The only issues I've had with my 597's is related to known problems with the magazines for the .22lr version. Proper lubrication/maintainance of the magazines remedies this. The .22wrm doesn't like the 50gr CCI/GoldDot ammo- sometimes fails to feed the last two cartridges.
Both rifles are exquisitely accurate, however. (both have been glass bedded, and triggers honed to 2lb's, crisp) The .22wrm is near MOA at 100yds, the .22lr will shoot under 1/2" at 50yds with ammo it "likes".
