I've had a couple of the ones just built on sporterized military Mausers. Both 12ga. I've seen ones in 16 and 20's.
I past up a beauty of a 12 with a custom sporter stock as nice as any rifle. Stupid me,,one of those moments.

The 12ga guns shot fine. The 98 bolt safety lug is the locking lug. The front lugs are gone in the conversion to make room for the 12ga shell.

The front portion of the 98 bolt is cut off and a larger separate shell head of a diameter suited for the 12ga fits into the front of the bolt.
The only thing that holds it in the bolt is the original 98 extractor, slightly modified itself,, as it snaps into a groove in the outside edge of this new bolt head.
That's all that holds it in place That allows the bolt to still rotate.

Here the problem can occur. The shotgun can still be fired with out that bolt head in place. Put a shell in the chamber, close the bolt and the firing pin (it's the original 98 bolt FP) reaches the 12ga shell and fires it with a pull of the trigger. Nothing in back of the shell but your face, as the bolt head is missing.

The bolt heads are often missing from these guns. They can also pop right out during use if a sloppy or worn fit.
During operation the ejection of a spent shell can also cause the bolt head to eject with it if poorly fitted. Chambering and firing another is as above noted.

One of mine had a habit of tossing the bolt head away during quick bolt manipulation. Trying for that double on Skeet,,that idea was quickly put to rest and a Mossberg 185 does the trick now.

In good condition,,these are perfectly fine for use with low pressure loads,,that is my opinion of course.

You have to be alert and attentive to what you're doing,,but that comes with any firearms use.

Many rifles have separate bolt heads and will fire w/o them in place. SMLE, Steyr's, GEW88, ect. They just don't have a habit of spitting them out as easily during use as the Mauser shotgun! ,,and are very well fitted to begin with in original design.

The 16 and 20 ga conversions still have some of the front bolt locking lugs for engagement. But if they still engage and how much is to be determined,,so don't count on the fact that they do w/o some investigation.

It's that Mauser 98 third safety lug that's keeping the bolt locked,,and it's as much or more than most commercial bolt action shotguns made.
Examine carefully though as the history of the guns are largely unknown. Many were built from nothing but war scrap.