I cannot imaging anyone shooting loads in a Rossi (an M92 Winchester basically) chambered in .357 Magnum, that could stretch the receiver sufficiently to cause excessive headspace sufficient to result in head separations. IMHO, that would leave a couple of other possibilities: 1) a manufacturing error from (a) the rim relief cut being made too deep, resulting in excessive headspace; 2) the barrel being incorrectly installed, again resulting in excess headspace; 3) an incorrectly cut chamber with a bulge or a bulge possibly caused by a squib load followed by shooting the next bullet with the first slug stuck in the barrel just ahead of the chamber; 4) bad ammo, which could happen with the two different brands on a random basis, even though less likely.
As was posted headspace can be measured by gages (Brownell's may have them, or Clymer Manufacturing makes them). I'm not sure of the costs nowadays. A full set comes in "GO", "NO-GO" and "FIELD". Actually the "GO" and "NO-GO" are for gunsmiths to use when setting up a rifle, though the "NO-GO" gives you a conservative measurement of excessive headspace. The "FIELD" gage is the absolute "do not shoot" measurement. I'd order the full set, however. You also need to know HOW to measure with the gages, so you don't get false indications. Without disassembling the rifle, you must hook the rim under the extractor, while holding the gage so it goes into the chamber. BE CAREFUL NOT TO SCORE THE TOP OF THE CHAMBER WITH THE SHARP EDGE OF THE GAGE! The ejector will probably give some pressure against the gage, and it would be best to remove it, but not absolutely necessary if you do it right! Close the lever with as little pressure on it as necessary to press the breechblock against the gage. If the lever does NOT close completely (to the point where it closes when the chamber is empty, then the headspace is NOT closing on the gage. If the lever closes completely (DO NOT FORCE IT), then the chamber is accepting the gage, which is NOT good with the "NO-GO" or "FIELD" gage.
A chamber casting is made using LOW MELTING POINT CerroSafe bismuth alloy metal. Browning and Dixie Gun Works carry the metal. You run a patch down the barrel until it is just ahead of the chamber. Melt the metal in a double boiler (using a tin can or a pot that your wife won't miss! Form a funnel from some aluminum foil and pour the melted CerroSafe into the chamber When the metal has hardened, punch it out with a cleaning rod. If there is a bulge in the chamber, this may take some force. Such a bulge would show as a scrape on the side where you forced the casting past the normal diameter of the chamber. You can measure the casting about 1/2 hour after hardening to get the exact chamber dimension...the stuff shrinks back slightly to the exact dimensions after this time.
Hope this helps!