It's all about pressure.
The Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) was founded in 1926 and set the standards for allowable pressures in sporting ammo. Guns made in North America since about 1930 have adhered to these standards and if in good shape should be safe to shoot with modern ammo from the point of view of pressures generated. Chamber length and hard (steel) shot are two factors which need to be considered before shooting, but a SAAMI spec gun should survive the pressures generated by modern ammo (given the correct chamber length).
Know the chamber length of your gun! 12 gauge guns have been made in various lengths, from 2 inch to 3 1/2 inch, both in North America and Europe. The chamber is the length of the FIRED shell, so yes, a 2 3/4 inch shell WILL fit into a 2 1/2 inch chamber. That doesn't mean it is good to shoot. If you don't know, measure.
With shotguns made prior to SAAMI, each manufacturer made decisions as to what pressure levels their guns were designed for. As a rule of thumb one should not use modern ammo in these guns. There are exceptions, but you had best know what you are doing.
Guns made in Britain and Europe are a different story. Today there is the C.I.P., the euro equivalent of SAAMI, but for most of history it was the manufacturer and the proof house which set the pressure limits, and they were different for different guns. For instance, an English 12 gauge could be proven for a 1 1/8th ounce load or for a 1 1/4 oz. load. Those loads had different peak pressures and were subjected to different proof loads. The proof marks were then stamped on the gun, and the owner is supposed to adhere to those loads. This is only valid so long as the gun is still in proof, i.e. the bores have not been honed out too far, or the chambers lengthened, or deeper pits have formed.
In general the pressures generated by the European loads are less than SAAMI spec. Today, the difference between SAAMI and C.I.P is negligible, but for guns built before 1950 the proof marks should be carefully examined and appropriate loads chosen. Remchester ammo is rarely the best choice.
GameBore (distributed in Canada by Kent) produces a line of ammo specifically meant for shooting these lower pressure guns. Many recipes exist for the handloader as well.
Promo loads from Can Tire or Wal Mort can exhibit the highest pressures of all. To keep costs down, these loads use smaller amounts of fast burning powder. This produces a short, sharp spike in pressure. Not what you want in an older gun.
Black powder burns very slowly compared to smokeless - so slowly that the 4 millisecond window of my pressure gauge cannot record all of it. Barrels made for black powder should NOT be used with modern ammo for fear of losing body parts! There do exist, however, a number of Damascus barreled guns which were proven or re-proven for smokeless powder. These can sometimes (if still in proof condition) be shot with low pressure smokeless loads. Do not do this without intimate knowledge of the barrels, the pressures they were proven for, and the pressures generated by the ammo. In these cases, it is not just the peak pressure but the total pressure curve which is important.
In summary;
1) If you want to use modern ammo, buy a modern (SAAMI spec) gun. Beware of issues with steel shot (wall thickness, chokes).
2) If you want to shoot older American or European guns, buy or make low pressure ammo. Avoid promo loads. Know the condition of your gun.
3) If you want to shoot guns made prior to WWI, especially Damascus, twist or laminated barrel guns, get knowledgeable, buy some measuring tools, hand load and be intimately familiar with the construction and condition of your gun.
Note: Velocity, high or low brass shells and other "folk lore" indicators have NO RELATION to the pressures generated by any given load. In addition, it is very rare for a manufacturer to tell you the pressure generated by a load. As an example, a 2 1/2 inch load I use and the Winchester 2 3/4" promo load (lead #7 1/2 shot, the 4 boxes to a pack stuff) both produce velocities in the 1200 fps range. The Winchester load has a peak pressure over 3900 psi higher.
Shooting older shotguns is very rewarding and enjoyable. In my opinion some of the best shotguns ever made were produced between about 1885 and 1920. It is not fun if you blow your fingers or face off. Don't guess, either learn for yourself or get the opinion of a knowledgeable gunsmith. Spend the time or money to obtain the appropriate loads and be sure the gun is safe to shoot. Then go out and have fun!
Sharptail