KiltedClaymore, sorry man, you have no idea what you're talking about here.
OK.
First, Pietta makes a pretty decent gun. About on par with Uberti, with strength right in the same ballpark as a post-WW2 Colt SAA or Ruger New Vaquero.
The term "Black Powder Frame" in this context has nothing to do with strength. It means they've duplicated a cosmetic feature found on Colt SAAs of the 19th Century, a retaining screw for the base pin instead of a spring-loaded cross-pin. There's no strength difference; if anything, the older design is actually better (resists spitting the base pin - I do recommend blue locktite on this screw...).
Next, if the gun is stamped "357" on the side, you can ABSOLUTELY shoot 38+P in it. Unlimited diets of same. 357 pressures can top 35,000psi, 38+P never tops 20,000.
Finally, if you compare the cylinders between an SAA-family design like this one in 38 or 357 and, say, a Ruger GP100 or S&W L-Frame, the SAA-family cylinder is fatter and has more overall "beef" in every direction. No surprise there, the cylinder was originally meant for 45LC. By our standards it's kinda borderline as a 45 (which is why the 44Spl is well thought of in these guns) but in 38 or 357, you've got GOBS of strength available.
Upshot: you can shoot the nastiest Buffalo Bore, Doubletap or Grizzly Ammo 357 fodder in this gun. Some of it hits 800ft/lbs energy. This stuff will be just fine in this gun, except that to eliminate the base pin retaining screw, use a drop of blue locktite. Firing a hot round with a loose base pin could be REAL bad.
Now understand, I'm NOT a guy who advocates "skirting the edge of safety" in any gun, any caliber. I'm pretty damned conservative on this stuff. This is one time I'm telling you that you can stretch this gun's legs and let it run.
The only way you could hurt it is with handloading at the ragged edge of the 357's performance envelope. THAT I don't recommend, even though this is an abnormally beefy 357.
For the record: my favorite gun (and daily CCW) is a Ruger New Vaquero in 357, which is in this same strength/size/heft range. And I've run monster factory ammo through it no sweat. I have a cross-pin base latch system; I've put an over-strength crosspin spring in, again, to prevent base pin jump under big recoil.
WARNING: one thing the Pietta doesn't have is a decent safety. Carry it FIVE UP, load one, skip one, load four, that should lower the hammer on the empty cylinder bore. Load it six-up, drop it and there's a fair chance it will go off...hammer down OR hammer back.