If in good condition, most of those guns are perfectly OK with modern ammo, since ammo makers are careful to keep pressures to the same level as the old black powder cartridges.
No, no, 1000x
no! If a gun is black powder only,
it should only be shot with black powder or black powder substitutes! The pressure curve of black powder is nothing like that of smokeless, even if the maximum average pressure of a given smokeless loading is identical to its BP counterpart. Smokeless loads will batter a black powder gun in ways the heat treatment of the era just isn't able to cope with.
Real-life example: I own a beautiful Colt 1878 double-action made in the 1880s. It has relatively little collectors value as someone went through an awful lot of trouble to refinish it, so I bought it as a shooter. When I got it, it had a lot of fouling that I immediately identified as being the product of smokeless. While the cylinder was okay, the soft BP-era frame had seen a bit of stretch (ergo, there was end-shake,) and the hand (which in the 1878 design is wholly responsible for locking the gun into battery) had a chip knocked off it. This was from smokeless loads the previous owner was convinced was safe to shoot in his gun. Sure, the gun is a sweet shooter, and locks up tight with Pyrodex and Triple Seven loads. It just locked up with not-insignificant throw-by.
The end-shake was easy enough to fix. A new Single Action cylinder bushing, appropriately trimmed and fitted, and a Belt Mountain base pin to lock everything in place without adding additional wear. The hand . . . not so easy. A new replacement hand cost me $100. All told, I spent an eighth of what I originally spent to buy the gun to repair the damage that had been done to it by shooting smokeless.
Now, I'll take a minute and have a beer . . . better.
Can anyone tell me about the old .32 and .38 H&R break tops, any good/reliable, how much are they worth, and so on? Oh also anybody know about Forehand, or Iver Johnson top breaks
The H&R and IJ top-breaks were the inexpensive alternative to genuine S&W top-breaks, and were made in mind-boggling numbers (from about the mid 1880s clear up to the dawn of WW2.) As a result, they're not worth very much (for 20th century examples in average condition, much less than $200.) The actions tend to be fairly robust, when considering the purpose for which these guns were designed, (save the occasional improperly heat-treated spring,) except top-breaks are especially prone to frame stretch. Pre-WW1 examples from the 20th century should be regarded as
black powder only guns, unless blessed to shoot smokeless by a qualified gunsmith. Pre-1898 antiques should never be shot with smokeless. Post-WW1 examples can be shot with smokeless, but should be done so sparingly. They'll take well to handloading, but as my Lyman manual states, keep your loads on the low side.