Just got a really nice 38 safety hamerless from 1890's time frame. Can I shoot smokeless ammo through it?

If the gun is tight, I see no problem. To my knowledge, the 38 S&W cartridge has always been loaded to be safe in the break-open revolvers.

Bob

Yes. Factory .38 S&W smokeless loads are lightly loaded to pressure and velocity levels safe for use in for the old revolvers. Don't believe the myths about smokeless powder "pressure spikes" as there is no evidence such things exist. The majority of these old top breaks have probably fired far more smokeless rounds than black powder ones, with no ill effects. Remember that smokeless powder loads pretty well replaced black powder loads very early in the 20th century, and for the most part, top break guns manufactured after 1900 are largely identical to those made in the 19th century in metallurgy and design. Heat treating of steel was not even practiced by S&W until after WWI, well into the smokeless powder era.

Yes, within reasonable limits and with proper caution. I don't have experience with S&W .38 topbreaks yet, but I own and have fired Iver Johnson, H&R, and Hopkins&Allen .38 topbreaks from the era, all of which were considered lower quality than S&W, and have had no issues. The problem with guns that old is never the strength of that type or model as such, but what abuse the particular gun has suffered over the past century from previous owners. When I've shot guns like these, I basically take the same precautions as when shooting muzzleloaders, i.e. inspect the gun, especially the cylinder, frequently while shooting and be prepared for the eventuality that something might come flying in other directions than just out of the muzzle. But manufacturers are aware that with an obsolete caliber people are going to be stuffing it into antiques, and keep pressures down accordingly.

DON'T use Buffalo Bore .38 S&W ammo.
It is loaded way too hot for the top breaks.

Other US made ammo is loaded light, in deference to the top breaks and should be perfectly fine.
Unless I'm really wrong.

__________________
What would Jim Cirillo do?

Most excellent point, sir.
I should have thought to mention that. The Buffalo Bore folks are on a mission to remedy the overly cautious approach of mainstream ammo manufacturers to classic calibers and load at the top of what the case will take. Great for modern guns in old calibers (I used them in my .45-70), not so much in antiques.

I think BB advises use of their .38 S&W ammo only in solid frame revolvers. If you happen to have one of the S&W .38/200 British Commonwealth WWII service revolvers (or other more modern solid frame revolvers), and handload for it, it's very simple to safely reach .38 Special performance levels. Just don't use them in a top break from any manufacturer.