Very nice, you will like your Uberti. I had a Uberti Cattleman 4 5/8” .45 Colt years ago, I gave it to a friend when he retired and moved to Idaho.
“Cowboy loads” are usually low-powered, lead bullet loads that are directed towards Cowboy Action Shooting. (I did this back in the 1990’s.) That shooting discipline does have a low-power minimum of 400 fps/power factor of 60, as you need enough to knock over a pepper popper on some stages. (There is a maximum of 1,000 fps, to avoid target damage and excess splatter from steel targets.)
Guys like lower recoil to stay on target at speed, so these lighter than standard target loads got that nickname.
Most factory .45 Colt will be safe in your Uberti, including the 255 gr lead loads and most others. There are some loads that are not safe for the Uberti, as it is a clone of the old Colt with the same limitations. These will be marked +P (more pressure) or “Ruger only”, as those guns are much sturdier than the Colt/Uberti design. I would not use these higher powered rounds in your Uberti, as it can prematurely wear or damage the gun, or possibly injure the shooter if the gun itself fails and bursts.
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All of these lead bullet loads are fine in your revolver. (Ignore the price tags, these boxes are pretty old. Expect to pay up to $45 bucks a box/50 now.
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Let us know how it shoots when you get a chance.
Stay safe.