A Model 10 will chamber any .38 Special ammo, but the older ones aren't strong enough for +P ammo (there'll be a +P in the headstamp). I'd recommend a 158 grain lead semi-wadcutter for a start. They're cheap and will most likely shoot to the sights. A second choice is a 148 grain wadcutter. Both are mild loads and everybody makes them.
If it's a Model 11 chambered for .38 Smith & Wesson, Remington and Winchester each make one very mild and identical load. They might be hard to find.
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