Because of the larger case volume, using 38 Special data in 357 cases will result in lower velocity and, possibly, erratic ignition.

Look in a good loading manual and you will see many "mid range" loads in the 357 section. Select one of these for your practice loads.

Even better would be to load with Trail Boss. Fills the case up well with powder puff loads. popular with the cowboy action crowd.

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-jwk-
US Army '72-'95