I'm not spotting #7 loads for .38 Special in any of my manuals. The problem is that it's not designed to burn well at .38 Special pressure, which is only about half the pressure you can load your 9 mm to. Powders that are burned at pressures below their design limits are prone to erratic burning and muzzle velocity and to squibbing out (extinguishing and leaving a bullet stuck in your barrel), and that's a hazard that has ruined many guns when the next round is unknowingly fired into the stuck bullet.
QuickLOAD predicts that between the low .38 Special pressures and the low starting pressure of lead (as opposed to jacketed) bullets, as much as half the powder could go unburned in a 6" barrel using the lighter bullet. If I tweak the load pressure up in QuickLOAD, it calls for loads too heavy for me to want to mention them online because I don't trust the prediction under the circumstance of erratic ignition and burning.
Frankly, I just wouldn't use it. If you had to in some kind of end of the world scenario, I would use only the heavier bullet, as that improves the burn. I would use a chronograph to get to the velocities you had with that same bullet using a faster powder, before. You might improve ignition further by using a magnum primer, but use it with your starting load and work up to velocity with it. Don't just switch after having worked the load up with a standard primer, as that could cause a pressure jump. I would not try to use these loads for any kind of rapid fire. Always check there is a new hole in the target after each shot so you know nothing is stuck in the bore. Don't be surprised to find unburned powder that escapes the barrel/cylinder gap in a revolver winds up clogging the mechanism or at least gumming it up and requiring detail cleaning to get it working smoothly again. I got that when I tried to use 2400 in low pressure loads 30 years ago.
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