You're too hot.
The closest thing that the Western Powders load data shows are couple of 110gr bullets (no 100gr).
One of them shows a 5.6gr-6.2gr range for True Blue for 38spl +P.
The other shows a 6.7gr-7.4gr range for True Blue for 38spl +P.
I know your bullet is 10gr lighter - but at your starting charge of 7.8gr you're above the max for a bullet just a tiny bit heavier.
Where did you get the 7.8gr for a starting load?

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