What causing this and how to prevent this.

[Che762x39]

I’ve gone as low as 3.8gr for a 125gr xtp.

Why? I look at the 125 XTP as a premium bullet and load it as a 357 Magnum or a 38 Special +P. Not a light plinker load. JMHO, YMMV

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[Che762x39]

Good point.

I just use plated bullets since CA Indoor Ranges have an issue with us using lead bullets. Going thru my supplies I found three boxes of XTPs and fortunately I had 357 Magnum brass which I have not used in years.

So shooting 357 Magnums was very nostalgic for me.

[stand125]

Titegtoup is pretty dirty. I switched to magnum small primers and it helped somewhat, but not a lot. I actually use small rifle magnum primers in all of my pistol loads.