Was trying SP out in .38spl. I want some light loads for plinking. I was using 125gr Xtreme CP bullets.
The load data sheet says 4.1gr max but take 10% off as a starting point. Hey that sounded good. NOT
What happened was failure to ignite. But only if the powder was towards the bullet. I was right on the cusp. Tilt the gun back and the rounds fired just fine. Tilt it forward and it wouldn't ignite. Fortunately I was using a 3", very tight gap S&W M60. When the the powder didn't ignite there was enough pressure to lob the bullet to the 5yd target. Disconcerting to the shooters around me, but I could see the bullet in flight and also see the very low holes in the paper.
As others have pointed out, SP isn't for small loads in longer cases. Works just fine in 9mm and .45ACPs