I have a Leupold Mark 4 LR/T 3.5-14x40mm on mine. It's a great scope. Careful with bigger than 50mm objective lens, it will make it difficult or impossible to lock the charging handle in the notch.
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I had a USO ST 10 on mine and it was a great scope for my SR9TC. They can be had used for $1000.
My SR9 wears an SWFA SS 10x42M . . . and loves it.
Sparkplugs are irrelevant.
Plenty of HK's and clones.
Reach out and touch the target, use the H & K SR9 Hidden Content Man I'm lucky !