.22 Brocock Commander Hi-lite and 17.5 gr NSA slugs.

The 17.5 gr. NSA slugs do well. The trick to get them to cycle reliably is to seat them into the magazine a few mm or so. Accurate and flat shooting. Yeah we get huge Fox squirrels here.

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In choked barrels I've found the smaller diameters work better for power, ease of loading and often accuracy. I run the heaviest slug I can reach 900-980 fps with. 

The fastest I could push the NSA 17.5 gr. out of the Hi-lite was 885 fps with the reg at 155 bar. The 17.5 gr. come in only .2165. The .217 20.2's would do 845 fps.

Remember to seat them. If you don't they can walk out of the back of the mag when you cycle them and become misaligned and jam on the way back in.