The SPC II has a longer throat so you can load/build hotter rounds. If you can't do it with the SPC, you probably can't do it with the SPC II either.
I built my bolt gun with the SPC so I could jam my bullets into the lands. So far, -1/2" groups seem to work pretty well for me.
If you need a hotter round in an AR, go .308 and be done with it. Or better yet, stick with a .223 and learn to use it.
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