Two feet from your chrono is likely the answer. Suffice to say that if you could truly pull 4,500 FPS from a 124-grain 9mm load from an M&P, we'd all be trying to replicate the feat and go in to commercial production of it.
To believe that you actually launched 9mm slugs at 4,500 FPS would also be to believe that some rich guy is going to step up and buy out the national debt next Tuesday.
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Attention Brass rats and other reloaders: I really need .327 Federal Magnum brass, no lot size too small. Tell me what caliber you need and I'll see what I have to swap. PM me and we'll discuss.
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