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I cut down a barrel for my MP5SD, cut it to 330mm length. That places it about 2-3mm inside the end of the SD muzzle. You won't find one cut to that exact length, just find one a bit longer, hacksaw the end off where you need it, then dress (crown) the inside and outside to remove burrs.
BTW, it added roughly 20-22fps to my velocity, not a bad thing! And it still works just fine with the ported cylinder.
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