I think all you get with that length barrel and super 300blk rounds is a lot of blast and you really don't gain anything over a 9mm small sized PCC. Second just look at the size of the magazines, it will be easier to carry the 9mm mags over the 300 blk mags.
I think a 8 inch barreled rifle in 300 blk or 7.62x 39 is the shortes that you can really go. I also think 10.5-11.5 is the smallest in 5.56. Now people will point out to me rounds that do ok in even shorted length barrels in 5.56, I know they exist. I just don't think they are as good a tool.
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