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Buy the 8” version and have it cut to 6.5”. That’s the way to go IMO since you get the extra picatinny space and extra MLOK mounting.

And also, DEFINITELY buy a Gen 3 Lingle lower for it. Gun runs 100% with that setup and there’s not an MP5 on the planet I’d take over it.

I think the longer one with the barrel shortened to where the handguard ends is the best option, but that's just my outside opinion, as an observer who doesn't own one.

Several members here have mentioned doing that though, and they seem to like it.

Basically this.  Unless the intention is to semi-permanently suppress it, the 8" is very definitely the way to go.  The 5" is awkward and a fingertip remover, for non-existent practical size savings, at the cost of a decent amount of ballistic power; and handguard space.  

As to Scorpion magazines, I've yet to see anything show they are any more reliable than the Stribog curved mags.  (yes, we all know the straight Stribog mags have problems).  But sure, the Scorp lower is a non-regulated item, so get one at your convenience, if you like.  

There really isn't an "Influencer" review of a basic 8" current roller delayed Stribog with curved mags.  I have a couple random Rumble video's of people using it to just blasting away for 20 seconds, sometimes even running straight mags just fine, with no commentary.    Note the near complete lack of recoil with the Euro 40 degree roller delay block in it.