Those are KNS anti-rotational pins. They were designed to use on 'registered' full auto lowers to keep from wearing out the holes in the lower receivers (since these lowers are $10K+). The side benefits of them are that they make the trigger pulls very consistent shot to shot and the pins can't walk out.
I use them because of consistent trigger pulls, Gen 1 pins work best in this regard.
Gen 1 pins on my LMT. (this is a register SBR, which if I had to replace it would cost me about $400 for a new lower and tax stamp).
Gen 2 pins on my 3gun rifle.
Chief Armorer for Elite Shooting Sports in Manassas VA
Chief Armorer for Corp Arms (FFL 07-08/SOT 02)
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