Eye box and the fact you cannot get true 1X with an optic that has magnified lenses in play. I loved Short-Dots and the red dot works well but will never be the same as an Aimpoint or EOTech (or other 1X sights).
Still, though, they are a better option than fixed magnified optics like the ACOG. The ACOG 4X was great for the time and in a military application because they are built like tanks but there's a good reason they tried to slap a RDS on top of the ACOG.
And agreed no reason why short barrels can't be accurate or employed well past QCB distances. They actually have some benefit to the shorter length being more rigid assuming the profile is the same as a longer barrel.
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