If you research this on the internet, you will find the reason is more of a liability issue. The ammo marked " Do Not Use In Ported or compensated Barrels" is a Plated Bullet and can sheer off a portion of the plating when it passes the ports. These can be thrown back into the shooters face!
Most ammo makers are going to FMC, as opposed to FMJ as it is considerably lower in cost to produce.

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It's The Carbonaro Effect!