Not sure that I buy such an explanation from the manufacturer. HKs, like Glocks, have polygonal rifling which seals gas behind the projectile more completely than conventional cut rifling which tends to result in higher muzzle velocities from barrels of the same length. Of the 8 HK pistols that I have in 9mm(3), .40S&W(2), and .45ACP(3), all of them produce higher velocities than pistol barrels of the same length with conventional rifling.
If polygonal bores are the culprit, then Seimic would've also issued a caution against using their ammunition in Glocks (and other pistols with polygonal rifling) since polygonal barrels seem to be the only difference between the excluded HKs and all others.
Something is just not adding up here...
