Onagoth,
I know the topic is a little bit older and I'm not sure if you got the answer that you were looking for but the answer to your POI difference is most likely a difference in muzzle velocity. The MV listed on the box of ammo is typically data from a 24" test barrel (per SAAMI spec) unless otherwise noted. The poster who had the 18" chrono data at around 2900 fps sounds accurate. I typically use an estimate of 50fps velocity loss per inch of barrel length on .223 and it has held up.
If you're shooting the ammo out of a 16" barrel (I didn't see if you had mentioned it), the the velocity that you calculated for your POI is probably pretty close to what you're getting out that barrel.
