Swifts often want less powder than other similar weight bullets. Their copper jackets are "grabbier" in the bore than the more commonly encountered gilding metal jackets.
On top of that the Interlocks have bearing surfaces toward the short end of the scale which reduces pressure.
Combine those two factors and the discrepancy you're seeing is readily explained.
If you're careful working up with the Sierras they'll come out pretty close to the Hornady numbers.
The node you're looking for may not land on the same velocity as you found with H4350. They're located more by barrel time than speed.
You stated something I forgot to put in my original post. I did some *ruff measuring of the bearing surface between the Hornady and the Sierra last night. They are very close to each other.
Not having a swift to measure I could not tell about them, but their website states they are about the same length as the Hornady. Having the copper inner part means they have to make up for the weight somewhere else. My theory is they have a much longer bearing surface than the Hornady's or Sierra's.
