25epps,
Welcome to the Campfire.
When I bought my first 5.3 Creedmoor in 2010, it didn't take long to notice the resemblance to the .250 Savage Improved. In fact back then 6.5 Creedmoor brass was pretty scarce, so when assigned to do an article for HANDLOADER magazine I tried blowing out .22-250 brass (far more available than either .250 or 6.5 CM back then) using the technique of using some Unique powder with cornmeal or Cream of Wheat (uncooked of course) on top. It worked perfectly, so the resemblance between the .250 Improved and the 6.5 Creedmoor isn't exactly news.
The Ackley version of the .250 Improved does indeed have a 40-degree shoulder, but the RCBS version (which many people do not remember) had a 28-degree shoulder, very close to the Creedmoor's 30 degrees.
