Any insight on their protocols testing velocities of their loads? 10 shot strings? Extrapolating based on a few actually chronographed loadings?

Looks to me that they test a low, middle and high loading and then derive the velocities in between.

Cold be, but they also don't retest every load for every cartridge in every edition of their manuals. Some of the loads can be a decade or more old since they were tested. Components change.

Did a short stint working for a large, well know commercial ammunition manufacturer back in the early '80's.

P&V testing was done on every load being developed. There were many data points that are never published.

Generally, they will find a point on the pressure scale below the SAAMI max where the pressure/velocity relationships are linear and publish that data. If I recall, none of our data was ever extrapolated.

As you approach and exceed min/max recommended pressures the pressure curve no longer becomes linear and some funky stuff can happen.

The ammo manufacturers normally do ballistic testing on every lot of powder they receive and develop loads for each caliber to stay within their peak pressure and velocity limits. Their labs have special test barrels and receivers for that purpose. They may use off-the-shelf guns for functional testing, but generally not for pressure and velocity testing.

Correct. Function testing was done on the commercially available production guns. P&V testing was done with test chambers and barrels.

Great inputs. We have very knowledgeable people on this forum and I appreciate very much the advice.

Best to all

I have visited the ballistics labs of Remington, Winchester, and Federal (also Lake City). I don't know about Lake City, but all the ammo makers have fairly impressive gun collections.