Soviet State Secrets remain secret. US secrets are all over the internet.
AFAIK, when destructive testing is needed, most manufacturers contract with HP White or do it in house. Prooftesting is also usually done in-house in the USA, we don't have government mandated "proofhouses" like Europe does. Most proof loads are either loaded by the gun manufacturer or are bought on a very restricted basis from commercial ammo makers. You can bet Colt and Marlin have tons of signed agreements with CCI or whoever before they can buy any proof loads. Same goes for load data, someone somewhere would sue.
There are several pressure transducer setups out on the market now, in the $600-$700 range last I looked. You glue the sensor to the chamber area, shoot, and a USB doohickey samples the signal and sends it to a laptop, running software that generates a nice graph for you.
They're also reuseable, so you can use the transducer on multiple firearms, or all day while you're developing loads.
Expensive, but you can use them over and over, like a chronograph.
I recall the one I priced was $300 assuming you just need the transducer and software. I'll supply my own laptop computer :) I was planning to get one of those sometime I have a chance to actually do some more testing.
Thought it was this one:
Ballistics Software, Chronographs & Pressure Instruments For Shooters
... however they only advertise their 2.0 wireless-only solution. I guess they have upgraded since I last priced them :(
You can use this information to compare to known loads as well. QuickLOAD software reportedly has pressure information attached to their application but I have not used it.