CCI 400 is safe for everything except maybe AR type rifles. The reason is that the firing pin in the AR is free floating in the bolt carrier, which could result in a slamfire. I have fired thousands of rounds in AR rifles and this has happened to me twice. The cause was a dirty bolt carrier causing the pin to become stuck. Cleaning fixed the problem. The armed forces decided to have a stiffer primer as in a combat situation there can be limited ability to clean it.
It's nothing to do with AR-15s. Early thin-cup CCI and Remington primers were designed for the original small primer rifle cartridge, the 1930s .22 Hornet which was both low pressure and saw many users adapt rimfire or other actions with weak firing pin strikes. When Remington introduced the .222 Rem cartridge in 1950 with its 50,000 psi MAP, it also introduced the first so-called SR 'Magnum' primer its 7 1/2 model which was deemed necessary to cope with primer extrusion and possible failure with the Triple Two's higher pressures. Although officially a 'magnum', it actually used the same explosive pellet as the thinner cup Rem 6 1/2. (It was later replaced by the 'hotter' Rem 7 1/2BR which was needed for reliable ignition in the .17 Remington cartridge in all conditions.)
The US Army requested less sensitive primers much later for the AR, the answer being the CCI-41 which is pretty well akin to the CCI-450 SRM in performance, but the 'anvil point' is placed further from the explosive pellet instead of nestling against it as is normal. This allows a small cup indentation to occur thanks to the free floating pin without pressing the anvil into the active compound risking ignition.
For a long time, Rem 6 1/2 primer sleeves bore a warning they were not to be used in any modern high-pressure cartridge such as .223 Rem. AFAIK, only Midway USA now prints this warning:
https://www.midwayusa.com/product/1601138078