Agreed, compared to a dangerous game rifle it is mild.Energy is not much use as a measure or usefulness. A Mini car traveling at 1mph may have more energy than a 300PRC but it is useless for deerstalking.
Very mild compared to a large calibre, 458 Winmag, for instance. It is very capable however, like a Martini Henry or similar calibres. I would not compare either with a 300PRC.
It can be shot very accurately long range and there is a following, shooting these to great effect out to 1300yds. Outshooting many modern rifles and cartridges, but as you pointed out, it is not useful for long range shots on deer due to the bullet drop. I would agree with a 150m MAX but better kept below 120m imo. It’s fine on known rages where the sights are set for that particular range.
We’re talking about lever actions where 400gr @2000fps can either be called a fantasy or attempted suicide. Only actions like the Ruger No1 or 3 or bolt actions can withstand those pressures. Lever actions cannot.
But, in the UK deer stalking context it has heavy recoil, 2-3 times that of most common stalking calibres, exacerbated in a light lever action.
Also, 2000 fps isn’t a fantasy, Hodgdon give load data for such in their lever action specific data.

