@malbro
OK, it just seems a very odd metric on a projectile device: instantaneous "power" on the moment of release. I've never heard of it in 35 years of obsessing over air rifles and their science.
But your lb.ft/s is more useful as a measure of momentum (mass x velocity i.e pounds x feet per second), rather than power (pounds-feet per second) even though the unit is equivalent.
The typical constant for ballistic calcs with imperial units is 450240, derived from the 1/2 in the KE formula (e = 0.5 m.v^2), the number of grains in a pound (7000) and the acceleration due to gravity (32.16 ft.s^-2). You need the acceleration of gravity value to convert from pounds (mass) to pounds-force (lbsF). From velocity in fps and mass in grains this gives energy in ft.lbsF (and almost nobody includes the F, instead using the incorrect but generally accepted ft.lbs "foot-pounds"). Problem is gravity varies at different points on the Earth's surface, so you never truly accurately know your air rifle's power unless you measure gravity where you are when shooting!
In SI you don't need to worry about this and can happily use metres, kilos and joules with the standard 0.5 m.v^2
I still use imperial by force of habit but generally switch to SI when things get complex.