Energy is energy, and it doesn't matter if you pump it up by increasing the mass or the velocity -- or both.
I have no problem dropping steel with .38 special, lead 147g at 700fps.
147gr @ 700fps is only about 150 ft/lbs of energy. It's a power factor (weight * velocity / 1000) of 103.
124gr @ 1050 is about 290 ft/lbs of energy. It's a power factor of 130.
Sounds like no matter how I slice it, these steel targets are a bit more discriminatory than the ones you guys have experience with. That .38special mousefart load above has the same energy as a .22LR (36gr @ 1300fps = 124ft/lb), but I bet it doesn't have the juice to bowl over the poppers at my range. .22's cannot accomplish it. They don't allow .380's or 9x18's to even compete. 9x19 is the minimum requirement.
I've been reading up on this, and it seems some people have a theory that hitting steel slower, with more mass, is more effective at knocking it over since it increases the contact time against the plate during bullet deformation, and the inertia transfer against it.
I'd love to know if a .223 could knock over one of these poppers, but there's a strict "no rifle rounds allowed against steel" rule.
I still have about 1500-2000 of these 124gr bullets so I'm going to play with increasing velocity for the next few months and see if that helps. Once I use these up, I'm going to go to a heavier 135-147gr bullet.
One other thing I've wondered about is my bullets have a strongly tapered nose/ogive and a pointy-rounded tip... I bet a flatter meplat would transfer more inertia against the plate and result in more mushrooming against the steel (and more inertia transfer to make it fall more reliably).