Don’t know your situation, but can’t imagine why enough dirt can’t be mustard.
I have a 100 yd range out back of my house
My berm is eight feet tall and about 12-15 feet wide
And probably eight feet thick
I keep my targets about 3-4 ft off the ground and very selective to who I let use my range .
I don’t do spray and pray at all
Shoot from a bench or field positions and if it’s a new gun do my zeroing at fifty yds to make sure I’m on the 4x8 sheet of ply board used as a backer plyboard last s long time if you use same size targets and pin them repeatedly in same spot when the top half gets shot out I simply flip it over , use t post o the ends drill holes in the board and wire it to t post
Have shot thousands snd thousands of rds into it
Evey few years have friend with back hoe come over and dress it up using bucket to tamp dirt tight
I would not feel inadequate using this berm at longer ranges if I could stretch out more .
Kenneth