My pistol berm (up to 50 yds) is 12' wide, 8' tall. That's more than enough for two shooters at pistol range. Rifle (up to 300 yds) is 16' wide, 8' tall.
Getting dirt up that high - with anything - is difficult. 2x base = height was not working for me in reality. After roughly about 4' height I could no longer just dump dirt on top of dirt and go higher. It just ran from peak down to bottom. Pile got bigger and bigger, but never taller. This was just using a backhoe front bucket and trying to compact the dirt as I went by "smooshing" bucket around. Very frustrating and I'm sure someone who knew what they were doing could have done it. I gave up.
So I built [ shaped frames out of 4x4 and 2x4 treated lumber. Won't last forever, but having a solid wall to pile dirt up against uses way less dirt and lets you have a nice thick top vs a peak.
Your sizes sound nice, but will significantly increase project dirt moving. Can always start smaller and expand. I agree with others and would not do L shape. Just a straight wall is going to be much easier to maintain. And no matter what you plant, you will have to maintain. Will still get a lot of erosion and you're continually chewing into the center (.355 and .451 inch holes at a time if like me

)
"nearest house is 100 yds away". That is extremely close. Add in a firing line to accommodate 5-6 shooters at once. I don't think you're going to do anything for noise abatement. nothing that will go any way toward substantially cutting down aggravation to a neighbor 100 yds away. I would look to Gemtech, SilencerCo, AAC et. al. to solve that problem.
Otherwise, I would build a box out of 2x8 or 2x12 8' tall, 24-36" deep, and fill that with dirt. Will help a lot with sound and you aren't trying to maintain another 8' berm - with both berms washing down into an overlapping spot where they meet. That "box wall" type deal could even be moved back some so it's out of the way of berm maintenance but still providing some sound and stray shot protection to neighbor's house.
Posted other threads, but in general this is how mine were done. Saves incredible amount of dirt. Just need dump clearance over that back wall for maintenance.
