Depth of rubber mulch backstop?

I'm off with my boys today & am hoping to get a backstop box put together. Right now shooting into a small Duct Seal backstop but want something bigger for the paper targets I ordered. 

I have access to some 1/4" steel so I'm not worried about passing through but do want to keep things quiet & never even reach the steel. 

Most of my target practice is sub 20 FPE shooting 22 pellets, not slugs, at 20 yards or greater. 
 
Deeper the better. However, if you are hitting the same spot over and over again such as shooting groups, you will eventually drill a hole through the mulch. The higher the power the gun the faster this happens. I find 6 to 9 inch deep cardboard boxes from Amazon work well. I shoot until the front of the box loses integrity and dump the mulch in a new box.

I also made a wooden box for shooting 25m benchrest. The back has a piece of 1x6 protecting from pass through. The front has a piece of cardboard tacked onto it to hold the mulch in. I haven't reached the 1x6 backing because I am not shooting groups.
 
Mine’s about 12x12x16

1579540092_1881108495e25de7cb280c8.78463850_D15BB810-6CCF-478A-AF11-7F000C15F6FE.jpeg



 
I have had a different experience with both of my rubber mulch traps. I have one outdoors that I shoot at 50yds and one indoors for chrony work and short range shooting. I use the same target paper 95% of the time so my traps get hit in the same spots repeatedly (thousands of pellets). The mulch in my traps is not packed in tightly and fills back in on itself. I occasionally "fluf" the mulch because the lead starts to stack up at the front of the trap and loose pellets fall out. I shoot .177 up to .25 cal into my traps and have never had a pass through. The more you shoot a rubber mulch trap the better it gets due to the build up of pellets inside. They do however start to get pretty heavy. 9 inches deep sounds about right. Mine are 12" deep which turned out to be a bit of overkill for me.
 
I like cardboard over plastic because it is quieter. 6 inches works for 30 FPE and below. I always pack them tight so that the mulch doesn't settle and leave a dangerous mulch gap at the top of the box. Regardless of the depth of mulch, boring through the mulch is the real danger. The deeper it is, the longer it takes to bore through.

Agree. 6" is fine for that power level and probably a bit beyond. The mulch absorbs energy very rapidly. I also use cardboard boxes and just move the mulch from one to another as they get "shot out" on the front. That fluffs the mulch up again and loses some of the lead.
 
I like cardboard over plastic because it is quieter. 6 inches works for 30 FPE and below. I always pack them tight so that the mulch doesn't settle and leave a dangerous mulch gap at the top of the box. Regardless of the depth of mulch, boring through the mulch is the real danger. The deeper it is, the longer it takes to bore through.

Not sure how it’s quieter? I can’t say the plastic is very loud at all. I use a cardboard front for mine. Also, I shoot regularly at 32fpe and sometimes at 62fpe into the same target, never a pass through. Reason I like the plastic is that I leave mine outside all the time, sun, rain, snow, and no problem. The mulch tends to settle, and I shoot in the middle, so the gap on top for me, is negligible. 
 
I designed my traps with settling of the mulch in mind. Even after settling the mulch is 5" higher than the top of the target area. I don't move the outdoor trap and the indoor trap sits on a dolly cart so the weight has not been a problem. At some point I will put new mulch in (built these traps 2 1/2? years ago). What do you guys do with your discarded fill media?
 
I like cardboard over plastic because it is quieter. 6 inches works for 30 FPE and below. I always pack them tight so that the mulch doesn't settle and leave a dangerous mulch gap at the top of the box. Regardless of the depth of mulch, boring through the mulch is the real danger. The deeper it is, the longer it takes to bore through.

Not sure how it’s quieter? I can’t say the plastic is very loud at all. I use a cardboard front for mine. Also, I shoot regularly at 32fpe and sometimes at 62fpe into the same target, never a pass through. Reason I like the plastic is that I leave mine outside all the time, sun, rain, snow, and no problem. The mulch tends to settle, and I shoot in the middle, so the gap on top for me, is negligible.

Logical. I use boxes because I've had a lot of them and they're disposable. If I run out then plastic would probably work better long term if left outside.
 
Not sure how it’s quieter? I can’t say the plastic is very loud at all. I use a cardboard front for mine. Also, I shoot regularly at 32fpe and sometimes at 62fpe into the same target, never a pass through. Reason I like the plastic is that I leave mine outside all the time, sun, rain, snow, and no problem. The mulch tends to settle, and I shoot in the middle, so the gap on top for me, is negligible.

You use a cardboard front. Is that because it is quieter than a thick piece of plastic? That is what I meant.

I don't leave mine out in the yard. I bring it in when not shooting to keep the yard looking nice.