How do you all make your indoor pellet traps?

Lots of replies coming. I have two basic types of indoor traps.

Some type of square enclosure, fill with duct seal, happy shooting. I recommend something with a metal backing so you don't blow the back off depending on the fpe.

The other is basically a wine box. I line the inside and the back with 1000 dernier ballistic fabric. One layer on the side to catch any stray pellets, about 8-10 layers on the back. 4 layers of this fabric will catch anything you can through at it under 100 fpe. 10 layers for additional safety in case I'm whacking the same spot a lot. Then I fill the rest of the box with old towels or throw away clothes the kids grow out of - this helps quiet the shots down as it slows the pellets before they hit the back. 

Both might run you $30-40 due to the duct seal or ballistic fabric. I haven't tried rubber mulch but that seems the most popular option, might get messy indoors. 
 
An empty Amazon Box and rubber mulch... cost me a grand total of 8 bucks after taxes for the rubber mulch. Cheap and super easy. I tried a ton a traps with the duct seal but over time you need to replace the duct seal and my traps started to wear and tear therefore needing to buy another trap. If a box wears out, I’m sure you have another box lying around somewhere. I use a staple gun to staple the targets on to the box, unless theyre adhesive targets. 

I haven’t had any issues with it, and I’ve never shot through the box. The mulch does a good job at stopping pellets and slugs. I’ve shot as high as 40 FPE with slugs and it still stopped it. To avoid messes I got the bigger cut of mulch, they’re about half an inch to an inch. I haven’t had any messes maybe 2-3 fall to the floor that I just pick up and put back in the box. 
 
I ordered 12x12 1/4" steel plates on ebay [cheaper in small pieces and able to be shipped] and used construction adhesive to glue them to a wooden backer and put 16 inches of rubber mulch in front of it. I have not heard a pellet get through the mulch, but the steel plates give me peace of mind. Cheap way to make a really solid trap.



https://www.airgunnation.com/topic/homemade-steel-backed-pellet-trap/
 
i found duct seal to be hard to work with. i was traveling for work most of my 20's and accumulated a lot of suitcases. I fill those with rubber mulch for inside. duct tape up the fabric once it gets too loose, until it becomes unusable (takes a while).

i did the cardboard box mulch thing as well. that required retaping the front a little too often (i think i need more hobbies) with how much i shot at it.
 
Here's my only concern with the cardboard and mulch. Eventually you'll go through it and what stops the pellet then? I'm just saying, for peace of mind, I'd still want to put something additional at the back.

Agreed. I put some plywood in at the back, then stuffed more mulch in behind it. Finally I have some more plywood behind and outside of the box.

Suggestion: yard sales often have plastic storage boxes on sale for just a couple or few dollars. Better than spending 15 at Home Depot or whatever. I got a quality plastic container more than a foot high with some scrap plywood for all of two bucks the other day.
 
For my 50 yard target box I used a Rubbermaid 7 gallon trash can filled with about 1.5 bags of rubber mulch. Just didn't want to deal with a cardboard box, especially outdoors. Behind the target box are 2 1/4" steel plates, covered by old rubber 'welcome mats'. But shooting my Red Wolf HP GC2 (.22) nothing penetrated the back of the trash can. I cut up an old ~5/16" 'anti-fatigue' floor mat which was out in the shed, after it had become grubby and worn out for any further use in the kitchen. This gives a bit of self-healing surface just behind the cardboard target backer. Just attached to the trash can with 3M aerosol adhesive. Cheap clamps to hold the target board on.

Real quiet target as it is built. No cover, but a couple of pieces of rubber with a brick on top will prevent the rubber mulch nuggets from potentially jumping out when the box is hit. Holes drilled in bottom of trash can in case I forget to cover it and leave it out in the rain. I figure I can ultimately shoot all 4 sides of the trash can before having to replace it.

The Rubbermaid trash cans are a little more expensive than some others, but I have found their material to be more flexible, and less brittle. Plus I bought them in a 12 pack, so they cost just over $5 each.

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For an indoor target just recently used for chrono testing of a loud gun, I used a rimfire bullet trap filled with duct seal. A few pellets bounced off, but ended up no more than 12" from the front of the box.

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What Intenseaty22 said👆, keep in the center or lower part of the mulch catcher and you will not get pass through,,the mulch shifts with each shot so you do not get the tunnel effect,, the duct seal will tunnel with one hole accuracy and is messy , it did not take me long to make the switch. cardboard box, duct tape and mulch, is all I have used. Someday I will get a plastic tub and mount a frame on the lid, that I can use cheap clipboard and patch it with tape as needed till it fails, then new clipboard

duct seals sucks in my world, unless u r sealing ducts
 
I had a nice oak target box full of duct seal bricks for years but a few to many 100fpe 9mm pellets took their toll and split it pretty good. Nothing came close to getting through the bricks of duct seal though. So, I just stacked a few bricks in the corner temporarily. It's been this way for several years now. I just set the targets up in front of the bricks and they don't even fall down. Had plans to make another nice oak box but it werqs great just the way it is so just another of the many projects permanently on the back burner.

You could easily get by with just three of the 5LB bricks I just collected more over the years so use them all. But the rubber mulch will be far less expensive.