Need to beef up my shooting box

Several months ago, I added a 10 gauge steel plate to reinforce my shooting box and I thought this plate was more then enough to stop anything my air rifles shot. I found out I was totally wrong and I was able to shoot through the 10 gauge steel plate in 49 shots with my power tuned .257 texan. I was so confident this box would stop any rim fire round but now I will have to weld on a very thick plate of steel. Here is a post I did on this box I made https://www.airgunnation.com/topic/stuck-at-home/

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Is the box empty? I don’t shoot anything that powerful, but rubber mulch does a great job of stopping lead. 
*I saw your link and the old clothes. That’s a pretty big box. I would definitely try the rubber mulch. If it was me shooting such powerful guns at it, I might be inclined to layer the metal with duct seal, then cover that with a piece of foam tile and fill the rest with rubber mulch. 
 
It's Hickock45 I think from the youtube that says more or less the same thing. He likes free-swinging steel because you get less bounce back, but the principle is similar. If the energy is dispersed at an angle, there is less to worry about re penetration and ricochet both. The pellet trap I got from Amazon works on the same principle. It has a slanted back to drive ammo back into the "ground," and an up turned lip to keep lead from bouncing out again.

https://www.amazon.com/gp/product/B0013R4DBQ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
 
I suggest you fill your trap with rubber mulch and use a few layers of landscaping fabric [or cardboard or old carpet or whatever] to hold it in place. I have about 12 inches of rubber mulch in my trap in front of my steel plates and I've never heard pellet hit steel. I also suggest 1/4 steel, I got mine on ebay for cheap in 12x12 pieces and used construction adhesive to bond them to a wooden backer. If you can weld it all the better.
 
On my box targets with steel backs I found if I slant the steel to the ground the pellets do not penetrate as bad, My wood traps are a lot more noise but nothing is going those 10X10 blocks of wood, My pellet guns anyway....

Mike

It's Hickock45 I think from the youtube that says more or less the same thing. He likes free-swinging steel because you get less bounce back, but the principle is similar. If the energy is dispersed at an angle, there is less to worry about re penetration and ricochet both. The pellet trap I got from Amazon works on the same principle. It has a slanted back to drive ammo back into the "ground," and an up turned lip to keep lead from bouncing out again.

If you go to the link where I posted about how I built the box, you will see I angled the steel plate but repeated shots in one spot at 211 ft'lbs at point blank range was able to pierce the metal.