The quietest full size backstop ever!

LMAO In that case I wonder how well one of the archery cubes would work like I use for my crossbow:

I use the Yellow Jacket YJ450 Plus:

https://www.academy.com/shop/pdp/morrell-yellow-jacket-yj-450-plus-archery-target

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Maybe add a piece of plywood behind it?

I have that one as well for archery. It'll stop pretty much anything I've shot at it up to .30. But it's really loud when the pellets hit that nylon weave.
 
I do surprisingly okay with one of those 50 buck pellet traps with a layer or two of Amazon's mailing cardboard clipped to the front of it. I think the sound gets dissipated somewhat by the curved back directing it down, the catch-lip helping to break up and retain some sound, and the sides helping to break up the sound too. Even the cardboard itself must dull the sound a little bit on the way back, since it covers the entrance. Then again, I'm not shooting anything super high powered.