catalogues
I used to save the Wards, Sears, and Pennys big book catalogues to shoot at in the basement. They pulp pretty quickly.So I would drive nails into the corners to hold them together a bit longer. Problem is that in the area you hit the backstop deteriorates quickly. The buckets of sand are a pretty good suggestion. Old tires, big, thick ones are also good. Slowed by this sort of thing, even the straw and the 3/4 or 1 inch CDX plywood should catch the rest. .22 rimfire, long rifle or less.
At the same time, I agree, if there is anyone in the house... Mr. Murphy will be out to get you--with a vengence! and you will have made it awful easy. God forbid a stranger wanders into the situation. Runs out, into the line of fire, to see what is going on... I don't think your insurance would touch the case.... So unless you are the only one living there... Then shoot when the house is EMPTY...
The "silencer" is really just baffles that delay and redirect the blast energy. If you want quick lessons, pick up at the junk yard car and truck mufflers and cut them up. If you were to shoot through several tires, muzzle blast caught in the tires... You might be amazed at the sound reduction, whatever direction you are shooting.
And, I was amazed the first time I fired regular velocity Long Rifle out of a target rifle with a full length barrel. 22"? 24"? I didn't think it was as loud as my pellet gun (pump up). I didn't feel a need for any hearing protection indoors. You might go looking for this sort of ammo. Target shooters use it alone. Probably not at Kmart. Less energy for your backstop to deal with too. luck.