Decided to upgrade my indoor range workshop's 20"x24"x4" solid wood backstop. Grew tired of the mess, noise and worst, too many ricochets from <600fps .177 pistols (max 5 fpe)! The solid wood\plywood target holders\backstops just were not the right tool for the job indoors! Using scrap wood I had on hand, came up with "Duct Seal" for the initial\main pellet catcher, a $10 1\8" metal plate backing from the local sheet metal business, and all mounted and backed by 3\4" plywood.
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16in.x17.5 in. Lead Pellet Back-Stop with .125 in. Metal Plate over .750in. Plywood.jpg
Installed Twenty-One 1.1"D x 2"W x 7"L "sticky clay like" 1lb Duct Seal bricks (total of 280sqin, just the size of my new target box). I packed them in tightly to insure tight sealing between each brick. Overall, ended up with a thickness of 1.25" of DS in my 16" x 17.5" shooting area. Put on the hanging clips, pulled out targets and started testing this new shooting box and backstop. WOW, no more .177 ricochets, no more target paper or lead pellets all over the workshop, or splintered wood fragments to clean up after a practice session. And, NO IMPACT NOISE, just the piffs and spiffs of the pistol actions or shot broadcast following each trigger pull...Nice!!
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16in.x17.5 in. Lead Pellet Back-Stop with .125 in. Metal Plate over .750in. Plywood-21# of Duct Seal Fill-In @ 1.25 Thickness-1.JPG
As for the targets, used 24#, 32#, 67# and 110# weight paper targets to test for "clean hole \ wad cutter" efficiencies! Surprised that the 64# paper performed as good as the 110# card stock we had, which is great since our laser printer likes that lighter paper product! However, was curious about the real sticky duct seal. As per my testing, even the 24# paper targets were left with clean holes, as long as I stuck the target directly on the duct seal!

Still, the cleanest hole penetrations were found with the 67# weight paper stock and with a full ream in the house, will be making my own targets from now on! After 200 or so rounds through my four .177 air pistols using Hobby 7.0g wad cutters, the P17's (410 to 420fps) penetrated the DS so that the skirt was even with the surfaces. The Trevox (535fps) and the Trail NP (565fps) penetrations went 1\16" below the DS surface.
Attachment:
16in.x17.5 in. Lead Pellet Back-Stop with .125 in. Metal Plate over .750in. Plywood-21# of Duct Seal Fill-In @ 1.25 Thickness-3.JPG
So far, seems to work extremely well for my intended purposes! Haven't tried heavier HP, pointed or domed leads yet, but believe even my Titian XS (1000fps) will not get through the 1.25" of DS...will see!
Ted