I saw this at another forum and thought I'd share: Amazon.com has duct seal 5 lb bags(2.2 kg) for about $13 with free shipping for Amazon Prime members. Seems like a good deal to me. I know that many people use the stuff for sound dampening and pellet capture in their backstops.
http://www.amazon.com/gp/product/B000VZPWZW/ref=pd_lpo_sbs_dp_ss_1?pf_rd_p=1944687442&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B000BO8XD6&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=1228VD7X6VMYQT4NYWW0
http://www.amazon.com/gp/product/B000VZPWZW/ref=pd_lpo_sbs_dp_ss_1?pf_rd_p=1944687442&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B000BO8XD6&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=1228VD7X6VMYQT4NYWW0