This, I believe, is the proverbial staw-man. What the heck does a grocery chain have to do with converting a rifle? You aren't buying guns from them, you're buying guns from H&K.
I would buy from Nissan. They have an extensive racing program, and recently brought-over Godzilla undiluted for the USDM. Major props, they respect their performance fanbase.
H&K make cool guns, but refuse to do what Steyer, Sig, FN, ect have done and produce 922r-compliant UMP/G36 rifles. And they have instituted a policy that will make it all-but impossible for civvies to un-neuter the rifles they sell. They are, essentially, urinating in our faces.