I think a standard sling swivel stud threads right into the threaded hole on a SIR. I havent had to do one like that in a very long time, but I swear I have done it for a guy before. Even if that doesnt work, you can get a pretty inexpensive bipod mount that attaches to the rail for about $30 or less. Yankee Hill makes one, so does Midwest Industries, Fobus makes a cheap plastic one, there's probably a few other companies as well.