bigbear,
Check out www.Hornady.com. They pretty are the only show when it comes to gas checks(they make them for Lyman I believe). They have 348's listed. Try using 35's for target loads, making sure they are seated center. I usually run the bullet through the sizer, nose first, so the hard gas check aligns with the softer lead bullet, not the other way around! I don't have a 348 but use 44 checks in my 416, cause you never know when they'll quit making 416's OR 348's! I want a good stash for serious work.