Generally .003-.005 on an open top. That advice from the Goon Hisself. Remmies can go a bit tighter; the less gap, the less powder buildup which tends to bind the axle pin on a Remmy. Also, Remmies have a fixed gap, as should a Colt with short arbor issues fixed...but most of them aren't. Just sayin'.
Also, when checking barrel gap at half cock, don't lower the hammer back down on the nipple from HC. Go to full cock then down to rest or you'll wind up with "ring around the collar" (drag line) on your cylinder.