SKS swivel stud conversion

Converts the leather strap sling rungs to use swivel studs, without any modification needed to the rifle

Be aware that you will need a sling that has swivels to use the swivel studs, the classic SKS sling can not be converted. (Files for swivels and a tutorial for a paracord sling: http://www.thingiverse.com/thing:1044946)