ABPOS,
Some of them are really tight. You have to "un-thread" the sling from around the oiler with the oiler still in place. You may have to do some wiggling around of the oiler to gain the necessary clearance. Some of the slings with thicker material, or the "D" tips can make it a tough job. The stocks can vary a bit too in the dimensions of the slot. I've got a couple that I have to coax a bit with a light hammer and a small wood dowel. There are other posts that will help if you use the search feature. I've got (2) carbines with this front sight "mod." It actually makes it a lot easier to obtain your sight picture, especially in low light - otherwise you might be drawing a bead using one of the protective wings. - Bob