To keep a long story short, I have the melted remains of a nylon sling in the slot underneath my front sight. I know I can remove the cancer once I heat up the barrel while shooting. My question is, would using a lighter or torch harm the finish on my m4? I was thinking of heating it up with a torch like the ones used for copper pipe or conduit? I feel like my m4 is suffering and since i'm so busy with work, I won't be able to get to the range for quite some time. HELP!

Try to get it out while it's cold, using an eye glass screwdriver. Trying it with a barrel hot enough to re-melt it will:

Be messy as hell, and next to impossible to get it all out.

Likely give you repeated opportunities to burn the shit out of your fingers.