Well first you have to figure out where it is tight..
Dropping it into boiling water will surely do a number on the sheath..
What you need to do is controled heat in the area where the sheath is tight...
With that being said, I suggest that you try a chemical approah to the tight fitting sheath before you apply any heat.
Get yourself some silicone spray. I use SNAP brand found at Meijers or just about any decent hardware store... Since the gap is thin and you really can't get anything in there to spread it around, I usually suggest spraying on a finger then wiping inside,, or a Q-tip, you will have to spray a shot ,just a little bit directly inside the sheath..
Swoosh it around and shake out the excess...
Immediately put the knife in and work it in and out to spread the silicone to the proper areas...
If this doesn't solve the problem,, then you can play with the heat...
You will need some type of focused heat though..
A heat gun works best,, however in a pinch, a hairdryer will also work.. Lay the sheath with knife inserted on the table,, and gently heat with a hairdryer..You won't get enough heat out of it on it's own,, so what you will need to do is cup the intake of the dryer,,only for a second, then release..This will super heat the air,, wait a couple seconds and do it again...
If you do it to much,, you will blow the thermal fuse in the dryer. No worries,,once it cools it'll reset and you'll be good to go again... Try this first on the back side of the sheath,, in the area,, just where the edge starts..This is where the lock is on this particular knife..You should be able to just adjust the back without moving to the front..This way if there are shiny spots afterwards,,they will be in the rear..Keep moving the dryer in circles or in an out of the area you want to heat to avoid shiny spots..
If you don't care about leaving shiny spots a lighter or candle will also work..
You have to be SUPER careful keep the heat directly away from the sheath,,and keep it moving...
All you want to do at this point is warm the plastic ,not melt it..
When after a few seconds of warming the plastic,, try and work the knife in and out..
This process may take you a few times, and is better to work up to the proper heat, then to over heat, and have to re-fit the sheath afterwards...
Play with it, take your time..
ttyle
Eric
O/ST