I have several HF O-ring kits different type and sizes in each, between the 4 kits I can have 80% of the O-rings I need, the others I measure and order from theoringstore.com , I know most don't want to buy so many O-ring kits, but I use them a lot, so it works out for me.
So, best thing to do is to measure with a Micrometer in metric the size of the O-ring, you want the ID, OD and CS , you want dura 70 or 90 buna nitril, 70 being more pliable and 90 stiffer depending on the application, I think the originals are dura 70, don't forget the Silicone grease....
btw, to take off the front sight there usually 2 grub screws, one on top of the other that hold the barrel, make sure to remove both or you will score the barrel, and finding a spanner tool to remove the cylinder end nuts will be impossible, you may need to make one or use a channel lock pliers with rubber jaw protectors to loosen them, good luck and be careful.
