How tight should the cylinder be?

My HW100 will sometimes need an extra snug up after attaching, just check it an hour or so later for leakdown (a small amount is normal, as it cools). You will see a small O ring on the valve, go ahead and order replacements. Since the cylinder has to be removed on that rifle to remove the barrel, and since the barrel has to be removed for a good cleaning, you will be removing the cylinder more often than most others.