You've been given the answer multiple times above, although there's been a few derailments of non-related or nonrelevant (or incorrect) info. "Rifle length," aka 12", will stop right at the barrel shoulder, not making contact with the gas block. "Extended rifle," 13.5", and 15" models are designed to cover low profile gas blocks, which is NOT what you want.
I've used exposed gas blocks without pins for many years, and I see no advantage in "protection" by covering the gas blocks. If you're banging into a gas block hard enough to cause it to come loose, then you either did a piss poor job installing the block, or you've hit it hard enough you'd have broken most free float handguards on the market anyway, maybe even hard enough to fracture the upper at the barrel extension mortise, or fracture the lower at the extension bridge. That one sounds like a Red Herring to me.
If you want a free float handguard with an exposed gas block, leaving room to use a fixed A2 sight base/gas block if you so desired, then you're looking for a 12" handguard, aka "Rifle Length."
An example - for
@boom boom - of a free float rifle length hand guard which ends behind the gas block is the Bushmaster Varminter Special or Remington R15 VTR Predator, pictured below. The vast majority of free float handguards are made in the same dimensions to give this same look - "rifle" ends behind a rifle length gas block, leaving it exposed, carbine end behind a carbine length gas port... Unless a guy seeks an "extended" handguard, over length (i.e. 15" or 18") or uses a size up compared to their gas length (i.e. a rifle length forend on a mid-length barrel), most forends are NOT designed to cover the gas block.
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