The float tube does not make the rifle more accurate. The float tube makes the rifle less sensitive to variable forend pressures and tensions when shooting in various positions.
Lots of folks have demonstrated that sling tension when shooting off hand and variable pressures when shooting from various supports will shift POI. On my old clamshell Service Rifle (which was an Hbar), I could easily pull a full minute into my sling, so shooting from a bipod, a sling, or a fence post meant my rifle might miss a coyote in the field by INCHES just because of the position.
Converting from clamshell to free float is usually a waste of money - most guys end up buying a new tube and barrel nut, new gas block and tube, and often by the time they do that, they replace the barrel while they are at it... Even when the barrel stays, usually clamshells are built around cheap chrome lined barrels, so it’s not a significant savings to keep. Building/buying a new upper usually pays off, in that case.
I build and have built a lot of AR’s, and almost always recommend away from clamshell handguards. For my own accumulation, I only own ONE clamshell now, specifically because of their sensitivity to forend pressure & tension; all of the rest of my AR’s are free floating. None of mine are built for Benchrest shooting, but they’re all expected to reliably deliver better accuracy in the field than simply hitting somewhere on a man-sized target, so a clamshell handguard simply doesn’t make sense.