In the strictest sense of the word free floats might not improve accuracy. But a non FF will have very large point of impact shifts in a field environment shot from conventional and unconventional positions.
In the not too distant past, we tested multiple issued M4A1's with the standard KAC RAS rail and a commercially available drop in extended FF handguard. The exact same rifles were used with the exact same shooters using the same rifles only switching out the handguards between tests. Shooter ability ranged from new members still in training to seasoned experienced members that are high level competitors.
There were two different tests conducted back to back. The first was a pure accuracy benched test. Ten rounds were fired from each rifle at both 100 and 200 yards. Groups sizes were measured for extreme spread. The second test was from ten different alternate positions with accessories mounted and dismounted throughout. Again 10 rounds were fired from both 100 and 200 yards with each rifle.
Benched there was little difference between the rifles. Alternate positions showed a huge difference.
The below target was one of the test ones, and produced the smallest groups from the NON FF portion. In other words- it is the best that the non FF produced.
20x rounds fired at both 100 and 200 yards from 10 different positions at both ranges.
Non FF on left, FF on right. Again, alternate position test.
Averaged out between all shooters, the non free float guns had a 100% hit rate (for group size) on a 8 inch target of less than 50 yards. The Free float guns had a 100% hit rate of around 200 yards.
I would rather have an M4/AR15 with an extended Free float rail and iron sights only, then a non FF with every optic, light, laser or sling I wanted.