I will tell ya this , out of a 26" 870 supermag i got 103 in the 10" and 163 in the 20"
With a Rob Roberts .660 recommended for a 3" HW 7 load.
Right after that i shot 2 3/4" HW 7 out of a Remington factory full rated for lead or steel in the same gun and beat it by a small amount.
Many other chokes in same gun with the 3" left the RR in the dust.
In one of my guns anyhow.