Exactly. You're not thinking like attorneys or juries, which is the problem. "The FBI had to go to a weaker round, the 10 was so powerful."
You might realize that the FBI 10mm load is almost identical to the .40 they use now. The change was guns with smaller frames allowing those with smaller hands to better grasp it. The recoil problem was due to a poor fit between the shooter's hand and the weapon, not excessive recoil in the cartridge. The proof of this is to look at what 10mm ammo was issued and compare it to what .40 ammo replaced it.
Sorry, but I have been down this path before. Your argument does not hold water. Certainly a prosecutor can try to make it but a COMPETENT Defense Attorney can easily tear it apart. The defense here though was INCOMPETENT on several fronts, caliber being only ONE.
There were SEVERAL ways to eliminate the attack on the gun, caliber and bullet, defense was just too stupid to do so.