.040 is certainly on the tight side for 20ga full choke. And standard constrictions, with modern shotshell technology, are certainly more open than they used to be. Jack O'Connor's "Shotgun Book" (1965 edition--just as shotshells were coming into the "modern" era) lists the following constrictions for 20ga full: Winchester, .027; Remington, .029; Browning (always seemed tight in the old Belgian guns), .033. Two current charts I have show .026 (Stan Baker) and .025 (Mike Orlen). So, with the exception of Browning, we were already pretty close to "modern" choke constrictions over 50 years ago.