Ideal is somewhat of a nebulous term.
Typical powders used in shotgun shells run the the medium-fast burning range, from unique to about Herco. This powder will be completely burned in 16-18 inches of barrel.
Longer barrels are primarily used for balance and ergonomics. The Benelli M-4 used by the USMC has a 14 inch barrel.
When defining ideal, we must ask, Ideal for what?
History fails to record a single precedent in which nations subject to moral decay have not passed into political and economic decline. There has been either a spiritual awakening to overcome the moral lapse, or a progressive deterioration leading to ultimate national disaster.
If you think a strong military is expensive, think about what a weak one would cost