If referring to a
20" barreled lever shotgun, might as well compare to a
20" rifled barrel lever gun. Henry claims 6 in the tube for the shotgun and 10 in the tube for the 20" barreled .357.
Other lever guns may be somewhat misrepresented by the factory for whatever reason. Fer instance, the current
Marlin 1894 in .44 mag claims 6 rounds fit in the tube of a 20" barrel. Which is a glaring difference from what the gun was spec'd to hold in the past.
https://www.americanrifleman.org/content/rifleman-review-marlin-1894-in-44-mag/
Or, Ruger is doing what Remlin did and they're stuffing mag tube springs from rifle length tubes into their shorter barrel offerings.
My Remlin 1894 CSBL* is a perfect example. From the factory, I could only stuff six .357 cartridges into the tube. With some research, it became apparent the Remlin used the same magazine spring in many of their lever guns regardless of barrel (mag tube) length. Due to that, the mag spring would stack solid on itself with only 6 cartridges in the tube.
As I learned from others on the net, I trimmed the magazine tube spring to the proper length and my 16" barreled 1894 CSBL now holds eight .357 cartridges or nine .38 special cartridges. And it feeds perfectly through the last cartridge.
And there is this article with the correct mag tube capacities listed.
https://www.shootingillustrated.com/content/is-this-the-ultimate-lever-gun-for-self-defense/
*From a batch made near the end of Remington making Marlins. Earlier batches may have been different.