The reason the 870 holds one shell less has to to with cosmetics.
When the gun was designed in 1950, the designers decided having the magazine about flush with the fore end simply looked better than having more of the tube sticking out.
So they went with the shorter tube, which won't QUITE hold 5 rounds.
When an extender is added, it too won't QUITE hold the additional round.
Interestingly, on the Chinese Norinco-made exact copies of the 870, they lengthened the tube just enough to allow one extra round.
If you look at one of the Chinese guns, the magazine tube sticks out farther, and the barrel's support ring is farther forward.
Although there is seemingly enough room in the tube on a Remington for an extra shell, remember, the spring takes up that room
On thing NOT to try, is to cut the spring trying to make room for an extra round. The gun will fail to feed reliably from insufficient spring power.
Other brands of shotguns simply used a different design, which allows a full five rounds in a standard magazine.
Either the magazine tube sticks out farther, or they lengthened the fore end to cover more of it.
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