A Winchester 1300 Defender with an 18 inch barrel has a capacity of 8. A Mossberg 590 20 inch has a capacity of 9. What limits the 20 inch 870 Police to 8 and the 18 inch to 7? In the 20 inch 870, its nearly possible to insert 8 rounds into the magazine, but the shell is just slightly too long to insert fully. If you take a dowel or similar and insert it into a fully loaded 870 magazine, you will see that there is sufficient space for an additional shell. What prevents the insertion of the additional shell in both 18/20 inch 870's using factory 2 and 3 shot extensions? There has to be a follower or a way to include the extra round as there is room and its nearly possible to insert the round now. Whats the deal?
The mag tube extension prevents it. If you look under the support band that goes around the extension tube and the barrel you will notice a crimp in the tube that allows the screw to fit between the extension and barrel and tighten the clamp. This crimp is a limiter. I have seen some mag tube extensions that do not have a crimp and will allow one more round.
Just my .02
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.