If you're serious about an extension I suggest you contact Remington parts and discuss their extension plus the 870P magazine spring. My 870 Express Tactical is one of the newer modela so it came with a single 6 round magazine tube. When I called parts to ask about an additional spring the parts tech told me that they use the 870P spring and it is rated for 3 years continuous load duty.....obviously I ordered one of those as an extra and the price is just under $4. That spring is 18 1/4" long.