Mag extensions are available, however, if like the 12 ga versions.........the new sporting guns have a mag spring retainer that is keyed to two dimples in the mag tube. Plastic, the mag spring retainer goes in and clocks to lock.
The old style retainer is a cup that is slotted, you have to pry them out, tamp back in.
The new style requires no tools.
However, those mag tube dimples mean no shell rim will pass them. So you have to pop them out, or drill away for extra capacity (w ext mag).
My 870 Supermag turkey gun (23" vent rib Remchoke bbl) has been drilled to use an extension mag. It has the old style mag spring retainer.
The older guns with such retainers.........kept the magazine cap on with a detent on the barrel ring.
The new style guns with plastic clocking mag spring retainer..........have a jagged crown on the spring retainer. The points of the crown correspond to notches inside of the new gun magazine caps.
So...........doing a conversion means you have no detent/ratchet action to keep the magazine cap (cap, not the retainer).......from backing off. You should be able to hand snug it and it stay put. Or use an old style bbl and cap (old bbls with detent, old cap with detent pockets on outside edge).
When hunting I just reach up and check the mag cap for tight halfway through the day. It was barely loose once, last couple yrs of hunting, it stayed put (dove, less than two boxes of shells burned in a hunt).
My 870 Supermag has the original non detent bbl, and old style spring retainer swap, mag tube tabs drilled out. I don't remember what cap it runs......proly the original. Not a problem, been shooting it, hundreds of shots per yr, for about a decade.
It's a Supermag Express, older pre cerberus......no probs.
Last edited by hookeye; 10/21/16.
