I have an older (90's) 870 Police that I would like to turn into a duck gun. I am getting mixed information about making this happen. I know about the "ball detent" on the guide ring vs the mag spring retainer of the newer Express models. What I am specifically wondering about is magazine tube length and guide ring location. I saw one vague post about a guy having to return a barrel because the guide ring was in a different location. Is this the case or is it possible to put an Express barrel on a Police model if the right spring retainer and cap are used. Or if I could find an older Express barrel with a "ball" on the guide ring?
Any help is greatly appreciated.
Turning 870 Police into Duck Gun
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Re: Turning 870 Police into Duck Gun
All standard 870 barrels & receivers are interchangeable. Police, Wingmaster Express, Special Purpose, they all have the same magazine tubes with the barrel guide rings in the same place. There are some 870s like the MCS or certain Express Tacticals that have nonstandard 6- or 3-round magazine tubes, and barrels with guide rings in nonstandard locations, but those are specialized combat/defense models. Virtually all hunting barrels (except for some rare old 870 Special Field barrels) are built for standard 4-round magazine tubes, and will drop right onto your Police receiver.

As for spring retainers, magazine caps, and cap detents, you don't really have to over-think it. If you have the option, look for a barrel with a locking detent in the guide ring, and a cap that's dimpled to engage it. But if the barrel doesn't have a detent, you can simply use a wave-style lockwasher to keep the cap tight.

Spring retainers of either type are not really necessary, especially with Police receivers, which lack the dimples in the magazine tube that the newer internal-ratchet spring retainers need to do their job. And the old press-in spring retainers are a real hassle if you want to be able to take a plug in and out, or switch back to a magazine extension. You can safely leave the spring retainer out, and either tighten the magazine cap down on the end of the spring, or if the cap is the internal-ratchet style with teeth on the inside, place a spare magazine follower over the end of the spring.

As for spring retainers, magazine caps, and cap detents, you don't really have to over-think it. If you have the option, look for a barrel with a locking detent in the guide ring, and a cap that's dimpled to engage it. But if the barrel doesn't have a detent, you can simply use a wave-style lockwasher to keep the cap tight.

Spring retainers of either type are not really necessary, especially with Police receivers, which lack the dimples in the magazine tube that the newer internal-ratchet spring retainers need to do their job. And the old press-in spring retainers are a real hassle if you want to be able to take a plug in and out, or switch back to a magazine extension. You can safely leave the spring retainer out, and either tighten the magazine cap down on the end of the spring, or if the cap is the internal-ratchet style with teeth on the inside, place a spare magazine follower over the end of the spring.
Re: Turning 870 Police into Duck Gun
Thank you very much for the detailed reply. Just what I was looking for.