RemingtonOwners.com

I have a 20 year old Remington 1187 that I literally LOVE
I have had the gun camo dipped and replaced many parts but
the gun continues to shoot and shoot accurately. I know that
time will eventually run out on this gun but I plan to shoot it
until it does. I have literally taken thousands of ducks, pheasants,
and dove with it.
One problem that I am not having is the magazine cap comes loose
(not falling off just loosens) when I shoot it several times. This
causes the forearm to have play or not be tight. Any insight on how
to fix this issue??

Buy a 1/8" cross section O ring and put it under the mag cap in front of the fore end. They also make a wave washer that will do the same thing, but I dislike metal on metal.
Why do you think it's going to wear out ? I have a 1963 Model 1100 with over 115,000 rounds thru it that has absolutely nothing wrong with it. Has shot everything in it's life, including many MAX 3" loads at waterfowl with a 3" barrel.

Have you lost the detent ball that the magazine cap ratchet's against? Hard to answer when you say this is a problem that you're not having.