THE ONES FROM NUMRICH WORK ON 1100 WITH THE DETENT BALL ON THE FOREND. MOST 1187'S HAVE THE DETENT ON THE INSIDE OF THE CAP.IF YOU BUY THE WEIGHTS AND DRILL OUT THE 1187 CAP THE DETENT WILL STILL WORK. YOU WANT IT TO WORK SO THE CAP DOESN'T KEEP GETTING LOOSE WHICH IS JUST ONE MORE THING TO CONSTANTLY HAVING TO CHECK. YOU NEED A SCREW WITH A THIN HEAD (OR GRIND ONE DOWN) SO THAT THE DETENT GROOVES ON THE END OF THE MAG MEET WITH THE GROOVES ON THE INSIDE OF THE MAG CAP WHEN YOU TIGHTEN IT UP.
The ones on ebay are the ones that numrich sells for 17 bucks. They are old stock remington. They work great on 1100's but not on 1187 because the cap detent is not on the forend, but the inside of the cap. best bet is to buy the weight only and install it on a 1187 cap. websit to buy weight only for 29 bucks is a few posts above. These also look nicer.
Fill a coouple of empty shells with shot, unscrew the magazine cap and put them in the magazine tube (be careful; the spring will put everything into orbit if not restrained). One may be enough--a lot cheaper than the screw-on type and this will let you know if the balance is what you're looking for.