I got the tacstar 1187 8rnd extension, but for whatever reason it would only fit seven. I put it in my 1187police 18" barrel. any same experience on this?

I don't have any experience with TacStar mag extensions, but try eight of the 2&3/4 inch shells.  If you are using 3 inch, it'll probably only hold six.  That's how Choate extensions work, and I bet that this is the same case.  

Edited to add: Choate's hi-viz followers also prevent inserting eight 2&3/4 inch shells, the followers are made with an extension to keep the spring from being over-compressed.  If you want to get the full capacity, you have to use the original follower.   ARKAR

Be aware that magazine extensions are often listed by the TOTAL number of rounds the gun will hold INCLUDING one round in the chamber.

So, if you bought an 8 shot extension, it's likely they meant it will hold seven in the tube and one in the chamber for a total of eight.

If you have a Remington shotgun with an 18" barrel, an extension that doesn't stick out beyond the muzzle can only hold SIX rounds actually in the tube.
Are you sure you have an 18" barrel?

As above, this also is based on 2 3/4" shells.  3" shells will reduce this by one.

dfariswheel is correct, I forgot to mention in my post that the extension counts include one in the chamber.  If it says "eight shot", it means 7 in the mag and one in the chamber, if it says "seven shot", it means six in the mag and one in the chamber.  I'm glad that he corrected me and pointed that out, my post was a little misleading.  ARKAR

I have the 18" barrel and yes the ext tube stick about an inch over the muzzle. It confuses me when the remington box say's seven round with +2 ext. when I tried it it's only six. that was the reason i got the tacstar and ended up only seven. I guess I have to live by this for my tactical shotgun match. for house and transport we have to place a plug to limit only 5 rnds. a way to live in NJ.

BTW I have a plan to change barrel to 21" vented but I can't find one, I tried remington only to find a rifle sighted barrels. Any info on this is available or i have to get the rifle sighted barrels, thanks......