Yes: A Remington 870 shotgun with a modified choke will shoot any commercial shotgun slug.
What do you want to know?
HR Ultra slug for sure.
Almost every gun shop worth its salt should have a slug barrel for the remington model 870,if not then they should be able to order one with no problem.
Can you shoot a standard slug threw a modified and or improved?
head
A rifled slug- yes.
Shooting rifled slugs is the ONLY way to shoot thru a smoothbore for deer. If you shoot sabot slugs thru a smoothbore, it will not spin and therefore not be accurate. For accuracy, the slug must spin out of the barrel. Either shoot a rifled slug thru a smooth barrel, or shoot a saboted slug thru a rifled barrel.
No.
yes
Remington 870 pump loaded with "OO" buckshot or rifled slug
You shoot at what is called a "patterning board" to determine the shot density at different distances (assuming that you are shooting shot and not slug). Your best bet is to buy a book. Check out Amazon.com or DBI books.