My dad has an old Remington 1100 that is pre-screw in chokes. He's got four or five barrels for it that are all normal choke names, except for "Modified Trap". Is this Improved Modified or is it something else? I guess I could just take a caliper and measure the bore width, right?
Yep, mic it out. Then you'll need to find a choke chart to see what the measurement is so that you will know where you fall in at. Here is something that you can go by.