[grunt]

Only a bore gage will let you know for sure. Some barrels years ago had the bore stamped on the bottom of the barrel.

 

[Oregunner]

You could put it on paper and see what it looks like. (see link) I have an older BT-99 that has a .726 bore. If yours is close to that, it sounds like yours might be a loose mod. Or, maybe the barrel has been bored out and is still a full choke with an overbore. You really need a bore gauge or need to pattern it to see. Or you could just shoot it and not worry about it. Put some shot on the target and it breaks.We have a guy that shoots games at our club and does really well. He was telling me that he shoots an IC choke all the way back. I didn't believe him, but after he let me unscrew the choke, it was an IC. Can't argue with success.

 

[trapshooterjoe7]

The bore could be .725 to .735 and it may have been back bored to .745, only the bore gauge will tell.

 

[coveybuster]

Trapshooter Joe is right. My '69 measures .725 and my '70 measures .730.

 

[copperhead]

According to the ser.# it was manufactured in 87 if that helps.

 

[Oregunner]

According to my Blue Book of Gun Values, Invector chokes became standard on BT-99s in 1986. Sounds like you have an unusual gun, maybe it has had a different barrel fitted?