Hi Friend,
It is okay to use slugs in either improved cylinder, or modified chokes. I don't recommend shooting slugs with a full choke, neither will any manufacturer.
I have used slugs in both chokes with a browning semi with no problem. If you are ever in doubt though, you should contact browning arms. Here is the info from their site.
Frequently Asked Questions--Can I shoot Remington® HEVI-SHOT® in my Browning shotgun?
If your question still remains unanswered, please call our Consumer Department directly at: 800.333.3288 or 801.876.2711
AJHunter