You don't state what barrel you have. I'm going on the assumption it is a 12 ga smooth bore with choke tubes.
Your barrel is "back-bored" which means it is slightly larger than a standard 12 ga bore. This means the slug will fit loose and may "rattle" down the bore and basically be inaccurate. I would suspect lead fouling may also be a potential problem.
My Mossberg 835, also with a back bored barrel, warned against using slugs in the owners manual for the reasons stated above.
Your best bet is to buy a slug barrel. Mount a good shotgun scope and use sabots. You will be amazed at the accuracy you can get with slugs in a rifled barrel.
Now, my disclaimer. You will undoubtedly receive several replies stating, "go ahead", I've done it for years" etc.
When all else fails, contact the manufacturer. Contact Winchester/Browning and see what they advise.