I have used a scoped Rem 11-87 rifled barrel for over 10 years with various slugs. Generally, Foster type slugs group about 3-4 inches at 100yds, but solid copper Rem sabots group 1.5-2.5 inches. This rig is dependable, accurate, and deadly on deer! I find that my hold needs to be consistent and firm because groups will spread if the hold or rest are inconsistent. My experience differs from yours. If you use a scope, check the mounts; check your crown and the bore diameter. I once had a different rifled barrel for this 11-87 that would not group under 4-5 inches.
best, nrb