I had the same in 2 different inline when I was shooting inline.
I find one gun to clover leaf 3 shots and then throw a flyer or 2 ,problem was resolved by swiching bullets.
next m/l always like you said maybe 2 together then a spread of 2" at the 100 yard target.I find this with just about any bullet and powder I used in this gun.I always new there was no prblem shooting deer with but a tack driver it never was.
The above gun mention was a 54 cal. t/c thunderhawk,one day while looking at some used m/l in the gun shop,there was a 50 cl. thunder hawk there price was $60.00 bucks as is ,seem it was not shot that much still had blueing in the bore,hey what the heck bought it.
I got home and switch the set up on the 54 to the 50cal. and to my surprise this gun will tack drive bullets at the 100 yd target,it tells me the barrel on the 54 cal.could of been the problem