Recently purchased a S&W M&P 15-22, I have put approximately 300 rounds downrange. At first all seemed well, tried to zero the iron sights and it progressively shot worse. The pattern was sometimes ok, somestimes wild flyers. Cleaned the gun and noticed some tightness near the bolt end of the gun. Noticed what looked like fouling. Back to the range, same issues only worse. Broke it down for close inspection again, ran a brush down the barrel and it was extremely tight again. Used a Hoppes solvent and pad, it became logdged in the barrel again near the bolt again only this time terminally stuck. Used soft taps from muzzle end to dislodge the cleaning flute only to find a wad of lead fall out. A large clydrical piece of accumalated lead that blew my mind as to how large it was and odd shaped, looked like the bullets were slivering off a little piece at a time until the end result described. I have thoroughly cleaned it a the barrel looks great, clean as a whistle with no areas of fouling.

Has anyone heard of this happening before? What do you think may have caused it?

Novice to the shooting arts of AR styled guns but love this gun, any help is appreciated.