While this is strictly speculation, it could be that a .308 chamber was cut into a 7mm blank. With the short SAAMI length of 2.8 and the typical long chambers of TC, the round probably chambered just fine.
The pressure squeezed the first couple of bullets down the tube, and the third may have left a jacket lodged near the muzzle. Shot #4 then encountered the obstruction, hence the jolt of additional recoil and the bulge near the muzzle end.
There certainly other possibilities, but without more details, this seems to be the most obvious that would fit the circumstances.
Incorrect ammo could also be a possibility.
Also, I know of a fellow who got the "ring" you mentioned. But he forgot that, in wanting to soak the bore, he left a wad of patches stuffed into the bore. One shot -- and a dose of severe forgetfulness -- ruined his range day in a hurry.