Yes, could be case neck - v.difficult to see. If so, you do not need to buy a special tool IF you have a plastic hammer, a 6mm or 1/4" brass rod, as least as long as the barrel, and a .36 lead round ball. Squirt lubricant into the barrel, whack the lead ball into the muzzle with the plastic hammer, then drive it on down with the hammer + brass rod. Once the ball is started, you will probaby find that you do not even need the plastic hammer - the weight of the brass rod will be sufficient to drive down the ball if you flick the rod down to hit the ball. As the lead will be formed to a close fit in the barrel, this will take out any neck remains when it reaches the chamber AND you have slugged the barrel at the same time.
Do it a second time, and you will have a good feeling for any tight/loose spots in the barrel.
Patrick