B767 Captain reported aircraft rolled back slightly at the gate after chocks were placed resulting in possible contact with a belt loader.

Narrative

I operated Flight XXXX from ZZZ to ZZZ1. It was a Company flight. We were empty. The aircraft was Aircraft X After landing we blocked in at XA36 at Gate XX. External power was connected. Chocks were inserted. After completing our cockpit duties; I got up from my seat and prepare to leave the airplane. However; before I left the airplane; I realized that I had never released the parking brake. As is my habit; I then stepped outside the airplane to visually ensure the chalks were indeed in place under the left mains. I then returned to the Cockpit and got into my seat; released the parking brake; and looked out the window to the left to verify no movement. As sometimes happens; there was slight movement as the chocks grip the aircraft wheels. It seemed like we rolled 6 to 10 inches. I can't be sure how far; but there was some movement before the chalks completely stopped us. I also reinserted the parking brake before I left the Cockpit as a precaution. Once I exited the airplane and went down the stairs; a loader said that the airplane had moved back and possibly struck the belt loader located on the right hand forward side belly. From the time I parked and chocks were inserted until the time I released the parking brake; there was a delay of a few minutes. During that time; a loader was placed against the airplane and a cargo door opened. While it would have been ideal had I released the parking brake initially after chalks were inserted; I also believe that in the future; chocks should be placed more snuggly in front and behind of each tire. That would minimize any rollback. As it was; there was a slight grade on the ramp. The First Officer and I examined where it the belt loader may have hit the door frame; but we could not be sure of any point of impact. The First Officer and I then got into the van and left the ramp and also the Company facility. I called Maintenance and explained the situation; asking if they could go check to see if perhaps there was any damage; which may may; or may not of occurred. I also called Person A to let him know of the situation. It is my belief that in the future; chocks should be inserted snuggly on the front and back of each tire. That would be the best solution to prevent any rolling when the parking. Factors that contributed to event/concern. As Captain; I should have promptly released the parking brake; and incurred the rollback prior to the loader approaching the airplane. However; I believe that the chocks should have been placed in a tighter manner to the tires. Have the chocks been placed tighter; the plane would not have rolled back. Had I released the parking brake sooner; there would've been no loader in place. Both options would've helped.