Devastated owner Valerie Brown, who keeps the cat at Fern Close in Atherton, says the vet who treated Oscar described the injury as the worst she had seen.
The 21-month-old tabby's eyeball was completely destroyed by the shot and the pellet is still lodged in the animal's neck, too deeply embedded to be removed.
Valerie, who got Cat's Protection League adoptee Oscar from Lowton when he was 10 weeks old, believes her treasured pet may have been held down and shot from close range on farmland behind the house.
She said: "It was absolutely horrendous and I'm furious. No words can describe how I feel. I was screaming and walking around for 10 minutes banging anything I could get my hands on when I saw it.
"There's nothing left of the eyeball.
"It'll never go out of my mind because every time I look at him I'll remember.
"The vet thought he'd been hit by a car but I knew he'd been shot, the wound was too clean.
"There's no damage to the bone. It was straight into the eye socket and shot from a downward angle. It must have been close because it's so accurate. "Oscar is so friendly he would have gone up to anyone.
"He's been a generally healthy, happy cat but he's not my Oscar anymore. He's usually bouncing around and really energetic. He's got a long way to go in life yet, but he's feeling very sorry for himself at the moment.
"I prefer my cats to be indoors but he's so adventurous and boisterous I can't keep him in.
"Everyone round here likes him and they stroke him and tickle his belly when he rolls over."
Cat Protection League (CPL) adoptee Oscar went out through the cat flap at around 11.30pm last Monday and returned with the terrible injuries the following morning at around 7.30pm.
He has been back and forth to the vets ever since and is currently unable to eat due to serious bruising to his mouth and throat, and he currently has a problem with blood running down his nose.
Retired bank officer Valerie, who also has an older house cat from the CPL called Monty, added: "There's always air guns going off over the back and when I got Oscar I was concerned about what might happen to him because he's a real hunter and he goes after mice in the field.
"I was worried something like this would happen. I've had it out with the police on two occasions this week but it has been going on for many, many years."
Police Constable Leah Hart, of Atherton police station, said: "This appears to have been a deliberate act of animal cruelty on a disgusting level.
"I would appeal to anyone who knows anything to come forward and speak to police.
"These types of actions are carried out by mindless individuals and will not be tolerated."
Anyone with any information can call the Atherton Neighbourhood Policing Team on 0161 856 7319.