I think I understand your reluctance to removing the barrel, if you are concerned about scratching the barrel with the barrel clamp. Don't be. Just be careful and it isn't a problem.
Even if the gun only has a box of ammo throught it, it should be cleaned and especially if it is going to sit for months and months.
I gave you a link to the AI&P Tactical sight. There is one aspect of cleaning it recommends with which I cannot agree; the use of paper towels as cleaning patches. That is fine if the barrel is off the gun. If it is on the gun you risk a small piece of paper tearing off and getting stuck inside your receiver. I had this happen by running a piece of paper towel down the magazine tube.
It only takes two minutes to remove and reinstall a barrel; 5 if you have the extension for the magazine.