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Unload , make safe, take apart like for cleaning.
- Chamber. Using 0000 steel wool or the finest Scotchbrite [tm] pad wrapped around a bristle brush, clean that chamber good. After a bit the plastic build up will be gritty and sticky and not allow a shell to extract.
- Gas Ports - Using a pipe cleaner with solvent , clean the gas ports in the bbl. Those two little holes accessed from the "sleeve" that bbl fits through.
-Extractor, clean with pipe cleaners or stiff toothbrush, do the bolt face while at it.
The most important areas in any firearm is the chamber, extraction and in this case the gas ports.
Clean the rest of the gun with solvent. Then CRC Brakleen to blast away and all dirt ,crud and solvent. Pay attention to getting ports clean Be careful to not get on wood or O ring.
Lube lightly. ATF ( Dexron II automatic Trans, Fluid) is GREAT for keeping a gas gun running.
Most important is the chamber. If using factory shells - ok. If reloads, make sure these hulls and cases are re-sized...otherwise they may not chamber / extract in your gun.
1100's will run, many of us have run 'em a 1k rds before "cleaning". I may have rarely cleaned the bbl, but I assure you I did clean chamber, extractor, ports. I used good ammo, meaning either factory or resized all cases in reloading.
HTH