Unless your gun has been subjected to adverse conditions (sand, mud, etc) where dirt has entered the barrel, just blow some duster into it from the chamber end and it'll blow out whatever dust/dirt might be in there.
Remember oil attracts dust and dirt. Also many higher quality (upgrade) inner barrels have a coating so adding oil on top of the coating isn't necessary and maybe even be detrimental to performance.
Edit: if the barrel DOES have significant sand/dust/dirt in it, disassemble it, remove the hop up rubber and run water through it it to clean it. Shoving a patch down the barrel and dragging dirt around will only scratch up the coating/inside finish of the barrel. So clean it out with water, THEN you can use a dry patch. If its just brass you can use silicone oil on the patch, it'll get extra dirt off; you shouldn't use so much it leaves a coating on the surface. Use the lightest weight you have, don't fucking grease the inside of the barrel.
Also don't get oil on the hopup. Be EXTREMELY careful about scratching it up, not only can it tear but scratching it up will affect how it performs (the oldschoolers who used sand paper on slick Marui hopup rubbers know what I'm talking about). You can clean the hopup rubber with water and dishwashing liquid to get the oil off. If you're using a GBBR you'll want to do this every once in a while since the oil in the gas system is being sprayed onto it (in extremely tiny amounts, you don't have to wash your hopup rubber after every game; you're going to do it maybe once every year or two depending how many rounds you put through that GBBR)
Last edited by Drake; October 4th, 2016 at 13:01..