I don't know what you mean by "extended period of time". Assume to begin with that the gun is perfectly clean, and you are going to store it for several years. I would use the product called R.I.G. in the bore and on the outside steel parts. I would NEVER use COSMOLINE, even if I could find it. R.I.G. stays soft, and is easy to remove. Cosmoline gets very hard, and is nearly impossible to remove from steel surfaces except with gasoline! The military gave up using cosmoline ages ago, and now for long term storage, they store gunswrapped in VPI (vapor-phase inhibitor) paper inside sealedplastic bags. This is by far the best method for storing anything made of steel. You can get these bags and the VPI paper from Brownells.

For storage of several months to a year, with the possibility of perhaps someshooting during that time, I start with a spotlessly clean bore, then use a patch saturated with Birchwood-Casey SHEATH to swab the bore and exterior metallic surfaces. Then I place the gun in a gun safe. You can shoot it next time without having to even swab out the bore, because SHEATH dries in the pores of the steel, and leaes no oily residue at all.

I don't trust bore butter. The only time I've ever had any significant rust in a stored ML bore in the past 40 years was when that crap was in it! All it is good for is for lubing bullets, and then in VERY SMALL QUANTITIES!