Yes they can. Where do you think 17 Mach IV's came from? There are some things to watch. If you over lube the case, you will get dents in the shoulder. If you do not lube enough, you will get a stuck case. When you "down size" a neck there are some other things to watch. By the nature of squeezing down a neck, there may be a ring that forms at the inside, base of the neck. The brass has to go some where and it does not all flow out the mouth, some of it tries to flow down the shoulder, but can't. It forms a ring inside the juncture of the neck and shoulder. Some times it is not enough to worry about, some times it is. If there is much of a ring there and you seat bullets down past the ring, you may have dangerous pressure build ups. You then have to "ream" the necks. You may also end up with uneven neck thickness, then you need to turn necks, to uniform them. You may have to turn necks anyway, because they end up too thick. In this particular case, probably not. Do not forget to trim all cases to a uniform length and deburr the inside and outside of the necks. One last thing. Be prepared to loose some brass along the way. There is always some thing that goes wrong. Some times it is a case mouth that crinkles or you get dents in the shoulder from too much lube on the case or built up in the dies, etc. If this sounds like a pain in the ear, that is because it is. Buy your brass in the correct size. Do not form from another case unless you can not get the correct brass or you have tons of the parent case and have nothing else to do. Do not forget to buy a neck reamer, neck turner and case trimmer to fit your particular application
