Relatively small sized, light weight, high velocity military service cartridges like the ×39mm allow a soldier to carry more ammunition for the same weight compared to their larger and heavier predecessor cartridges, have favourable maximum point-blank range or "battle zero" characteristics and produce relatively low bolt thrust and free recoil impulse, favouring light weight arms design and automatic fire accuracy.  Tests measured the free recoil energy delivered by the ×39mm AK-74 rifle at J ( ft·lb), compared to J ( ft·lb) delivered by the ×39mm in the AKM. 
AKs offer several buttstock options besides the classic fixed position. You can find an under-folder and right- and left-side folders. Those are now legal in most states since the sunset of the Clinton 1994–2004 Assault Weapons Ban. If you want to leave behind tradition, you also can buy an AK with an AR-15-style collapsible buttstock . The under-folder is perhaps the most recognizable version, but the side-folders have certainly been around for a while. Many side-folders come in a triangular shape, approximating the basic shape of the wooden buttstock. Or you still can find what is called a wire stock, a single rod extending from the rear trunion and ending at the buttplate. That is also called a crutch-folder since it resembles the end of a crutch that goes under the armpit. The beauty of the side-folders is you can retrofit them to a conventional buttstock AK with little work. Because of its unique rear trunion design, the under-folder is an under-folder for life.