The lowers for an AR-10 and AR-15 are different although some parts maybe interchangable. Uppers are also different length and there parts are not interchangable. Best visual referrence is to stand a 308 cartridge and 5.56/223 cartridge side by side. Height difference has to be accomandated in one by making everything larger i.e. magazine, bolt, housings (upper & lower), barrel, bolt, carrier etc. Additionally there is the difference in pressure each cartridge opperates at during firing with components need beefing up to withstand.