AR-15 in .223, going from 16", to 14.5", to 12.5" etc, where do reliability issues come into play, if at all? What other issues or considerations? Does the lower need to be re-sprung and/or buffer change? Let's assume factory geissele complete 14.5 and swapping a factory geissele upper in 12.5 or shorter (+nfa). No suppressor. Thanks!