I have a question, i bought an sbr upper thats 6.8spc,my question is if i have to have a lower marked 6.8spc too?The lower reciever im planning on using is a new rra 556/223 will this mess up the paper work on form4?Do i need to get a reciver marked for 6.8spc or can the 556 one work as far as paper work goes.thanks