both, actually. I tried to do my own flat-top first.. definitely did not go well, so i shelled out the ridiculously expensive price for a second upper. The stock SP M16 comes with the old style A1 forward assist, and no brass deflector. there was a guide on classicairsoft.net at one point.. or maybe even here on arnies, that ran through doing your own flat-top, adding RIS, and adding a deflector.