All National Firearms Act (NFA) engravings for individual weapons will be done "while you wait," so that the weapon is never out of your possession. While we have a SOT 2 license, this just makes it easier for all parties involved. For this reason, it is critical that you call in and schedule an appointment so that we are certain we can do it as soon as you are here.
Typical NFA engraving times are 20-45 minutes depending on how many pieces and the state they are brought in. It is preferable that AR-15 type receivers be stripped completely, as we have fixtures that make this a very fast engraving. We can work with receivers that are not stripped, but it will take a bit longer to fixture and level.
You should NOT wait until the stamp is back. You can have the engraving done at any time and usually the sooner the better.