Since it is legal to engrave the barrel would it not be legal to do this? law just states that it must be readily available which this is as readily available as doing it on the barrel is it not?
Buffer tube isn’t part of the rifle.
technically neither is the barrel? but it’s a common SBR engraving point and OK in the ATF’s eyes. One would argue that the buffer tube is at least more a permanent than the barrel is it doesn’t come off with the upper and is staked to the lower (when installed properly anyway)
On the part where the extension screws into would be neat. The charging handles would make it out of sight. Might be hard to do on the curved surface.
Not sure about all of the above. Engraving has to be on the registered weapon part itself (AR that would be the lower, if a Hk MP5 it would be the upper strangely), not a part connected to, and able to be viewed with the weapon in it’s functioning state. Inside the magwell for instance wouldn’t cut it either. If you want your engraving hidden out of sight I suggest having the engraving done on the under side of the receiver along the trigger.
… Or you could engrave on the front of the magwell but I’ve found that spot to be tight fit for a trust.
With the right milling centre, that’s almost trivially easy, but those sorts of things are usually making way more interesting parts. Honestly, if unobtrusive is the goal, trigger guard is still ideal.
Not legal
Why is everyone always so obsessed with hiding their engraving? This comes up monthly and I still dont get it.
That is because it isn’t up to the ATF. The actual law says where and what needs to be engraved. Serial number on the frame or receiver, other markings on the barrel, frame or receiver.
The law specifies that the barrel is OK. Not any other detachable part, not the receiver extension. Shaky ground, IMHO, if you’re concerned that you might get “interviewed” by someone from the ATF.
Send it to Orion Arms, have them engrave the front of the receiver & blacken it. It’s so neat and small that you would never, ever see it unless you looked hard. Legal and easy.
Better yet, inside the trigger guard. The lettering only has to be 1/16 inch high. Not exactly intrusive.
That’s where I did it on my noveske lower
ident marking did the same place for mine. Looks professional and not noticeable unless you actually are looking for it.
Paranoid types.
Just engrave a non removable part of the lower. It removes the bullshit factor and your following the reich order…
But it might just be me…
I think I am going to have someone engrave something for me and I am going to do it diagonally starting in the lower and up to the upper. Then I am going to weld the pivot pin so it is non removable just to CMA.
I’ll give you 20 bucks if you do that and post a picture of it. 
+1, Orion did the same thing on my CMMG SBR lower and it looks great (in that you can’t really see it unless you’re looking for it).
Orion Arms is good, but I have found Ident Marking to be a little cheaper and a little faster. And Orion is only about 100 miles from here.