What am I getting for my $200 tax?

Exactly how much shorter are the SBR's I am making?

The overall length of a 16" carbine with a fiberlite stock is 35".

The XM177E2 clone (11.5" barrel) is 33".  Replacing the faux moderator with a birdcage flash-hider would shorten that to 30.375".

The 14.5" barrel will be 33.4".

Remember, the length of the flash-hider doesn't count for the legal length of the barrel.  If you drill a hole through the flash-hider and into the barrel a bit, stuff a pin into that hole to lock the flash-hider in place and then weld over the top of that hole you get to count the length of the flash-hider as part of the barrel.  This is called pinning and welding.

An 11.5" barrel with a pinned and welded 4.5" moderator comes to 15.375" long.  Going with an anachronistic 12.125" long barrel with a pin-weld job makes it legal without the tax stamp and is hard to tell apart from the NFA version.  Overall length would be 33.625".

The 14.5" barrel with a permanently attached flash-hider is even closer; 15.875"!  A 1/8" spacer behind the flash-hider or a slightly longer one will easily bring it up to legal length and still more than an inch shorter than a 16" barrel with a flash-hider.

There are several places selling the 14.5" barrel with an extended flash-hider permanently mounted too.

The only advantage to the 14.5" SBR is I can take it apart without cutting anything off.  A dubious privilege.

What I am doing it for is the visual accuracy.  If you put Sabrina next to a real XM177E2 the only difference you'll find is the roll-marks and my selector has an A2 tick on it.  A bayonet will be in the correct spot on Kaylee.

That tax stamp also lets me go out and get spare uppers that don't fit the current narrative.  There's no legal way to make a visually close 10.5" barrel with a permanently attached birdcage.