Heya folks I am building a few AR's and hopefully can get these questions answered:

For the XM177E2 I have an innovative industries moderator on order and a grenade ring I am trying to figure out barrel choices for building: Either a 11.5" two tax stamp gun or what is the shortest barrel I can get away without having to do two tax stamps?

Regarding the aluminum/fiberlite buttstocks what era/year did they change the collapsible stock hardware from a C-clip retainer for the stock stop pin to the cross pin style for the stock stop pin?

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My ‘177 barrel is a cut-down 6520 barrel w/ a Brick moderator & original grenade ring. The barrel is 14.0” from the rear of the barrel extension to muzzle w/ the fake moderator pinned-n-welded, bringing overall length to a non-NFA 16.1”. Upon further measuring, total length is 16.75”.

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Sounds like you are going with their NFA moderator https://innovativeindustriesllc.com/xm177-moderator/.
I would bite the bullet and stamp an appropriate lower and run the correct barrel length. I think most of us built our XM177E2 clones with faux moderators permanently pinned and welded to bring our barrels to non NFA length. I know I did. If I were to go NFA I would use a 10" barrel and build an XM177E1.

The stocks with the c-clip on the adjusting pin are newer commercial, the original stocks has a slotted hole through the nut and were pinned through.

Thank you for the info so far.

Why I chose the XM177E2, is I am cloning a specific rifle that a friend of mine carried when he was in the service in the early 80's and I always thought it was a neat variant. Originally I was going for a XM177E1 but I decided to change it around after talking to him.

Regarding the collapsible stock nut: Who started using the C-clip if it is not a military modification? I am genuinely curious as to when this happened? I used to believe that it was just pinned; but I found a C-clip that was in use recently. I later saw a vendor (Retro Rifles) selling them so that got me thinking that maybe this happened? But if I am wrong in this supposition which I admit is quite possible let me know please. I am trying to just get to the bottom of this.

This stems from me not knowing much about the minutiae of these older carbine variants. I am trying to avoid the frustration of buying the wrong parts for my builds.

If you are putting the effort to put an NFA moderator on the gun, than I would bite the bullet and do a proper length barrel. Besides, all other longer barrels look weird IMO.

Several years ago, Brick made a lengthened replica of the XM177E2/GAU-5AA moderator that was identical, but was 5.5" long. Combined with a 12.5" barrel length, it kept the look and did not require a tax stamp, but it was a P&W job.

Here is some information on the Model 629/XM177E2. You may find it helpful. If you already have this, disregard.