Ok, 1st true gunsmithing question from Feinstein Project


May 3, 2013, 04:34 PM
In an AR-15, what will happen to the buffer spring, buffer, & bolt if the buffer tube is not cylindrical, but octagonal (roughly) and is larger in cross section than the buffer spring?

We're trying to design a thumbhole stock to take some of the stress off the lower. It takes a lot of small plastic bricks (LEGO, Mega Blocks, etc) to build an AR thumbhole stock, & such blocks are not good at precisely locating things such as aligning buffer tubes. What if instead of precisely aligning a buffer tube with the upper, the buffer tube & spring rattled around in a hexagonal channel made of ABS plastic bricks? Ceteris paribus, would the upper still function if affixed to such a buffer system?

Eventually, of course, we'll have to replace the liner bricks - given that would be a couple of $ worth of bricks on the used market that's not the issue.

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May 3, 2013, 08:23 PM
Just guessing?

But, I would guess the spring would flex out of line with the bolt carrier, and the buffer would bounce out of direct alignment with the Lego buffer tube.

Which would rather quickly turn the Lego buffer tube into raw plastic material to injection mold more new Lego blocks.

The thing is though?
I don't believe anyone has ever made a working AR-15 out of Lego blocks.

So you will have to be the one to answer your own questions by finding out what happens, when you try it.
If you want anything more then a WAG answer that is.


4v50 Gary
May 3, 2013, 10:26 PM
Uneven compression of spring will lead to kinks, impairing the spring's effectiveness. Said kinked spring can also begin to gouge into the plastic, eating it away and causing the part to fail.

ETA: I only finished one year of gunsmithing school. Onto the third semester next fall. Then again I'm taking seven classes in summer school.

May 4, 2013, 11:16 AM
rc, I am 99% sure you are correct about no one ever having done it, so we are trying.

In the pine lower, the upper tilted due to the failure of the upper front pin holes, and on the 3rd shot the bolt jammed in the buffer tube. Obviously the buffer tube could be slightly larger than it is now, and the mechanism would still work, but I guess no one's ever used oversize buffer tubes before, so we don't know how slight. Thanks for the quick response.

May 7, 2013, 12:31 AM
One of the most important things will be that the buffer base plane remains perpendicular to the axis of travel of the recoil spring. The coils already over the buffer when the bolt is in battery are captured and controlled so your next concern is that the remaining free coils are not allowed sufficient bowing outside of the optimum cylinder of travel so as to allow the "inside" coils to cause a degrading friction on the buffer not to mention possible outright lock-up at or near the buffer nose.
The less rigidity you have - the less tolerance for off axis travel there will be.
If whatever you've got going on allows the free coils to easily lay into the buffer - then your problem will be to keep the carrier from binding in either direction due to less axial control.
I think in short, whether cylindrical, hex, octa or whatever is less important than how much axial variation you end up with and then, how consistent is the variation?

May 7, 2013, 02:42 PM
Are you allowed cardboard or aluminum liner tube to keep buffer spring from kinking?

4v50 Gary
May 7, 2013, 10:15 PM
Just thought of it. Is it cheating to fill the spaces with Acraglass or cheaper yet, Bondo?

May 8, 2013, 10:41 AM
Just thought of it. Is it cheating to fill the spaces with Acraglass or cheaper yet, Bondo?

How 'bout melting some legos, putting a mandrel the size of the actual buffer tube in the stock and pouring it in. if you chill the other blocks and your space to be filled is small, you should not have to worry about heat.

If this is against the rules, grind up some blocks to a fine powder and then mix it into a slurry of abs cement and plastic particles, kind of like making a wood filler from sawdust and glue. Pour that around a mandrel covered in release agent. Straight, strong and still legos.

May 8, 2013, 11:41 AM
Gentlemen, there are no rules. Just trying to come up w/ a no drilling, no milling, no 3-D printing way to build a lower from scratch. I'm liking the Bondo idea - the trick is to have it perfectly in place before the Bondo sets up. However, if you do screw up - the buffer is not perfectly aligned to the upper - you remove the buffer tube, Bondo, and one layer of bricks - break off the bricks, sand down the worst of the Bondo, & start over.

May 8, 2013, 04:51 PM
And Loki, the roommate, just came up w/ how to hold it in place while the bondo dries.

Build half the stock. Set the buffer tube in place. Fill remaining hole with Bondo. Use scaffolding made of plastic bricks to hold forward part of buffer tube in place. When bondo is dry, remove scaffolding, finish building stock. Add more Bondo when complete.

May 8, 2013, 04:56 PM
oh you have to post pictures of your progress.

May 8, 2013, 05:15 PM
sota, this is a crowd-sourcing project. Get the kids' plastic bricks out & start building. I do need to take a pic of the upper part of the buffer tube/bondo receptacle, however - but not tonight.

4v50 Gary
May 8, 2013, 07:19 PM
On second thought, bondo may be too hard and may shatter from constant impact. Something a bit more ductile would be better. Epoxy/Accraglass would be my choice. BTW, you can also wrap the exterior with duct tape.

May 8, 2013, 09:54 PM
If there are no rules, make plaster casts of the pockets, mag well and trigger group, and cast some fiber reinforced epoxy around them. Drill some holes and voila.

May 9, 2013, 04:15 PM
25, sounds good, are you still allowed to home brew your lower in the People's Republic of Colorado?

May 10, 2013, 09:41 AM
I'm only going to be here another year.

May 10, 2013, 10:11 AM
no laws about building a lower but you cannot build a large box with a spring in it after July 1.

Good luck with you project. fun exercise if little practical use.

July 6, 2013, 01:54 PM
For sota:


The pictorial on how to build the stock core, to solidly capture the buffer tube.


July 7, 2013, 06:44 AM
Have just been emailed a digital jig which I will post on the website soon. That will allow us to correctly align the 5 pins - two upper lug pins, FCG, & safety - and fill in the rest of the lower w/ plastic bricks & glue.


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