AR Build

[whipped]

from plans or what? supersonic projectile probably means extra math to cancel wave reflections. I don't even know.

[Indy]

from plans or what? supersonic projectile probably means extra math to cancel wave reflections. I don't even know.

Damn keyboard ate my long post. The NFA tax stamp for a suppressor is $200 plus the fairly long wait time for the form to be approved. So I did a bunch of research and found the most promising design to which I could add some refinements so as not to waste my NFA tax stamp on a bad design.

The shock wave caused by a supersonic bullet is pretty much unavoidable. The only solution to that is to use subsonic ammo, something I won't be doing. The suppressor's mission is to capture the high velocity gasses and give them a place to expand, as well as to disrupt any uniform pressure waves.

I figure 6AL-4V Ti or 303 stainless (less $$$) for the baffle stack and 3AL-2.5V or 6AL-4V (if I can find it cheap) for the outer tube.

[whipped]

I don't think you can build one without the stamp...

[EBSB52]

I don't think you can legally build one without the stamp...

Fixed it :thumbleft:

[Indy]

Clarified my post, I'm getting the stamp. Federal prison isn't terribly appealing to me. :thumbleft:

I'm pretty amped about it, Kansas just legalized MG's and suppressors last July. Thinking about having the baffle stack permanently attached to the barrel so I can get around the SBR stamp.

[The Dark Side of Will]

The shock wave caused by a supersonic bullet is pretty much unavoidable.

I saw a show on the military channel a while back about a sniper competition at an Army base. They were using the .50 cal rifles and the shooter could SEE the shockwave from the bullet at it tracked downrange. They even showed it on camera. It was pretty wild.

Of course they were shooting at things 2000 yards away, so there was plenty of flight time...

[Indy]

Yeah Will, I saw that episode! The shooter had enough time to recover the target in his own scope and follow it. Wicked! The shot looking out from the scope with the delta spreading out from behind the bullet was one of the cooler things I've seen.

Got a little bit more done on the new rifle today since the bolt came in yesterday. Ordered it at the end of April - things are settling down a little bit with the AR craze. Figured I'd do a little install writeup.

Step 1: Gather all the parts - or all 5 in this case. Bolt carrier (large piece), bolt (short little lugged guy), cam pin (cross-drilled with the square on top), firing pin, and firing pin retaining key (a simple cotter key). This represents the largest collection of parts in the AR15 action.


Step 2: Insert the bolt into the carrier. The only critical part is to make sure the extractor and ejector pin are facing the right way so the empty cartridge doesn't try to eject into the wrong side of the upper - the side without the ejection port in it.


Step 3: Insert the cam pin through the bolt carrier and into the bolt itself.


Step 4: Rotate the cam pin to allow the firing pin to slide through it.


Step 5: Insert the firing pin.


Step 6: Push the firing pin all the way forward and insert the firing pin retaining key.


At this point assembly is complete! The firing pin retaining key is held in by the upper reciever when the bolt carrier assembly is slid in. No tools needed.

Bolt in its locked position in the barrel:


Unlocked, rotated while chambering a round and during it's trip from and back to battery:


And a unique view into the barrel boss and gas port (top) without the barrel installed:

[The Dark Side of Will]

Now that I know a bit more about the way an AR/M-16 works... How well do the aluminum uppers wear? The ones in service sure look like steel to me.

Do you start losing accuracy to thermal expansion once you put a lot of rounds through it and get it warm?

[Unsafe At Any Speed]

That last sequence of pictures reminds me of having to clean M-16s after Army FTXs. Firing blanks through them really dirties them up. That sucked.

Cool build you have going though.

[Indy]

Now that I know a bit more about the way an AR/M-16 works... How well do the aluminum uppers wear? The ones in service sure look like steel to me.

Do you start losing accuracy to thermal expansion once you put a lot of rounds through it and get it warm?

Mil-spec calls for 7075-T6 for both the upper and the lower, although I suspect that material spec on the lower at least is for corrosion resistance only. Several companies make 6061 lowers.

AR uppers still wear very well even though it's a steel bolt carrier on aluminum. The carrier is pushed back from the center by expanding gas behind the gas rings of the bolt. This is along the center line of the upper/carrier and as a result radial forces are very low. Alot of the wear that occurs is when the bolt carrier is charged with the charging handle, which is off-center.

Thermal expansion...Loss of accuracy occurs when the bullet goes someplace you're not expecting it to. It's been my experience that thermal effects on the upper are far outweighed by those upon the barrel. Even in a geometrically symmetric barrel, residual stresses have a very large impact over the thermal range that the barrel traverses. Not to mention it's 24" long. The upper can get warm but I've never shot it to the point where I couldn't hold it, while I can't say the same about the barrel. So getting back to accuracy, thermal effects usually manifest themselves in "stringing" groups, where the impact point of the bullet moves in a line farther away from zero as more rounds are fired. Loss of a predictable impact point, aside from environmental and ballistic factors, generally stems from barrel harmonics and mechanical slip/shift in the action or bedding. AR's have a very free-floating action because the only thing constraining the lock-up of the bolt in the barrel is the barrel itself, and the only thing constraining the position of the barrel is the 1" long hole that the barrel slides in and the barrel nut itself. The genius of this is that the forces applied to the barrel are very predictable. Gun designers in the last 20 years have started figuring out that the barrel with the least number of forces acting on it will be the most accurate one. So at least this is the case with free-floated barrels. M16's use a handguard that mounts on a ring attached to the front portion of the barrel near the gas block, so pressure on the handguard in this case applies pressure directly to the barrel and further affects barrel harmonics. I'm not sure whether or not the issued M4's are free-floated.

So to finally answer your question, shooting at the relatively short range (<300 yards) that I have been so far, I haven't experienced any shift due to heat that could be differentiated from the standard deviation in accuracy. But I don't do full-auto mag dumps

Here's a pic with the handguard and gas block removed:

[The Dark Side of Will]

I have a friend who's a hardcore gun nut... and a machinist... You can fill in the blanks.

He's mentioned barrel harmonics before. Each barrel has a combination of bullet weight and powder load (whatever the jargon for these two quantities is) that makes it fire the most accurately. What surprised me is that this differentiation is at the "serial number" level rather than the "model number" level.

I imagine that's a deep field, because the shock/sound wave propagating down the barrel is travelling at constant speed (related to the powder load?) while the bullet is accelerating. Is the accuracy combination when the bullet and the wave reach the end of the barrel at the same time? Will any combo of bullet weight and powder charge that gets the bullet to the end of the barrel in that same time take advantage of the same effect?

[Indy]

Can't answer that one. I'll only offer a couple of observations. First, if the projectile is supersonic the gas flow is likely supersonic also. In a typical gun barrel this flow is not fully stopped by the bullet, some inevitably seeps past. So the shock wave present at the leading edge of this flow is probably also accelerating, though at a rate related to both the acceleration of the bullet and the burn rate. Once the bullet leaves the muzzle, the gas also usually has enough pressure to temporarily pass the projectile, so the bullet also travels through the shock wave from that twice as it is passed then passes.

Making any sense?

[TheFieroBoy]

Accuracy comes from consistency. If your cartridges have exactly the same amount of powder charge and exactly the same bullet weight, and the barrel vibrates at exactly the same frequency every time you pull the trigger, your weapon should be highly accurate.

HOWEVER...

You also have to consider the bullet weight versus the rate of twist in the rifling grooves. If the bullet is too heavy (or the rifling has the wrong rate of twist, depending on your point of view), the bullet won't "spin up" fast enough before leaving the barrel, and will be more unstable in flight. Also, the higher the rate of twist, the more powder charge you need to overcome the extra drag.

[Indy]

Got a Daniel Defense rail system! It's a used 9" instead of the 10" that I was planning on, but it was also only $240 shipped with a bunch of other stuff included. There still seems to be something missing though

[Indy]

[AkursedX]

Home-designed suppressor?

[Emc209i]

Is it illegal to own a suppressor, or to buy a suppressor, or to use a suppressor?

[Indy]

Is it illegal to own a suppressor, or to buy a suppressor, or to use a suppressor?

In my state of residence: No, no, and no. Kansas repealed some of its laws banning MG's and suppressors a couple of years ago. YMMV.

This is a good place to start:
http://www.atf.gov/publications/firearms/nfa-handbook/

Cliffnote is that if you're going to become a ninja assasin murderer, you have to pay a tax.

[crzyone]

Should cut down on the noise complaints

[Aaron]

Cliffnote is that if you're going to become a ninja assasin murderer, you have to pay a tax.

Tha man always be holding a ninja back.