AR-10 Build - Copied from Biopar.com

[kombi1976]

How's this project going now, Paul?
Got it shooting?
I'd imagine forming 8-08 is easy enough.

[gitano]

Yes, it's shooting, Kombi, but not in 808 form. It's currently sporting a 16.5" barrel chambered in 8x57. I'm working diligently on a subsonic load at the moment. See this thread: http://thehunterslife.com/forums/showthread.php?t=17116 particularly posts 17 & 18 for discussion of how that's going.

Because the large case of the 8x57 makes auto operation a bit more challenging in subsonic mode, I am back to seriously considering not using the AR-10 in subsonic mode. Instead, I am fairly well convinced that I will constrain subsonic application to the AR-15 and use the .223 case necked up to 8mm. BUT... There are still other cases being considered: 8x33 Kurz and 8x39 from 7.62x39 are at the top of the list. I'm going to get an 808-chambered barrel, for supersonic use, installed on the AR-10, then make a decision on the chambering for subsonic use on the AR-15. I have enough 8mm barrels to play with that I might make all three chamberings, and see which one I like the best. The only real challenge at the moment is cutting chambers.

Paul

[kombi1976]

Have you thought about 300 Blackout? Cases are easily obtainable, just a bbl change and the rounds fit into a standard .223 AR15 mag.

[gitano]

A .30 caliber! GASP!!! :frown:end::angry: God Forbid!

Kidding, and my fundamental distaste for .30 caliber aside - I didn't give the Blackout much consideration for the following reasons:

1) I like to 'plow my own furrow',
2) the 30 Blackout is way too "cool" for my tastes,
3) I have a slew of 8mm barrels with which I can, for the most part, rechamber as I wish,
4) 8mm is bigger than .30 caliber, and in subsonic that matters,
5) the .223 case necked up to .323 also fits just fine in a .223 clip.

Reasons 4 and 5 are the most important ones.

There is one, and only one, .30 caliber cartridge I care for - the .308 Winchester. Everyone else can have all the rest of them with my blessing.

Paul

[Jorge in Oz]

I think an AR-10 in 8x57 is pretty cool. I'm sure the Krautz would appreciate it.
 
Pity we can't have those black evil rifles here as people in Down Under get scared at the sight of them. Not even allowed to have an AR-15 airsoft rifles here.

[gitano]

That is bazaar, and sad, Jorge.

I am wrestling with the supersonic cartridge. The 808 is "out". It's too anemic with the constraints of the AR-10 action. The 8x57 is "doable", but the case is "long" relative to its capacity. The case I am focused on today is the .284 Winchester necked up to 8mm. The only "problem" at the moment is a reamer. The 8mm x 376 Steyr reamer is oh so close, but the head is 0.010" too big at the length of the .284 case, and the shoulder of the .284 is considerably sharper.

I was thinking of making a "short" 8mm x 376 Steyr. That way I could use 376 Steyr brass. THEN the problem is the rim diameter of the Steyr brass. I need nominal 0.473, the Steyr is nominal 0.510".

I am going to try to find a used .284 Win reamer. I have an 8mm necking and throating reamer, and can use them to cut the 8mm neck and throat on the .284 Win chamber.

I'll keep you posted.

Paul

[gitano]

Today, I shortened the chamber to the overall length of the .284 Win case. The only slight modification was the 'hybridization' of the shoulders - somewhere between the 8x57 and the .284 Win. I have fired one test shot, loaded from the clip. It fed perfectly, but proper feeding must be conducted with at least 3 rounds in the clip. Here's a picture of the fire-formed case from the final chamber:

 
The final case is second from the right. On the right is the .308 Win. On the left is the 8x57. Second from the left is the "long" 8x284 cartridge with the short neck. THE case is to its right. The neck on the final 8x284 is 0.275" in length. Not a full caliber, but longer than the minimum of 0.216".
 
The barrel finished at 23.5" long. Since I've only taken one test shot, I don't have any MV information other than that predicted by QuickLOAD - 2525 f/s for the 220-grain Sierra GameKing, and 3300 for the 125-grain Ain't No Varmint Bullet (ANVB).
 
Since the AR-10's gas operating system operates in the vicinity of 17,000 PSI, I had to use a "rifle" length gas tube in order to get out far enough on the barrel so that the pressure at the gas port was in the 17KPSI vicinity.
 
I have decided not to attempt to get anything done with regard to subsonic on this rifle at this time. My subsonic itch can be scratched on the AR-15 platform.
 
About 15 to 20 years ago, the hunting community as a whole FINALLY started to grasp that terminal performance was a more complex issue that Jack O'Connor had everyone believing for the previous 50 years. Among the elements of terminal performance that got at least some consideration was momentum. While momentum STILL isn't widely acknowledged by the hunting community as a whole, hunters that give at least some thought to the physics of the matter understand that it is MOMENTUM, not kinetic energy that determines "penetration". Of course bullet design also determines penetration, but momentum is what must be overcome in order to stop a bullets penetration, not kinetic energy.
 
In the two bullets in consideration above - the ANVB and the Sierra 220-grain GameKing - the momentums (at 300 yd) are 1.137 and 1.8 respectively. (By the way, the formula for calculating momentum is: momentum = mass x velocity. The unit of mass in the Imperial system is the "slug", so the unit of momentum in the Imperial system is slug-f/s.)
 
As you can see, the 220 has more momentum - 58% more - than the 125-grain ANVB. Therefore, the 220 will be more difficult to 'stop' than the 125. This should be relatively obvious when the two bullets are so far apart in weight, but let's compare some other "heavy" bullets that don't have the ballistic coefficient that the 220 GameKing does.
 
I put together a couple of Xcel tables because I know the words get tedious pretty quick. As most of you that have known me for a while know, it is important to know the "starting conditions". In the tables below, the cartridge in question is the 8x284 seen above. Other cartridges will produce different results. Second, in order to compare apples to apples, the muzzle velocity chosen was the fastest one that was ON a timing node. Lastly, the trajectory figures were determined by allowing the bullet to travel 2" above the line of sight, and be sighted in at the range that produced the smallest drop. Each bullet got its own sight-in range.
 
I broke the bullets into three categories - Light, Medium, and heavy. These are arbitrary divisions, so divide them differently if you like. Here is the table for the "Light" category:

 
Red signifies the highest value, green the second highest, and yellow the third highest.
 
Nobody shoots game at the muzzle, so comparisons start at 100 yd. At 100, the ANVB "beats" the other two bullets in impact velocity and delivered energy, but comes in a relatively close third in momentum. It's only about 6% down from the "winner".
 
At 200 the ANVB still "wins" the velocity and energy, and moves to second in momentum.
 
At 300, the ANVB "wins" all three categories, AND has the best trajectory. The difference in the momentum between the ANVB and the 170 is just over 12%. NOW, ask the average hunter which bullet will "penetrate" farther at 300 yd, a 125-grain "Plastic Tip" or a 170-grain RN, and you're likely to get a heated argument if you say it's the 125. Of course bullet design has a big effect on "penetration", but the REAL issue is "work". See here http://www.thehunterslife.com/forums/showthread.php?t=566&highlight=Work for discussion of that topic.
 
Here's the table for the "middle weight" 8mm bullets.

 
No color coding here because there are only two bullets to compare.
 
Here is the table for the "Heavy" bullets:

 
 
Without going into all of the comparisons, let me make a couple of comments. First is the drum I frequently beat, and that is that "heavy" does NOT mean greater penetration in MANY cases. The only thing "heavy" does is make for rainbow trajectories and usually LESS energy and momentum. Round noses just exacerbate the problems.
 
That said, let me remind everyone of the fact that this analysis is for a specific cartridge. While the 8x284 case has more capacity than an 8x57, it has significantly less than that of an 8mm Rem Mag. What that means is that while the 250 Woodleigh doesn't fare too well in this comparison, it would do much better if it were being launched from an 8mm RM or even a 325 WSM.
 
Using these tables, you can see why I chose the ANVB and the 220 Sierra. The Nosler 180 BT shoots pretty straight from my Rem M700 Classic in 8x57, so it will get 'the nod' over the 220 Sierra for that case, but the 284 case 'handles' the extra weight pretty well. The .376 Steyr case handles it very well with a 300 yd drop of only 4".
 
Looks like the AR-10 is "done". I just have to pin the gas tube in the gas block and neck-up some cases. Next in line is chrono data.
 

Paul

[gitano]

It's alive!
 

 
And it's finished. All I need to do now is fire-form a bunch of cases and take it to the range and sight it in. :D The 'scope, provided by j0e_bl0ggs, is a 20x42 SWFA with mil-dot reticle.
 
Paul

[kombi1976]

Very, VERY nice, Paul. How many cases have you got for it?

[gitano]

Thanks, Andy.
 
At the moment, I've only fire-formed 5, but there's really nothing to making these. I can simply neck up to 8mm and fire. I have a couple hundred .284 Win cases in hand. I intend to neck up and fire-form 50 to take to the range. 25 with the 125 ANVB, and 25 with the 220 Sierra GameKing.
 
Paul

[kombi1976]

I reckon you'll get some looks when you turn up with this sucker, especially when they see the rounds.
"That aint no ordinary NATO round!" :D

[j0e_bl0ggs (deceased)]

Kewel....!

[gitano]

I wuz thinkin' tho'...
 
I should have a barrel chambered for .308 Win/7.62 NATO available "just in case". The 8mm cartridge is "fun" and a better performer, but in the end, for 'some' applications, it's better to have a large supply of ammo than it is to have a 'good performer' but not much on hand. Good performance ain't gonna mean diddly if you run out of the "good stuff'.
 
As for "looks" from folks at the range: My preference would be that no one even notices me. I paid $150 so I could use the range when the public wasn't allowed in, just so I could do 'my thing' unnoticed. "Show-boating" never really was my 'thing'. (Except when I played HS B'ball. ) The truth be told, I really don't even like to talk to people at the range.
 
PauL

[j0e_bl0ggs (deceased)]

I should have a barrel chambered for .308 Win/7.62 NATO available "just  in case". The 8mm cartridge is "fun" and a better performer, but in the  end, for 'some' applications, it's better to have a large supply of ammo  than it is to have a 'good performer' but not much on hand. Good  performance ain't gonna mean diddly if you run out of the "good stuff'.

Prepper?

The truth be told, I really don't even like to talk to people at the range.

I know that feeling brother!

[kombi1976]

I guess I'm blessed that way. The only people who use our part of the range are our club members and most members are genuinely nice guys, most of whom hunt more than they target shoot so there's no time for owning big fancy rifles that are clearly compensating for something. Plus we live in the country and people here buy guns as tools, not status symbols. I try and be friendly and ask what they're shooting, find something complimentary to say and generally make their visit nicer. But it's part of my role. I'm a club committee member and a range officer so on the days I'm not on duty it's good to build relationships. As a result I've had a chance to fire some interesting things and had a couple of invites to hunt.