AR Build - Moved from Biopar Site

[gitano]

Actually, "Midnight Blue" was one of the color samples I got, and it is indeed quite shiny.

While I didn't order the "Graphite Black" sample, it is one of the "black" colors at the top of the list. The "Burnt Bronze" looked good on the website, but not 'in person'. It was considerably more drab - and by that I don't mean "matt" - in reality than the web pictures.

I did purchase a small hand-held sand blaster in order to provide the 'rough' texture required for the CeraKote process.

At the moment, I'm concentrating on getting the 8mm subsonic cartridge chosen and the barrel fabricated and installed. There is really A LOT to that decision and fabrication. For instance: Using the .223 case has several advantages. However, if I use the .223 case, I have to cut the entire chamber off of the existing take-off barrels - that I have in hand - in order to get to a place on the barrel I can cut a .223-sized chamber. The 7.62x39 case has essentially the same problem. Using the Kurz case (7.92x33), that has a "standard" .470" rim and head, means I can salvage some of the existing 8x57 chamber which in turn means that I have enough of the chamber OUTSIDE diameter remaining to provide the shoulder necessary for the barrel extension to butt up against. However, the Kurz case MAY be problematic with respect to feeding from the "standard" magazine, AND it requires a ".308 Win" bolt face which is much harder to come by in the AR-15 platform. If I go with a .308 Win case shortened to 45mm I can shoot 150-grain bullets at very nice hunting velocities - 1000+ ft-lb of energy at 300 with impact velocity of 1800+ f/s - and have 'good' chambering, but... I'd have to fabricate cases and use the .308 Win bolt. And those issues are just the tip of the cartridge decision iceberg.

I'm accumulating the ancillary parts - gas tubes and blocks, barrel extensions, index pins, hand-guards, barrel nuts, etc. - while I contemplate the cartridge choices. Cartridge choice is a BIG commitment as it impacts every aspect of the long-term future with this rifle. I think I'll have a decision on the cartridge within a few days.

Paul

[gitano]

From 22hornet

I thought that .308" size bolt size was common in AR's in the USA? Is the problem getting them or the $$$ they are charging?
Are you able to make up some dummy 8mm Kurz cartridges and send them to someone who has an AR to check for any feed problems?

You mentioned using the .223 case. Is an 8mm/.223 one of the options you are looking at? Wouldn't that be too long to fit into a standard .223 magazine?
I would have thought the .308x45mm with a 150gn projectile would be too long as well?

[gitano]

22hornet wrote:
I thought that .308" size bolt size was common in AR's in the USA?
They are "common" for the AR-10, NOT the AR-15... EXCEPT that the 450 Bushmaster (made specifically for the AR-15) is made on the .284 Win case. It therefore has a .473" rim, meaning that it has a "standard" .308 Win bolt. This is just one among MANY silly "things" about the AR platforms.

Is the problem getting them or the $$$ they are charging? Yes. :)

Are you able to make up some dummy 8mm Kurz cartridges and send them to someone who has an AR to check for any feed problems?
Already done it myself with my own magazines. No apparent problems, BUT... That's only with respect to magazine fit and load. I've got to get the barrel mounted to test actual feeding. When looking at the loaded mag and the barrel extension mounted on the 8mm barrrel, it LOOKS ok. We'll see.

You mentioned using the .223 case. Is an 8mm/.223 one of the options you are looking at?
Very seriously.

Wouldn't that be too long to fit into a standard .223 magazine?
Nope. Keep in mind, one can always adjust the seating depth (up to a point) to accommodate cartridge overall length constraints.

I would have thought the .308x45mm with a 150gn projectile would be too long as well?
Again, seating depth can be adjusted as needed. However, also note that the OAL constraint on the AR-15 (magazine dimension) is 2.26". 45mm (also the .223 case length), is 1.772". That's plenty for both the 8x.223 with any 8mm bullet including a 3-caliber-long Flying Beer Keg.
The shortened to 45mm .308 case is getting "big", but it's not as big as the 450 Bushmaster cartridge, so I am encouraged.[/QUOTE]


Paul

[gitano]

Maybe some pictures of some of the cartridges being considered will be helpful...



Starting from the left:

1) 8x57 with "Flying Beer Keg" (FBK) simulator (pretty close actually). This is loaded to the COAL of the AR-10 (2.800"). It fits the magazine.

2) 8mm-08 - .308 Win case necked up to 8mm. FBK loaded to AR-10 COAL.

3) "8x45" which is .308 Win case shortened to 45mm. Loaded to .223 Rem AR-15 COAL of 2.26". Fits magazine.

4) .357 Maximum. Loaded to AR-15 COAL.

5) 8x57 Maximum. Loaded with 220 Sierra BTSP. Longer than Ar-15 COAL.

Maximums are not really 'in the mix' as the case is rimmed. I don't particularly have to deal with that issue since lots of other non-rimmed cases are available.

6) .358x39 - 7.62x39 necked up to .358 and loaded to AR-15 COAL. Fits and cycles in AR-15.

7) .338x39 - 7.62x39 necked up to .338 and loaded to AR-15 COAL. Fits and cycles in AR-15.

8) 8x39 - 7.62x39 necked up to .323 and loaded to AR-15 COAL. Fits and cycles AR-15.

9) Actual 7.62x39 - Fits and cycles AR-15. Not a candidate, but in for comparison.


There are two more x39 cartridges being considered - the .375x39 and the .416x39. I just haven't made dummy cartridges up for them.

10) 8x223 Rem - .223 Rem necked up to 8mm and loaded with FBK to AR-15 COAL. Fits and cylcles.

11) .338x223 - .223 Rem necked up to .338 and loaded to AR-15 COAL. Fits and cylcles.

12) 8x223 Rem - .223 Rem necked up to 8mm and loaded with 150 Spitzer to  AR-15 COAL. Fits and cylcles.

I thought .338 was the biggest one could blow a .223 case out to, however, recently I have read about someone blowing a .223 out to .358. I have inquired about how it's done, but have not received an answer.

13) 7.92x33 Kurz - Loaded with 180 cast spitzer. Loads and cycles in AR-15.

14) 7.92x33 Kurz case. There are four more cartridges based on the Kurz case 'in the mix' - .338, .358, .375, and .416. No dummies made up for them yet. However, with FBKs loaded to .67 calibers deep, they are shorter than the AR-15 max COAL of 2.26".


Below are some graphs that consolidate the data in a spreadsheet I developed to estimate the volume of the suppressor needed for each cartridge in consideration for this project. If any of you have MS Excel and want the spreadsheet let me know.

There is a graph for each CALIBER being considered. Within each graph, data for each separate CASE is presented for barrel lengths from 16" to 29" for the 8mm and 16" to 26" for all other calibers. The 8mm gets analyzed on out to 29 inches because I have 29" 8mm take-off barrels on hand.

The first set of graphs is the required volume for a suppressor for a specific case, with a FBK loaded, versus barrel length. As you can see, as the barrel length increases the volume of the suppressor decreases. For the subsonic application, the MV is constrained to 1050 f/s. For the cases shorter than 51mm (.308 Win), the COAL is 2.26". For the 51mm case, the COAL is 2.800". The lower the value, the smaller the suppressor. The vertical scale on each graph is the same so the data can be visually compared. For comparison, it is the relative values that are important. The actual values are only important in design of the actual suppressor after a cartridge is chosen.



The next set of graphs are the same, except in supersonic application. Here, the bullet is NOT a FBK but rather the one that gives the most delivered energy to 300yd.



The next sets of graphs compare the cartridges in the context of the ratio of the delivered energy - to 100yd for subsonic application, and 300yd for supersonic application - to the required volume of the suppressor. Since in the subsonic application, the muzzle velocity is constrained to 1050, all the cartridges within a caliber have the same terminal energy. However, for the supersonic application, the delivered energy is dependent on the MV AND the bullet used. The higher the values, the more energy delivered per cubic inch of volume of the suppressor.

First subsonic:


Then supersonic:


These look small to me. If you want to enlarge them on your screen, hold the "Ctrl" button down and tap the "+" key until they are as large as you want.

Paul

[gitano]

From 22hornet

Good work Paul. As they say a picture is worth a thousand words.
And looking at the ballistics you have posted these cartridges should work for you, provided you say within reason. Which I know you will.

And just to throw it in the mix too the little .280/30 British should work well on an AR platform provided you use suitable projectiles such as Speers 115gn HP.

[gitano]

Actually, 22hornet, I think you could use any bullet you wanted in that case on the AR platform. The specs I see state that a max chamber pressure of 50,000 is "fine" for the AR-15. That should launch just about any .30 cal bullet at reasonable MVs from that case.

It's a 'spiffy' firearm platform. I am genuinely disappointed that it has been commercially hi-jacked by "Madison Avenue" - AKA the advertising industry - to the point that prices are truly absurd with respect to the quality of the item vs. the retail cost. I know of no other product in the US in which the gouging is so blatant AND so extraordinary.

Pau

[gitano]

When I first started this project Midway had a replacement barrel on sale. It was an 18", "bull" profile, 1:7 (fast) twist rate (TR), and most important, the cheapest on the site. I was most interested in the 1:7 TR as I wanted to shoot some of the really heavy .224 bullets, and maybe subsonically. I would certainly need that TR if I had any hope of gyroscopically stabilizing an 80-grain .22 bullet, and I would probably need it even for supersonic use.

Rockinbbar impressed upon me the absolute requirement for a 'floating' handguard if I wanted the best precision I could squeeze from the rifle, so I bought the least expensive one Midway had: a Hogue aluminum "carbine-length" one. $50! And that was the CHEAPEST ONE!

The 1:7, bull barrel came with a gas block, but not a gas tube, and it had its gas port at "rifle" length, (as opposed to "carbine" or "pistol"), so I needed a "rifle" length gas tube as well. Cheapest one - $13.

I'm getting ready to get out and do some predator calling, and the AR-15 is coming along, so I was installing and sighting in the new 'scope AND installing the new barrel today. I did everything, (not the sighting in), while sitting in a chair in the den. This platform is amazing. Anyway, here are some pictures of the piece as it sits ready to hunt. (You can compare them to the pics in post #36 if you want to see the changes.)







It shoots straight with 'cheap' federal "military" (5.56x45 NATO) ammo. I just got it "on" at 100 so I don't have any targets to show, neither did I run it over a chronograph. I'll do all of that when I start handloading for it. For predators, cheap, FMJ 55 grainers out of "military" brass shooting about 1MOA will be fine for the time being.

Paul

[gitano]

From j0e_bl0ggs

Coming along nicely....

what cartridge will you decide on???  (8mm)

[gitano]

Still wrestling with that...

Short list is .223, 33mm Kurz, and x39mm.

Paul

[gitano]

From 22hornet

Now it's all coming together Paul. Looking like a real rifle now.

[gitano]

Thanks, 22hornet.

I forgot to mention that I replaced the original "factory" trigger springs with a "4 lb" set.

Makes a difference.

Paul

[gitano]

Since I had to psychologically cross the "ugly bridge" to even consider this project, I have felt a certain emotional liberation with respect to what I can hang on this thing and not feel like I am making it any uglier.

The 'scope you see in the above pictures is a brand spanking new Simmons 6-18x40 that I recently got off eBay for $40. I LIKE the new Simmons 'scopes with the side focus. A LOT. I was a little bummed by having to put that scope on the AR-15, but it was 'OK' because I was getting the gun in 'fighting form' in prep to take it 'varminting'. However, when I was down in the sanctum sanctorum, I happened to notice that one of my Ruger No.1s had another Simmons 'scope on it that I really didn't like on that gun. I had put it on that rifle just so I could work up specific handloads. (The No.1 is chambered in .308 Win.) I also got that scope 'for a song', but I don't 'like' it because it has a 50mm objective. Also, if a scope is going to have a 50mm objective, to my eye, it needs to be longer than this 'scope is. I don't particularly care for Hunting gun 'scopes with objectives larger than 44mm, and 40 is 'better', aesthetically, to my eye. I bought this one because it was cheap, and I intended its use only for load development. It is a 3.5-10x50 with a stadia crosshair and some kind of 'bullet drop compensator'.

As I looked at it, I realized that it would be 'perfect' for the AR-15. 'Looks" don't matter, and the 50mm objective would help with light gathering after dark on moon-lit snow. (No artificial lights allowed in AK for hunting ANYTHING.) Conversely, the 6-18x40 would be 'perfect' for the No.1. :D

Here's what they look like now.





Paul

[gitano]

From drinksgin

More Buck Rogers by the minute! :D
nra, tsra, goa, 2nd amm. jpfo, def con.

[gitano]

I used that exact term to describe to Susan what the appeal of the AR was to the 'younger generation'.

Paul

[gitano]

From Alboy

I agree on the scope swap and am glad the thing shoots for you guys.

In my case I entered the service shortly after these things and had to shoot the early models which I fear ruined me forever on the gun. To fragile and persnickety for my tastes. Jams on the firng line were common and the guns were clean.

My boy has a couple of new ones and they shoot fine but it just does not twist my crank.