I was going to make this part of the AR-15 thread, but I decided it should have it's own. Among the 'things' I have learned with these projects is that unless you REALLY want something bigger than the AR-15 platform can offer, DON'T EVEN THINK about an AR-10. Everything about them is twice as expensive and twice as difficult to find. It's genuinely ridiculous.

You know I milled the receiver from one of Tactical Machine's "80%" blanks. I then started looking for parts. The lower kit is identical - IDENTICAL - to the AR-15 lower kit, but it costs $15 more. When I went to get a butt-stock for it, the one I wanted was "out of stock" everywhere 'on the planet' OR it was twice the price of Midway. I settled for a REALLY UGLY adjustable one.

Paid a ridiculous price for an "assembled" upper. That's a real stretch of the imagination to call an upper with NO BOLT CARRIER GROUP "assembled". I paid $125 for the bolt CARRIER ALONE - no bolt. Everyone is "out of stock" of AR-10 bolts unless you want to pay obscene prices. So I'm waiting for one to come "in stock" so I can only pay a ridiculously stupid price for it instead of an obscene price for it.

I think you can see that I am not particularly 'happy' about the prices associated with the AR-10.

I was planning on chambering the rifle in "8-08". The .308 Win necked up to 8mm. The primary reason being that I have many 8mm milsurp take-off barrels in hand from which I could make an AR-10 barrel. However, when I got to thinking about it, it occurred to me that the 8x57 cartridge MIGHT fit in the AR-10 magazine. If it did, I could simply rethread the milsurp barrel and be ready to go. I checked the max overall length of of the cartridge for the AR-10 as dictated by the magazine size, (2.8"), and found that the 8x57 would fit and "work" even with a 195-grain Hornady loaded. Now I just needed to determine if a milsurp barrel had enough 'meat' on it to allow me to rethread for the AR-10 barrel extension (hereafter "BE"). (Cost of AR-15 barrel extension - $20. Cost of AR-10 barrel extension - $44.) After some measuring and 'cyphering', I decided that it was close enough to give it a whirl and see what it looked like 'in the hand'.

I start with a "large ring" Mauser barrel. (The "small ring" Turk '38 bbls won't work.) Here's a picture of the one I was going to use, chucked in the lathe.



And


(I know it's VERY ugly, but I'll come back to that issue later. Remind me if I forget.) I want you to note specific 'parts' to the stub sticking out of the lathe's chuck:

1) On the breech of the barrel there is a tiny bit of barrel diameter that isn't threaded. That is the diameter of the root of the threads. It is 0.980" in diameter.
2) The threads are right at 1.110" in diameter. That means that the threads are about 0.060" deep.
3) There is a 'shoulder' muzzleward of the threads. It is 1.110" in diameter.
4) The area muzzleward from the shoulder is 0.980" in diameter.

The ID of the Barrel Extension (BE), was 0.930". To the best of my ability to measure them, the BE threads were 0.050" deep. That would mean a MAX OD of the barrel stem of 1.030" to fit the BE. That seems a little too large to consider for the "root" diameter - 0.980" - of the milsurp barrel. Small by 0.050". However, male threads don't extend to the absolute root of the female threads, so I thought that MAYBE the combination of the threaded part of the milsurp barrel, plus the FULL contact of the shoulder of the milsurp barrel, plus at least 'some' of the rest of the barrel, would provide enough 'grab' to reasonably fit the milsurp barrel to the BE. And that's why you see the barrel chucked in the lathe.

I 'pared' everything down to the 1.03" dimension suggested by the measurements on the BE. Here's what that looked like.


As you can see, that got down quite a ways on the threads and provide for what would be at least 4 threads worth on the shoulder. I was hoping that would be 'enough', but knew I would get at least a little bit of 'bite' on the barrel body ahead of the shoulder.

I cut until I was just cutting the top of the threads with the "full form" shoulder on the threading insert and then tried the BE. The barrel was too large. Great! I continued cutting until the BE would just screw on with hand pressure. Here's what the barrel looked like:


As you can see, there's plenty of bite from one end of the threads to the other. This will definitely "work". Here are a couple of pictures with the BE attached, AND a picture of the barrel on the AR-10 receiver.







I am not tightening anything down until I can get a bolt and make sure the headspace is correct. The BE stops abruptly where it should, but I'm not sure that's where it should be in order to put the barrel breech against the face of the bolt. It seems a little too far off right now. I can't really do anything until I have a bolt.

It's a ugly beast no doubt. The barrel is the proverbial "sewer pipe". That's just fine by me. It is my intent to show that "sewer pipes", as long as they don't have mangled muzzles, and aren't bent, can shoot JUST AS GOOD as $500 dollar Lilja barrels. (Or whomever, I'm not picking on Lilja.) This barrel, after cutting off about 4" from the muzzle, is now almost exactly 24" long. I will 'clean it up', true up the muzzle, and have full expectations that it will shoot as well as I can aim it.

More when I get the dang bolt.

Paul

From j0e_bloggs

QuoteHey Paul,
How about threading the stub longer and making a locking nut to fit in front of the barrel extension à la Savage?
You then have a shoulder and an adjustable switch barrel.

Not a bad idea as long as the 'nut' was the same OD as the barrel extension. Of course, the AR is kinda of the "ultimate" switch-barrel. That is one of its traits that endears it to me.

Paul

From 22hornet

Quote

QuoteJoe_bloggs wrote:
Hey Paul,
How about threading the stub longer and making a locking nut to fit in front of the barrel extension à la Savage?
You then have a shoulder and an adjustable switch barrel...

Great idea.
This one is coming along nicley too Paul. It's a Maus-AR.

Maus-AR - I like it!

I have since received the Bolt Carriage Group (for ONLY $209). I haven't gotten around to tuning everything up as I have been focused on the boat purchase.

Paul

Originally posted on April 30,2012:

I am very close to shooting this rifle. Still need to drill gas port and install gas block and tube, but that's about it. I was noticing that it was kinda heavy so I weighed it. With everything on it including 'scope and rings and magazine but no ammo, it weighs 10 pounds and 4 ounces with the 24" 8x57 barrel. That's heavier than I think most people would imagine. I wouldn't want it to go over 11 lbs, but I don't mind that 10lb 4oz weight. Maybe I'll get it test-fired this afternoon.

Paul

Gas port is drilled - gas block and tube are installed on barrel - index pin is installed on barrel extension. Nothing left to do but load some rounds up and test fire.

Here is the barrel in the receiver with the gas block and tube in place:


Here, with the barrel nut on:


And here with it 'ready'. The gas block and tube are hidden inside the handguard.


More after I have test-fired it.

Paul

Originally posted on May 1, 2012:

It's test-fired! I'm still alive and I have all my important parts. No trip to the ER and the gun is intact and no worse for the wear.BigGrin

I loaded two rounds (wanted to test the autoload function). I used 40 grains of I3031 behind Hornady 150-grain flat-based spitzers. According to QuickLOAD that should have produced about 2500 f/s at a max pressure of just over 30,000 PSI.

It worked like I knew what I was doing. Both rounds ejected properly, but... the bolt did not stay open after 2nd round. What that means is that the bolt didn't move far enough to the rear to 'catch' the follower on the empty magazine, which strongly suggests that I need a LITTLE more 'oomph' from the gas system. I didn't have the adjustable gas block all the way open, so there is some room to play with, but...Instead of opening the gas port, I could just up the charge. I've probably got close to 20,000 PSI of pressure headroom.

I loaded the cartridges to the 2.8" OAL spec for the AR-10. That put the 150-grain bullet into the case right up to where the case mouth was at the bullet ogive. Wouldn't want to go any deeper. Might have trouble using a heavier/longer bullet. I wouldn't mind the 2500 f/s if that turns out to be the real MV.

In the final analysis, I'm thinking the 8mm-08 needs "doing". The 8x57 has it's advantages - the primary of which is no rechambering of the milsurp barrels. However, there is no ballistic advantage to the 8x57 when you have to make OAL the same length as the .308 Win OAL. Therefore, the 808 can "do" whatever the 8x57 can, AND the brass is cheaper, AND I can use longer bullets with the shorter case. I'll have to check on whether the milsurp barrel 8x57 chamber can be shortened and still have all the 'meat' I need for threading for installation of the barrel extension.

Paul

From Nelsdau

QuoteLooking' good.The switch barrel feature is mighty intriguing. If you get the hankering for something in 7mm, Montana Rifleman has AR-10 barrels in 7-08, 20" for $65.

Nels

WHOA! An AR-10 barrel for less than $400! It's a miracle!

Thanks, Nels. I'll have a look. ANY AR-10 barrel for less than $250 is worth a look!

This barrel has a very slight bend that I didn't notice until I was spinning it between centers to profile it for accepting the gas block, so I am going to take it off and make another. (This barrel may work for a subsonic chambering if I shorten it back to the first step.) I've got another take-off that I have checked for 'not bent', and will cut some of the chamber off and make an 8mm-08. Now that I have one of these under my belt, I am confident that I can cut the threads for the barrel extension and poke the gas port well enough that everything will function correctly. I am equally confident that I can 1) make the 8mm-08 reamer, and 2) cut the 8mm-08 chamber. I think I will put the gas port out at "rifle" length on the new barrel, instead of back at "mid" length like on the 'old' barrel. I'm not sure yet though. The rifle-length gas port location puts the gas block right off the muzzle end of the rifle-length handguard. The "mid" length positioning hides the gas block inside the handguard and gives it a 'cleaner' look to my eye. The adjustable nature of the gas block allows me to put it pretty much wherever I want to.

Here are some close-ups of the 'naked' 8x57 barrel:






By the way, careful measurement of the barrel extension, and just plain dumb luck yielded the correct headspace. When the barrel extension was screwed on as tightly as I cared to, the full-length resized cartridge fit the chamber 'perfectly'.

Paul

So tell me this isn't a "sign"...

I was doing some paper-whipping for the 8x51 cartridge. The barrel will finish at 23 7/8ths (23.875"). The best bullet for down-range delivered energy is the Hornady 195 International, a flat-based spitzer. Cartridge overall length is essentially fixed at 2.8" - the limitation of the AR-10 magazine. There are two powders that burn up completely in that length barrel: I3031, and Accurate 2495. The truth is, I don't trust QuickLOAD's (QL) numbers for I3031. It pretty much says I3031 is the "best" powder for almost every cartridge/bullet/barrel-length combination. Using Acc2495 and a max pressure of 50,000 PSI, QL predicts a muzzle velocity of 2430 f/s. It burns 100% of the powder in 18.02" of bullet travel. As most of you know, I rely heavily on barrel "timing" to start my load work-ups. The QL-predicted exit time for this load is 1.302 milliseconds. I opened excel and entered 23.875 for the barrel length into the optimal barrel timing node calculator. The theoretical node timing for the 6th node is 1.302 milliseconds. That NEVER happens. I'm taking that as a sign that the 8x51 is THE cartridge. :D

Paul

From DavidLT89

Quote

QuoteThe QL-predicted exit time for this load is 1.302 milliseconds. I opened excel and entered 23.875 for the barrel length into the optimal barrel timing node calculator. The theoretical node timing for the 6th node is 1.302 milliseconds. That NEVER happens. I'm taking that as a sign that the 8x51 is THE cartridge.

will be interesting to see how the QL prediction turns out! God Bless.

I doubt it will end up there after all the actual measurements on the finished chamber and barrel are input, but it was interesting to see it hit right on the button from the 'generic' numbers.

I have a really hectic week ahead, and I seriously doubt I will get ANY work done on this project for at least 10 days. It could easily be a month. Next up on the 'to do' list for this project is the chamber reamer. Then barrel profiling, and gas port placement. That should get it shooting. Once it's shooting the way I want it, I'll put a "finish" on the barrel. I'm thinking phosphate, but I'm not sure yet.

Paul

From Nelsdau

QuotePaul,

With a barrel with a gas port, any thoughts how that might affect OBT? Or are the are the waves down and gone before any effect from gas porting come into play?

I found from playing with QL for my 8mm wildcat that given the bore and case volume that the reduction in velocity by decreasing barrel length was surprisingly low. Less than I expected anyway in crunching numbers. Have you found that to be the case for the 8-08?

Nels

Truth be told, Nels, I haven't got a clue how all the "stuff" on an AR barrel effect its harmonics. The combination of the various "lumps and bumps" of the AR profile, plus the gas port, and significantly the gas BLOCK AND the "flash hider" or suppressor. Without going through the considerable effort of generating a new node mesh for an AR barrel and then doing a whole new simulation, I really don't know how one could guess what the timing relationship was relative to a 'smooth' sporter barrel. For the time being, I'm just going ignore the "timing" and see what shoots the straightest.

Before Chris Long came up with his optimal barrel timing theory (OBT), Dan Newberry was using the optimal charge weight (OCW) technique to come up with optimal charges. Turns out, the OBT 'theory' and the OCW 'technique' work to the same end. Therefore, absent the availability of the OBT timing data, I will use the OCW process and expect to get good results.

Paul