"You" can have a look a this thread http://thehunterslife.com/forums/showthread.php?t=16262&page=1 to see a similar discussion with respect to the AR-15 platform. That thread is a good read to get most of the background on the considerations and constraints one faces when building a purpose-driven firearm. This thread will be about the same subject, but focused on the AR-10 platform instead of the AR-15.

A significant difference between the two applications is that I have decided that both the AR-15 and the AR-10 will each get at least two barrels: One for supersonic use, and one for subsonic use. I have come to the following conclusions:

1) The goals of subsonic and supersonic shooting are too disparate to try to make a single barrel or cartridge optimum for both.
2) It's too easy to have, and change to, a second, (or third, or fourth...), barrel to worry about "one size fits all". I'm pretty sure I could complete a barrel change in the field in something near 5 minutes, and a 16.5" subsonic barrel fits easily in a day-pack.

In the AR-15, the supersonic cartridge is the .223 Remington. That's what that platform was made for, and that's what it's going to get. Subsonic, I think the .416x33 will get the nod. There are 500-grain .416 bullets available, and as I pointed out, when a muzzle velocity ceiling is set, (subsonic), the easiest way to increase delivered energy is to increase bullet weight. The 500-grain .416 at a MV of 1050 f/s, delivers 1100 ft-lbs to 100 yd.

So now the questions for the AR-10.

The AR-10 was made for the 7.62x51 NATO round, (.308 Win), and while I REALLY like that cartridge, I have an aversion to .30 caliber. So the supersonic AR-10 barrel will most likely be chambered for the 8x51, (8mmx.308 Win). (My "aversion" to the .30 caliber is complemented by my affinity for the 8mm.) I've always thought the 8x51 was a great cartridge. However, niggling at the back of my mind is the 8x284 Winchester. The reason I am not conclusively settled on the 8x51 is that the .284 Winchester CASE is in the front-running for the subsonic chambering. Of course the .284 Win case has more capacity than the .308 Win case (but only 5 grains more) and I hope to launch 8mm bullets faster.

In the subsonic version, of course speed is not a factor, but case capacity is. The larger the case, the more difficult it is to get a bullet going SLOW. Of course, the heavier the bullet, the easier it is, and 500 grains is fairly heavy for .416 caliber. However, some of you may recall the 500-grain BTHP Hornady produced in .416. VERY cool looking cartridge. However, the bullet is almost 2" long, and to use it in the AR-10, I need to shorten the .284 Win case by about 0.3". SO... It's not really a .284 Win any more anyway. Here is a picture of some of the candidates:


On the left is the .416x284 Win with the 500-grain Hornady BTHP seated. To its right is a factory 7.62x51 cartridge for reference. Notice that the .416x284 is longer - by about 0.3". That's because I didn't want to butcher a .284 case when I don't have a way to properly size/resize/form it. Just try to imagine the case and cartridge the same length as the 7.62 cartridge.

Next to the right is the .223 Rem necked up to 8mm and a 250-grain 8mm bullet seated to the proper length for the AR-15. To its right is a .30 M1 case "necked up" to 8mm with a 237-grain bullet seated. And on the far right is a .45 ACP necked down to .416 with the 500-grain bullet seated in it.

There is no difficulty, (on paper), getting the bullets in these cases to 1050 f/s muzzle velocity. However, in the .45 ACP case, the SAAMI max pressure is 24,000 PSI. It requires 33,00 PSI to get the 500-grain bullet to 1050 in that case. I'm not too worried about that at the moment because this was more of an illustration exercise than a real candidate. However, I don't think the extra pressure is an issue, because the SAAMI max is solely a function of the ACP action, not the cartridge.

These cases, especially the M1 and ACP, help illustrate the fact that for the most part, a cartridge's case is PRIMARILY just a device to hold the powder charge. As long as you can get enough powder in the case to achieve the MV you desire, case shape and "origin" are immaterial.

Anyway, that should be enough information to start a conversation. I seek your thoughts on this. There are no "wrong" answers, but I may - am even likely to - have a different opinion/preference on any particular point. Remember, I am asking for opinions, not instruction. If I disagree or don't use your suggestion, it doesn't mean I think you're wrong.

Paul

You might consider also, the .338 federal with the 300 gr offerings that carry fantastic BCs, or the .358 Win offerings.

Or maybe develop a "Max" version of the .44 Automag, which is based on a cut down .308Win case, and could carry a 310-500gr cast boolit (with minor resizing) of the .429-.430 diameter.   A close relative of the FBK.

I'm envisioning this, (on the left) with a slightly longer (1/8"-1/4") casing.


Just an idea or two.

Jamie.270 - I did consider the .338 Federal or the .338x51 with the very bullet that you note. However, I figured if I was going to "go big", I might as well go really big. Of course that begs the question: "Then why not the .458 SOCOM, or .450 Bushmaster or even .50 Beowulf?" I don't have a good answer other than with regard to those three particular cartridges, if you have to ask "How much", you can't afford it. ($1295 for upper alone!)

I thought the .458 SOCOM was based on the .284 Win, but apparently not. Here http://www.military.com/entertainment/outdoor-guide/ar-hunting/rifle-review-458-socom.html is some info regarding the relative "ease" of use of the SOCOM over the Bushmaster and Beowulf. Here's a site with lots of info about the SOCOM http://teppojutsu.com/welcome.htm

Bottom line is: I'm just not interested in the SOCOM, Beowulf, or Bushmaster. I simply gotta plow my own furrow, donchaknow. I do NOT think anything I might come up with, or settle on, will be "better". "Better" is not my goal. My goal is... have something that I thought up and made myself.

Of particular note is the ease with which one can change barrels on an AR platform. No reason - except money - not to have a .338 Spectre (.338x.223 Rem for the 15), or a .338 Federal (.338x51 for the 10), or much of anything else one wanted to buy a barrel blank for and make. Of course the 'devil in the details' are the magazine/feed issues, reamers, and dies.

Thanks, and "Keep those cards and letters coming in..."

Paul

Strange as it looks I like the 45 acp conversion. Carrying two barrels and thus two ammo types I kind of like the idea of being able to easily tell the cartridges apart no matter what the conditions and that little jewel is certainly unique enough.

QuoteStrange as it looks I like the 45 acp conversion.

Couldn't agree more. Just somehow appealing to the eye...

QuoteI kind of like the idea of being able to easily tell the cartridges apart

Good point. Something I hadn't considered. This could also be a place for the lye-stained brass. http://thehunterslife.com/forums//showthread.php?t=17087

10mm brass might be a better choice. Don't know.

Bye the bye... Jamie.270's comment about the FBK (Flying Beer Keg), was not without notice. Since we're talkin' subsonic, I looked at the FBK in all the calibers of interest. Of course it looks 'good'. Problem is, I don't have any in hand, and I've got enough on my plate without adding another "to do" to the list.

Paul

I just passed on my altered mold for the .452 , 315 gr gc, which is now .460.
A person I know found it was 1 moa in his newly acquired 1885, .45-70.
I did pour about 30 before sending  it off, so I could send you some if you are interested in it in the .45 acp case.
I cannot understand why that, with the 500 gr.,  .416 reminds me of a mortar bomb!


Just more fun making up weird combos.

Keep the .460s handy, Don, but I think I'll wait on them at the moment. The .416 is looking "good" for now. The real problem with the .416 is bullet availability. Hawk has plenty, but they are VERY proud of them. After shipping, they're about 75 cents a piece. Including brass, powder and primers, it'll cost more than a dollar for every trigger pull. That's not an issue once one has all of the kinks with the action worked out and a good load found, but getting there gets expensive when you're spitting one dollar bills out the end of a semi-auto firearm to "test" it.

The Hornady 500 BTHPs were a "test run", and I haven't seen them since.

"Just more fun..."

Ain't it the troof! However, there is a lot of learning that comes with "what if I did this". The learning comes from first figuring out the 'why', and more learning comes from the 'why NOT'. This is part of the 'soul killing' that draconian gun laws produce. It is difficult to kill the creative human spirit, but highly restrictive gun laws severely inhibit all the learning and innovation that comes "What would happen if I did this..."

Paul

Is a cast bullet not an option?



Hmmmmmmmmmmmmm maybe should have read previous posts..... apologies!

I want to get something in the AR-15 or AR-10 platform shooting a subsonic bullet - SOON. As we have wandered through this exercise twice, it has become clear that at least part of the 'problem' is too many choices. I'm about to remedy that. SOON - meaning SHOOTING in a couple of weeks - puts everything in a different light.

I'm going with an 8mm, and here's why:

1) I have many 8mm barrels IN HAND. That means no reamer if I choose to use the 8x57 cartridge. I do.
2) I have IN HAND, molds for casting 250-grain 8mm bullets.
3) I have IN HAND 8mm swaging dies. I COULD make a solid lead 8mm bullet of almost any weight. All I have to do is determine the maximum length that a milsurp barrel can gyroscopically stabilize. The point-forming die I HAVE will make the necessary ogive to facilitate feeding from a magazine.
4) I COULD use any of the 8mm bullets I have IN HAND. The 220 Sierra is an excellent candidate, and I would have to do NOTHING to implement that.

I should be able to get to a shooting rifle in relatively short order.

I DO intend to chamber a barrel for the AR-10 in 8mm-08. I ordered the reamer from Pactific Tool and Gauge today, but it will be 6 weeks for delivery.

I think for the AR-15 platform, I'll use the 8x223 Rem, for most of the same reasons mentioned above. I will have to get a reamer though.

Once I get a couple of monkeys off of my back, I can essentially devote myself to some of the firearms projects I have currently got on hold, among which will be some 'large caliber' subsonic cartridges for the AR platforms. In the short run, I need to get a subsonic-capable rifle in hand.

Paul

PS - I did some 'calculating' with the BC calculator which also calculates required twist rates for a given bullet. Using the 8R point forming die I have, I can swage a  pure lead 300-grain 8mm bullet of length 1.52". That bullet will be "fully" stabilized by an 11.4" twist. A 300-grain projectile doing 1050 f/s has about 735 ft-lb of energy. With the calculated BC of .612, it will deliver about 675 ft-lb to 100 yd.

To illustrate how insignificant BC is over 100 yd and subsonic projectiles, a round ball has a BC of about 0.085. At 100 yd, it would still deliver 465 ft-lb.

Paul

OOOf slap on head, forgot all about the 8mm dies... man I hate this CRS!

As some of you may recall, (and you'd have better memory than I do if you do), I got an 8x57 barrel on the AR-10 for testing functional operation of the finished build. It worked fine. However,  the first cartridge in a clip doesn't have the tip of the bullet down "in" the magazine. Therefore the extra length of the 8x57 vs the 7.62x51 was mitigated for a single round. That's how I tested the live fire function of the rifle using a barrel with an 8x57 chamber.

While an 8x57 case fits in an AR-10 clip, if you put a heavy (long) bullet in it, there are 'problems'. The first problem is overall length. The clips I have limit the cartridge length to 2.839". A 57mm case is 2.244" long. That leaves only 0.595" between the case mouth and the inside of the front of the clip.

"So" you say. "Just seat the bullet deeper.  You're sub-sonic anyway so you don't have to worry about case capacity in a 57mm case."

"Not so fast", says I. The heaviest JACKETED bullet I have is the Sierra 220 Boat-tailed Spitzer. If I seat that bullet such that the COAL is 2.835", there is precious little bearing surface held in the neck. Most of it is BELOW the neck in the shoulder and case body.

"OK", says you. "Just use a .308 Win case necked up to 8mm. It will fit the chamber and the clip, and who cares if it blows the shoulder forward."

"That would be OK", says I, "but it would blow the case almost straight." (The "neck" would be about 0.150" long.) Instead, I will use an 8x57 case, and push the shoulder back to where it is on a .308 Win case. That will make a very long neck - about 0.6" long - but that long neck will hold on to the longer, heavier bullets. When the cartridge is fired, the case will be fire-formed back to 8x57.

Those that fear "excessive" headspace should be shouting "the sky is falling" about now. This headspace is about 0.594" "excessive". And yet, there is NO problem, let alone ANY risk of "danger". The supposed "danger" of "excessive" headspace is ABSURD!

A lesson I have learned about modifying AR barrels is that once you drill the gas port, you cannot rechamber to a shorter cartridge - say 8x57 down to 8x51 - because the gas tube would be too long after you shortened the chamber. While you COULD shorten the gas tube, that's not a trivial matter. The tube has a crook in it, AND it has a slight thickening at the receiver end. I think that thickening is to help it "deal with" the slamming of the "key" that operates the blowback of the bolt.

Here's the upshot: I can't simply shorten the chamber on the barrel I already have on the AR-10 to 51mm (.308 win length), because the either the gas port would be in the wrong place, or the gas tube would be too short. Pick your poison.

The solution? The "long-necked" 8x57.

Here are some pictures.

Here are the three cases in question and the Sierra 220 BTSP.

On the left is the bullet of course.
Next is the 8x57 case.
Next is the "long-necked" 8x57 with bullet seated.
On the right is the .308 Win case.

Notice how short the neck would be if the .308 case was blown out with the shoulder length of the 8x57.

Here are two pictures of the cases loaded in the clip.





There just ain't no "free lunches" in REAL life.

Paul

Like I said above, I ordered the 8mm-08 reamer yesterday. For those interested, there is a schematic drawing of the reamer at the bottom of this post. Note that the outline of the reamer is NOT the ACTUAL outline. This is a generic outline. One has to look at the numbers in the boxes to see the true dimensions of the actual reamer. "TPI" is "taper per inch".

Paul

Are you going to have Dave "short throat" it?
I was thinkin' since you are looking at the longer 8mm bullets, that a longer throat might be better, but if they're buried in the case anyway, it may not make much difference.

A short throat would be better for those ANVBs though, should you choose to have an "either/or" gun from a single barrel.

Just curious.

How are you holding the case in place for the firing pin strike with the shoulder set back?
Does the ejector/extractor grip the base?

Jamie.270 - I was going to, but at only 0.090", it's already short enough as it is. You may recall that I had the 8x376 Steyr reamer made with NO throat and bought an 8mm throating reamer to cut the "long" and "short" throats separately while cutting both chambers with the same reamer. If I want a longer throated version of the 808, I can always extend the throat with the throating reamer.

Don - there are two mechanisms 'holding' the case for the firing pin strike: 1) The case body near the head fits snugly in the chamber. That "snugly" might not be sufficient to ensure a good firing pin strike EVERY time though. 2) The extractor does indeed grip the rim.

I probably should reiterate that I do NOT suggest that large headspaces, or "sloppy" ones are "good", or even OK for that matter. There's a very practical reason that most headspace specs are plus or minus 0.006". It's the very issue you raise; CONSISTENT firing pin strikes. Combine an "excessive" headspace with a short firing pin protrusion from the face of the bolt, and you get a mis-fire, at least now and then.

No, my "complaint" is the assertion - mostly by "gunsmiths" and idiot gun-writers (ptooey) - that "excessive" headspace is DANGEROUS. BALONEY! PROPER headspace is "good" and "right", and if one is having custom gunsmithing done, the headspace SHOULD be exactly on the specification. There's really no excuse for anything else. But the "danger" ascribed especially to old milsurp, (Mausers mostly), rifles due to "excessive" headspace is a BALD-FACED LIE. The only reasons being GREED by "gunsmiths", STUPIDITY by gunwriters (ptooey), or LIES by manufacturers to prevent people from buying or using less expensive firearms and thereby getting them to buy the manufacturer's firearms. This is EXACTLY what happened to the 8x57 cartridge in this country following WWII. If you can't tell, let me be clear - those LIES rub me the wrong way...

Paul