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Subsonic 8mm AR-15 Cartridge
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Old 10-11-2013, 12:11 PM
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Default Subsonic 8mm AR-15 Cartridge

In another thread - http://thehunterslife.com/forums/showthread.php?t=17116 (see post #19) - I point out that I am abandoning for the foreseeable future all efforts at making a subsonic round for the AR-10, and constraining that use to the AR-15 platform. Also, I am focused on the 8mm caliber because I have an 'armload' of old milsurp 8mm barrels 'laying around'. Given that, I have decided on the 8x223 Remington.

Having made that decision, I asked Pacific Too & Gauge (PTG) if they had a drawing of an 8x223, and they responded "No", so they sent me an "editable" drawing of a .223. I modified that to what I thought were reasonable and appropriate dimensions and sent it back with the request that Dave Kiff - the proprietor - have look at my dimensions and make sure there wasn't anything 'funny' with respect to reamer fabrication or function.

Instead, I got back drawings of two "new" 8mm cartridges suitable for use in the AR-15. The first one is the "8mm Blackout". The name gives the origin away. It is simply the .300 Blackout necked up to 8mm. This cartridge would make getting/making brass easy, it'd still be a "Blackout". I don't want a "cool" (popular with all the 20-somethings), cartridge in a rifle I MAKE. Here's the drawing:


The second was the "8mm Commando". This IS a cartridge I could own. If you look at the title panel you will see "British Infentry" beneath the 8mm Commando name. Ignoring the misspelling, I don't know whether this is something the British Gov't is considering or just the pipe-dream of a wannabe. Here's the drawing:


All of that considered, I decided to just make my own 8x223 - sort of. I started out with the intent of making my own single-flute reamer. However, 1) I hate to have to make something if I have a 'suitable substitute' available, and 2) I haven't had much success with making my own reamers. The reasons why are not mysteries, but nonetheless, I didn't want to spend a lot of time in fabrication and have YET ANOTHER one not work.

As I was getting the drill rod indexed, I thought I might look through all of my reamers and see what I had in hand that might allow me to forgo making a reamer. AHA! I have a reamer for a .17 Predator. This is a cartridge that is a .223 Remington blown out to almost straight body, 40 degree shoulder, and necked down to .17. Other than going the wrong direction in neck size, this reamer would work. The neck wasn't an issue. That could be reamed separately. Making a reamer was no longer necessary. That simple substitution saved AT LEAST 6 hours of shop time. Probably more like 10 or 12. Now I could focus on the barrel.

I started with a '38 Turk take-off. First was desoldering the front and rear sights. Next, the breech was cut off at about the point where the 8x57 shoulder is, then cut the muzzle off 22 inches in front of that. (When I dropped an 8mm bullet down the muzzle, it dropped in 2", and the muzzle hadn't been reamed!) Next I trued the OD of the chamber area, cut the diameter of the stub that would be threaded into the barrel extension, and cut the 3/4-16 threads 0.625" along that shank. The barrel extension fit with the proper 0.500" of "headspace" between the barrel breech and the breech end of the barrel extension. Now to chambering.

The neck was a bit of an issue. With a bullet diameter of 0.323" and neck walls somewhere in the vicinity of 0.015", I needed a chamber neck diameter of about 0.353". 9mm is 0.354", and that would work fine, but I don't have a 9mm chucking reamer. Because I was necking UP, the neck walls would get a little thinner. In fact they went down to about 0.012". That meant I could get by with a chamber neck diameter of about 0.348". Unfortunately, there aren't any non-custom reamers with that diameter. The next Imperial size down from 9mm is 11/32nds (0.34375"). That, was gonna be a bit small. However, I was "on a mission" by now.

I decided to use the 11/32nds reamer to cut the chamber neck, and shave the neck walls down to a thickness that would give me at least 0.001" of expansion. (I have a Forster neck "lathe".) It was going to be a close shave (pun intended), but, like I said, I was on a mission to get this DONE.

The importance of the "order of operations" cannot be overstated. When one is "making do" with the tools "in hand", it's difficult to get the order of operations right the first time. Especially when one is as inexperienced as I am. While you can 'adjust' to getting the order of operations out of sync a little, it doesn't usually leave a satisfactory product. Especially to someone that prefers to do the best that they can.

The first 'pass' at all of this (6 hours of shop time) left a chamber and barrel stub that I just wasn't happy with. The next day, with the experience of the previous day in hand, I cut the stub off, reindexed everything, and started over. Here is the proper 'order of operations':

1) Run 11/32nds reamer into barrel the proper distance for the chamber length to the mouth of the case. This cuts the neck, and reams most of the body of the chamber close to final dimension. That not only makes it easier on the actual chambering reamer, but helps keep it centered as well.
2) Cut chamber with chambering reamer - In this case, the .17 Predator - fine-tuning true headspace.
3) Profile stub for 3/4-16 threads getting both diameter and length of stub right. Length of threaded stub, as barrel extension indexes on stub shoulder, sets actual headspace.
4) Use 8mm "throating" reamer to cut throat and leade appropriate for 250-grain cast lead bullet seated to proper depth for fitting in AR-15 clip.

Voila' - an 8x223-chambered barrel for an AR-15. The proof is in the pudding as they say. It was time to test-fire. First I had to make some cases.

Because the shoulder on this case was more like snake hips than 'shoulder', the primary task was necking up to .323 from .224. I used the following dies:

1) .243 Win,
2) Hornady "universal" .25 caliber neck-sizing die,
3) 6.5x55, (.264)
4) .270 Win (.277)
5) 7x57, (.284)
6) .308 Win
7) 7.92x33 (.323)

I annealed only before starting. Unfortunately, when the case being resized doesn't fit in the body of the resizing die at least fairly close, the neck can get pushed to one side. This happens because the necks are not PERFECTLY symmetrical in thickness. When getting 'pushed around', the thinner, weaker side gives more and the neck ends up lop-sided. Not TOO big of a deal for testing because case would be fire-formed anyway, but unacceptable in 'production mode'.

After necking up, I used the Forster neck "lathe" to thin the neck walls so that the case just fit in the chamber with 'finger pressure'.

After the first case for necked up and the neck walls thinned, I used the "Blue Dot and Cream of Wheat" (http://thehunterslife.com/forums/showthread.php?t=14377), method of fire-forming. The first two attempts were fruitless. Not enough "oomph" to blow the case into the shape of the chamber. I decided that a bullet was needed to provide the necessary resistance so the case would be properly formed. For the third attempt I used a 125-grain Hornady. Worked like it was 'supposed to'.

Finally, I wanted to try the 250-grain bullet with the 1050 f/s MV charge calculated by QuickLOAD. QL wasn't designed to work with a system such as this, but I was hoping it would get close enough, then I could tweak charges using chronograph results to get the desired 1050 f/s MV. However, my greatest concern was for feeding from the clip. The issue of MV was irrelevant if I couldn't get the thing to feed from a clip.

(I'm going to digress a moment here and discuss terminology. There is of course a lot of 'heat' over the proper use of the term "magazine" vs "clip". I don't give a 'tinker's damm' about terminology AS LONG AS there isn't confusion about MEANING. I learned that there was a difference between "clip" and "magazine", and the difference was that a "clip" was the thing that fit IN a "magazine", and the "magazine" was the FIXED part of the firearm. Now I'm not going to get into a spitting match over this. As usual, I DON'T CARE what term is used as long as
there is clear understanding of what is meant. I think that separating "clip" and "magazine" into to "portable" and "fixed" makes sense, and that is how I intend to use the terms.)

I resized the neck so that it would hold a bullet, primed the case, charged it with about 6 grains of Blue Dot, seated the 250-grain bullet, put it in the clip, put the clip in the magazine, released the bolt, and the cartridge seated perfectly.

When I fired it, it was louder than I thought it 'should' be for a 1050 f/s MV load. However, I don't have much experience with "heavy" bullets and 1050 f/s MVs. I do have some experience with the 8mm Max - and that was not loud.

Here's a picture of the case after firing with the 250-grain bullet:


Chrono results tomorrow.

Paul
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Last edited by gitano; 10-11-2013 at 12:50 PM..
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8mm, 8x.223, ar-15, ptg, reamer, subsonic, wildcat


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