Subsonic 8mm AR-15 Cartridge

gitano · October 18, 2013, 11:33:58 AM

Don,

Don't head to the PO yet! I have a whole armload of .357 Max cases from the "8mm Max" project.

Quotea quick pass in the lathe to remove the rim and reshape the extractor groove

Considering the other hoops I have suggested, that's not really too much. I'm OK with trimming rims, but I'm a bit nervous about cutting extractor grooves. The case manufacturers may have counted on that part of the head remaining intact. Also, some of these "pistol powder" charges are up in the 35 to 40 thousand PSI MAX pressure range.

The "simple" solution is the 7.92x33. All I need is the 'right' bolt. The next best is necking the 7.62x39 up to .323. I have bolts for that cartridge. BOTH of those solutions require me to make, rent, or buy a reamer.

Paul

drinksgin (deceased) · October 18, 2013, 01:34:29 PM

25 on the way, just add to the collection.
I hope you do not have too much fun whupin' those color added fish!

;<)

The case does have an extractor groove, it may be enough but a few thousands deeper should not be a problem.
I believe the .357 max. loads at about 35,000 psi from the manuals I have.

Jamie.270 · October 18, 2013, 02:19:02 PM

Paul,
Check your e-mail inbox.

j0e_bl0ggs (deceased) · October 18, 2013, 09:27:16 PM

QuoteI'm OK with trimming rims, but I'm a bit nervous about cutting extractor grooves. The case manufacturers may have counted on that part of the head remaining intact. .

Section the head to see how much meat there is and if suitable grind a HSS tool to cut relief in one pass...

drinksgin (deceased) · October 19, 2013, 06:00:38 AM

Joe, you got there before me,although I was going to suggest doing both a .357 max case and a .223 case as well.
I really do not believe the very small amount of metal removed would present a safety hazard.
I believe you will have to make sure the modified case will fit the shell holder you are going to use.

gitano · October 19, 2013, 11:20:33 AM

Quote from: j0e_bl0ggs;128507Section the head to see how much meat there is and if suitable grind a HSS tool to cut relief in one pass...

That was the plan. But... looking down the case I can see that the head walls are definitely thinner than the .223 walls unless there is some optical illusion involved.

I may have found a source for "Basic" .223 brass with 0.025" walls. If that pans out, I'll be grabbing those in a heart-beat.

Paul

gitano · October 20, 2013, 12:10:20 PM

Here is a picture of the 8x223 side by side with the 8x357 Max:

The 8x223 has been fire-formed and neck-sized. The "Max" has not been fire-formed.

The "Max" looks good. I'll trim the rim and enlarge the extractor groove and see what things look like.

I am also working on getting the AR-10 operational. I chose to chamber it in 8x284 Win. To that end, I rented a .284 Win reamer. As I was preparing to cut the chamber, it occurred to me that I COULD leave the chamber 57mm long instead of cutting off the breech by 0.050" and chambering to .284 length. Leaving it "long" and fire-forming would give me a shorter neck, but a longer body - larger capacity - case. The reason wasn't that I wanted the extra capacity, it was that I am a little worried about the sharp .284 shoulder feeding from the clip. The extra capacity was just 'gravy'.

I knew there would be some kind of a "joint" where the 8x57 shoulder met the .284 shoulder, but this isn't a rifle for "someone else", and I can live with cosmetic 'anomalies'. Here is a picture of the fire-formed case :) with an unmodified .284 Win and an unmodified 8x57:

You will note the very short neck. Probably too short. It makes me 'nervous' at least. You can also see the juncture of the 8x57 neck and the .284 neck on the fire-formed case. I put this here in this AR-15 thread because this 'stuff' is all related as far as I'm concerned.

Using the measured case capacity of the 8x284 case, QuickLOAD estimates MV for the ANVB (Ain't No Varmint Bullet) at 3350 f/s. On my AR-10 with the 23.625" bbl, that means about 1350 ft-lbs at 300 yd, and about 6" below line of sight. With the 220 Sierra GameKing, the energy at 300 is about 1850 f-lbs, impact velocity is 1900+ f/s, and it's 9" below line of sight.

That's all "good". However, unknowns remain: 1) How will it feed from clip/magazine, and 2) how will that short neck work especially with the long 220 bullet?

A VERY simple 'solution' is just to make the chamber the correct length (to shoulder) of the .284 case. What 'worries' me about that is the sharp .284 shoulder and feeding properly. So the "no free lunch" is, take the short neck but "good-feeding" shoulder, or take the long neck and maybe NOT good-feeding shoulder.

It is not correct to say that the "complete" fix is just to use the cartridge for which the action was designed - .308 Win/7.62x51 NATO. That cartridge is "anemic" with respect to the 8x284 Win OR 8x57 cartridges.

There just ain't no free lunches...

Paul

drinksgin (deceased) · October 20, 2013, 01:56:56 PM

The .223 rim is .015" thinner than the max rim, so much of the reshaping can be done there,not further out on the body and the extractor/ejector may work well with much less of a groove than the factory design which may have been over designed to assure positive functioning in various military rifles, you can try it and see.
That should alleviate your concern about thinning things if it does not require as much thinning as you thought.

gitano · October 20, 2013, 03:11:16 PM

Of course I noticed the existing extractor groove, but your idea about taking most off of the rim, and minimally off the body is excellent.

J0e_bl0ggs has made the point that one of the nice things about the .223 case is that one doesn't particularly have to pay attention to where the spent cases go. If you loose them, not too big of a deal. Conversely, the more you put into making a given cartridge, the more you have to "worry" about where the empties go.

The reason most folks 'go with the flow' is because plowing your own furrow comes at a price - time, effort, money. While I do not seek to spend extra time, effort and money, I am willing to. Up to a point, and that point can move around a little depending on what season it is, and how badly I 'want' something.

Tomorrow morning, I'm checking into some "basic" .223 brass that supposedly has neck walls of 0.025". If that doesn't pan out, the Max is next in line, but using the .357 case does mean recutting the chamber.

Paul

drinksgin (deceased) · October 20, 2013, 04:15:54 PM

How much do you have to recut it?

j0e_bl0ggs (deceased) · October 21, 2013, 10:44:37 PM

Quote from: gitano;128533Tomorrow morning, I'm checking into some "basic" .223 brass that supposedly has neck walls of 0.025".

Paul

Just a thought when using expensive brass - would it be worthwhile to make this rifle single shot?

gitano · October 22, 2013, 08:07:58 AM

I don't know. The thought certainly crossed my mind. However, you are the one that told me you didn't really care to get your hands on a SS AR.

Don, you can see how much shorter the Max case is than the .223 case. That's how much I'd have to shorten the chamber.

Paul

j0e_bl0ggs (deceased) · October 22, 2013, 10:37:39 AM

'Tis true but that was a personal thing and we were talking about 'pipe dreams'!

Now you are talking about premium brass and not range pick-up.

gitano · October 22, 2013, 12:10:48 PM

It's really irking me that I can't find an AR-15 bolt that can handle a 0.473" (nominal) rim.

I may have to break down and use the 7.62x39 case.

Paul

drinksgin (deceased) · October 22, 2013, 01:21:11 PM

Can't just consider it a chamber with an extra 4mm of freebore?

I mention this because I have 2 rifles , .357 mag, reamed to .357 max and .44mag, reamed to .445 SM, both use more than one length case and I do mot have a problem with them, at least as far as I know.
If you were to consider a ss, you could do as I did, make a lump and fit an 8mm barrel to a Handi, you could even use your old Topper if you kept it below 45,000 psi, as the factory fitted that action with .30-30 and .44 mag. barrels.