Help Develop a New Cartridge: The .358 Ocelot


January 26, 2012, 01:21 AM


I call upon you to help me develop a new wildcat cartridge, henceforth known as the .358 Ocelot. The .358 Ocelot is intended to fall between the heavyweight .458 SOCOM and the much-discussed .300 AAC BLK in terms of ballistics and terminal performance.

The Goals for this Project:

-Bridge the gap between the smaller alternative AR15 cartridges (6.5 Grendel, 7.62x35mm AAC BLK, 6.8 SPC etc.) and the heavyweight calibers (.458 SOCOM, .50 Beowulf).

-Create a relatively (compared to .458 SOCOM, which is like shooting a 12 Gauge when run through an average carbine profile AR) soft shooting cartridge with a large bullet (.358 Diameter) that is suitable for defense, assault (LE/Military), and the elimination of medium game, which is to specifically include reliable one shot stopping of the hogs that plague Texas and the American South.

-Attain ballistic (bullet drop, effective range etc.) performance superior to the .458 SOCOM and .50 Beowulf

-Utilize standard AR15 Magazines

-Utilize a widely available parent case to speed adoption.

-Utilize .358 Caliber Bullets

-Utilize a fairly common rifle powder to speed adoption

I need recommendations on what parent case and powder would be suitable for this new caliber. I hope that this thread is long lived, and though my financial situation prohibits me from investing in the development of this I dream of seeing my concept realized even on a small scale by a better equipped handloader. Note that though the requirements are rather AR-Centric, I would love to see/create bolt guns chambered in .358 Ocelot and could see a definite market for it in shorter barreled "brush guns" especially since the case size limitation imposed by the AR platform means that the .358 Ocelot will likely work in micro-length actions, allowing for very quick bolt operation.

General thoughts on my ideas are welcome.

Consider this cartridge and its development to be fully open source, I do not plan to make money off this and am not in the firearms/ammo industry.

Timeline (will be updated as progress is made):

-Within 6 months I'd like to see standard case dimensions and pressures drafted for the .358 Ocelot.

-Within 2 years I'd like to see factory barrels, bolts and upper receivers for the .358 Ocelot.

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evan price
January 26, 2012, 03:28 AM
Take a 243 WSSM parent case (which is already possible in the AR15 platform) and just open her up to .358" neck ID. Might need to massage the shoulder a bit.
Recoil is always a factor.

January 26, 2012, 09:57 AM
I'd base it on .30RAR

January 26, 2012, 02:33 PM
I'd base it on the 9x39mm.

January 26, 2012, 07:06 PM
Since the 9x39mm is based on the 7.62x39mm round, would it have the same feeding problems in AR's?

January 26, 2012, 07:27 PM
I'm not really an expert at this, but there isn't much between, in terms of base size between the 6.8 SPC (.42) and 50 Beowulf (.51). Seems like the best parent case would be something like a .308 Winchester (.47). 450 Bushmaster is .50.

A shortened, necked up .308 has the right sound to it.

January 26, 2012, 07:31 PM
How about a .308 WIN necked up to .358? There's bound to be somebody building AR-10 hardware that would handle that.

I've been thinking about a .358 Win "Lion Scout", but an AR might be just as fun.

January 26, 2012, 07:36 PM
The AR10 is a heavier and less common platform with varying standards, a such I do not consider it to be suitable for the .358 Ocelot. AR15 alternative calibers see much faster adoption because of how common the platform is becoming.

January 26, 2012, 07:43 PM
Have you looked at the .338 Spectre? Seems to fill most of your requirements already.


January 26, 2012, 07:49 PM
The 6.8 SPC cartridge already has good bolts and mags available, and will give you decent powder capacity. However, the already developed .338 Spectre doesn't have much shoulder left and I am not sure there would be enough left for the .358.

Jeff H
January 26, 2012, 07:58 PM
Since the 9x39mm is based on the 7.62x39mm round, would it have the same feeding problems in AR's?

Looking at the Wikipedia link, it seems to have a much straighter wall than the 7.62x39.

January 26, 2012, 08:06 PM
The more I think about using a 308 Win as a parent cartridge, the more it makes sense for a bunch of internet commandos embarking on a new cartridge: lots of available parts due to the AR10 style platform. There will be much less need to make custom parts. Make a reamer and dies and turn the BCG to fit a 308 AR bolt.

January 26, 2012, 08:25 PM
6.8 SPC (.42) 6.8 is .270 and necking it up to .358 would just be rimless .357Herrett.

January 27, 2012, 07:43 AM
When taking about engines there’s a saying, “there’s not substitute for displacement.” The same is true for firearm cartridges; when it comes to performance -- case capacity is everything. It’s really as simple as that. Performance potential equals how much powder you put behind the bullet.

Consider the overflow capacity of the following existing cartridges:

.458 SOCOM -- 61 grains H20
.25 WSSM -- 52 gr
.30 Rem AR -- 45 gr
6.8 SPC -- 36 gr
7.62x39 -- 36 gr
6.5 Grendel -- 35 gr
6x45 -- 32 gr
.223 Rem -- 31 gr
.300 BLK - 26 gr

Effective case capacity would be something less once you get a bullet seated, but in general, that’s what we have as a starting point.

Midway between the .458 SOCOM and the .300 BLK (61 + 26 / 2 = 43.5) is the .30 Rem AR. Neck that up to .358”. If you seat a 180 gr Hornady spire point, keeping the COAL at 2.260“, then there’s now about 37.5 grains of empty space behind the bullet.

Keeping with the .30 RAR’s 55 kpsi SAAMI pressure spec, work up a load with some Varget or 4064, two powders that would be ideal for this cartridge’s expansion ratio. You should be able to safely get to about 2250 fps from a 20” barrel, giving 1960 ft-lbs of energy at the muzzle.

Recoil energy would be about 11.2 ft-lbs. Compare that the .458 SOCOM firing a 300 gr bullet at 1900 fps -- it has the recoil energy of 19.5 ft-lbs. So, we're not quite half that.

According to the Matunas Optimum Game Weight Formula this combination (180 gr, .358" bullet, at 2250 fps) would be adequate for animals up to 530 lbs. That encompasses everything up to and including Caribou and all but the very largest Black Bears. Hogs? Not a problem.

Is this superior performance to the .458 SOCOM? Strictly speaking, it can’t be since it’s case has less powder capacity, but it really depends on how superior is defined.

Personally, I’d like to see Remington take the .30 RAR and neck it DOWN to 7mm. Offer up cases, uppers, and all the goodies. Now, that would be something useful and something I’d lay some cash out for. But, you know, we Americans be lovin’ us some thirty cal, so I doubt it ever happens.

January 27, 2012, 07:56 AM

I guess I'm thinking along the same lines as Zane. The .358 Win is a necked up .308 Win. They just didn't have a need to shorten the case for their application. You could however use that as a model. There are known loads and ballistics data for that cartridge.

If you really want it to be an AR15 cartridge you could shorten the case down to the length of a .233 Rem and then reform the neck and mouth to maintain the 20 degree shoulder and neck length of the .358 Win. From a design perspective you'd be removing length from center of the body. Now determine your case capacity and look for a starting load from the .358 Win load data using a powder charge that will fit within that capacity limit.

I hope to hear more about how this works out for you.

Best of Luck.

January 27, 2012, 08:19 AM
Since the 9x39mm is based on the 7.62x39mm round, would it have the same feeding problems in AR's?
7.62x39 has feed problems in AR magazines and magwells. That is due to the 7.62x39 case taper causing the round stack to want to curve, and the AR magwell being, well, non-curved.

Blowing out the 7.62x39 case walls to a straighter design will alleviate that issue.

I'd base it on the 9x39mm.Looks like we're done here; the desired round already exists.


January 27, 2012, 08:27 AM
I would be concerned that .358 pistol bullets might not hold up to the velocities the round could generate. I would also be concerned about feed issues in that platform

January 27, 2012, 08:35 AM
I don't know that common .358" pistol bullets are the intended projectile.

The original 9x39mm load used a 250gr. bullet.

January 27, 2012, 09:44 AM
There really isn't much new under the sun.

Lets call it the .35Remington, and be said and done.

Accomplishes what you want, and was originally offered in a semi-automatic. And 100yrs ago, at that.

highlander 5
January 27, 2012, 10:23 AM
Many years ago Guns and Ammo created a wildcat cartridge the 357x39,a 7.62x39 opemed up to take .357 pistol and .358 rifle bullets. Chambered in a Mini 30 it equaled or slight beat 35 Remington ballistics. Sounds like that's the cartridge you may be looking for.

January 27, 2012, 02:02 PM
Many years ago Guns and Ammo created a wildcat cartridge the 357x39,a 7.62x39 opemed up to take .357 pistol and .358 rifle bullets. Wait! G&A was in cahoots with the Soviets? :what: no, the 9x39mm was totally different. Never mind! <Whew!>


January 28, 2012, 11:15 PM
Sam1911 said:

I'd base it on the 9x39mm.

I'd just make it the 9x39, call it a day, and have a tasty beverage. (Which is what I think you meant!)

January 28, 2012, 11:25 PM
Actually, I am curious about the 9x39mm. In looking around the 'web I've not been able to find out whether the Soviets blew the case out to .356" like the western style of 9mm, or up to the .365" diameter of the 9x18 makarov round.

One is technically smaller than a .358, and one is slightly larger, obviously.

January 29, 2012, 03:32 AM
Is anyone acquainted with the .30x47mm cartridge? It appears to be a shortened .308, used in hunter-benchrest competitions? Would necking THAT up to .358 leave sufficient case capacity to develop desired velocities within acceptable pressure envelopes? And MUST it be put in an AR or AK platform? Would it not work well in a FAL (which is slightly overstretched with the 7.62x51, a detriment to its accuracy), with the reduced case dimensions perhaps allowing the rifle to become a better-than 2 M.O.A. implement?

January 29, 2012, 11:27 AM
30RAR is the best choice for this.

January 29, 2012, 11:51 AM
Nothing new about it. Pretty sure they did this on Sons of Guns

January 29, 2012, 01:16 PM
Okay, I forgot that this was meant for boar hunting, but it still might be a nice cartridge to have in a F.A.L. or C.E.T.M.E. or even that funny French Gas-Auto that they used before they adopted the F.A.M.A.S. "bugle". My experience with boar-hunting inclines me to believe that a medium weight (200 + 20 grains well-constructed mid-caliber projectile launched at no less than 2200 f/s (2400 f/s would probably be better, but 2700 is probably too much) from an unobtrusive carbine disposed of one or more instantly available follow-up shots might be made-to-order for the pesky swine.
Depending on the ballistics obtained, it might make for a dandy "S.H.T.F." gun for the northwestern U.S. And by "S.H.T.F." I mean against overly territorial FOUR-legged adversaries. We already have many very nice solutions for the two-legged ones, though this would surely work for those, also.

January 29, 2012, 01:31 PM
There really isn't much new under the sun.

Lets call it the .35Remington, and be said and done.

Accomplishes what you want, and was originally offered in a semi-automatic. And 100yrs ago, at that.

At 2.58 it is about .25 too long for AR-15's.

30RAR is the best choice for this.

If a 30RAR is necked up to .35, with a 200 gr bullet you get something like this:

I bet that would work very nicely.

January 29, 2012, 10:53 PM
On the other hand, 7.62x39 bolt faces are much easier to work with. Here is what a 35 PPC might look like:

funny - it does resemble a 300 BLK scaled up.

35 Whelen
January 30, 2012, 01:07 AM
Yep, you essentially have recreated the 35 Remington. Problem is you're going to be limited to bullets for that cartridge as those intended for .358" pistol cartridges won't be stout enough and those intended for other such as the 358 Win., 35 Whelen, etc. will be too stout.
Sounds like fun though!


January 30, 2012, 02:06 AM
Okay, WHY are we limiting case length to 39mm? The Original Ar-15 cartridge ran 45mm in length. I'm not sure what the max C.O.L. is, that can be run through one, but I'll wager that it'll be very close to the same, whether the developed cartridge launches a .22 or .35 caliber projectile. Tossing away available cartridge length surrenders case capacity for no real gain. Case capacity translates to higher velocity/lower pressure, which is something we should not give up too willingly.
Yes, it IS true that it would be easier to adapt existing species of cartridge to the task, but if the mission is to "create NEW", then LET US "create NEW". To base the firearm on the "mighty MATTEL" strain of rifles already concedes more to convenience than one might consider prudent.

January 30, 2012, 05:38 AM
Actually, the max oal for the .35Rem from a Marlin Mod 336 is 2.550".
And, like .35Whelen stated, the pistol bullets are too soft, and the .35Rifle bullets too hard, except those intended for the .35Rem.

But this oal is like you said, about a 1/4" too long for the AR15 platform.
I don't think your ocelot will hunt..... but plink, perhaps

I would use the AR10 platform and chamber for the .358 and call it done, if you're wanting a .35 cal AR....

January 30, 2012, 08:12 AM
Okay, WHY are we limiting case length to 39mm? The Original Ar-15 cartridge ran 45mm in length.

One reason would have to do with the shape of bullets. The ogive of a .308" or .358" bullet would taper a lot farther back, longitudinally, before it reaches the full-diameter "driving band" section of the bullet than does a 5.56/.223". So in order to keep the overall length of the cartridge short enough to feed in an AR-15, you'd have to stuff that .358" bullet way down into a case that was 45mm long. The neck of the case has to be in the right position, lenght-wise, to grip the fat part of the bullet.

That may not quite be as short as 39mm, but it sure isn't 45mm.

Now you can get away from this problem -- or at least help it a little -- by specifying that this cartridge must use very stumpy bullets shaped more like handgun hollow-points or SWCs, but few rifle bullets really look like that anymore. Even .35 Rem. bullets for use in lever-actions tend to have a fairly long tapered nose in front of the driving band.

January 30, 2012, 12:00 PM
If you base it on .30RAR you'll be close to 358win velocities so the tougher bullets will work. Same way if you base it on a WSSM. Though that's been done already.

January 30, 2012, 07:49 PM
So many excellent responses! Good work gentlemen.

January 30, 2012, 08:23 PM
So this is a more technical question about wildcatting a cartridge. From what I've read looking at the cartridges mentioned in this thread, the .30Remington AR is a necked down .450 Bushmaster, which is a necked up .284 Winchester. For reforming purposes, wouldn't necking down be a better option since necking up thins the brass? How about necking down the .450 bushmaster to .358? Would the necks then need to be turned? As for working up the powder charge could you do that by plotting the .30RAR loads and .450 bushmaster loads on a graph and finding a curve that runs about 35% of the difference between them?

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