A Wildcat Cartride I like: .30 ARX

Now here is a wildcat that I can get behind. The .30 ARX is an immediately semi-auto AR-15 compatible cartridge. It’s nothing special from a manufacturing standpoint; as the cartridge creator claims, the .30 ARX is simply a necked-up 6.5 Grendel to accept common .30 caliber bullets.

But, by changing the case from a .223 base to a 6.5 Grendel base, the user immediately gains 14 grains of case capacity (no bullet loaded) over the 300 BLK. This gives shooters using the right powders the ability to get to nearly 30-30 load performance while still being able to use the relatively common 6.5 Grendel bolts.


Personally, I like this over the 300 BLK, simply from a mechanical design standpoint. While there is the issue of less common components, the higher-taper cartridge will make feeding and extraction more reliable. Plus, it solves what I consider one of the detriments of 300 BLK, its lower supersonic velocity (again, relatively speaking).

The .30 ARX is still a wildcat loading, but tools are available for the reloader to easily manufacture this round at home. Those interested in the cartridge can see more details, purchase dies and other components, and check load data at AR-X Enterprises. 

Nathan S

One of TFB’s resident Jarheads, Nathan now works within the firearms industry. A consecutive Marine rifle and pistol expert, he enjoys local 3-gun, NFA, gunsmithing, MSR’s, & high-speed gear. Nathan has traveled to over 30 countries working with US DoD & foreign MoDs.

The above post is my opinion and does not reflect the views of any company or organization.


  • sam

    So, like a 7.62×39 Ackley-improved?

    • Yeah, basically, with .308″ bullets.

      It’s got a couple hundred feet per second on the 7.62×39, but immediately I think “what does it do that x39 does not?” Silhouette shooting or something? I could see it being useful for that.

      • The US Army Marksmanship Unit has reportedly used a .30 Grendel wildcat in 3-Gun matches. It can make Major Power Factor for rifles.

      • Vitsaus

        Not that I’m rushing to defend a flavor of the week AR15 wildcat, but the benefit of this would be a true “intermediate” cartridge for the platform. Even if it only slightly out performs 7.62×39 the advantage would be simplication of components in the AR platform, so no weird magwells, no oversized bolts, etc… Also, potentially brass cased reloadable ammo versus the steel cased non reloadable junk.

        • In what way does the AR-15 not already have true intermediate cartridges available for it?

          • Vitsaus

            I should have said “another true intermediate cartridge” I didn’t mean to imply that there aren’t a few already. When I think of “true intermediate” I think of stuff like .280 British, 6.8 SPC, 6.5 Grendel, etc…

          • Why isn’t 5.56 an intermediate in your view?

          • uisconfruzed

            I’ve got a 300 BLK & Grendel & hunt with a 7-08. I had not heard of the 280 Brit, it appears to be an excellent design.
            I’m blaming you to my wife for the purchase of an EM-2 if run across one.

        • Echo5Charlie

          You just described .300 BLK

      • BrandonAKsALot

        I would say it offers what the BO promised and struggled to deliver: versatility. It should be able to operate sub or super sonic with correct loadings. Again, like the 300 BO, if you don’t plan to suppress, there probably isn’t much purpose. You can’t readily get sub-sonic 7.62×39 and while you can reload it, the .311″ bullets are not exactly staggering as far as selection. And let’s please not dive into the .308 vs .311 bullets as I know they CAN be used, but it’s always better to use the purpose built option if possible. You raise another point that would be my main concern in another post, that you are decreasing bolt face area and increasing thrust and wearing locking surfaces faster.

      • iksnilol

        You can use .308 bullets in 7.62×39. Finns have been doing it for a while (same for 7.62x54mmR).

        Using a 308 bore with regular .310 bullets shouldn’t be a problem (7.62×39 has a decent deal of less pressure than 5.56 or 7.62×51). So building a gun with a 308 bore and feeding it with .310 or .311 bullets shouldn’t be problematic (at least at 7.62×39 pressures). As always though, check the cases for pressure signs and whatnot + I am not responsible.

        • FarmerB

          My Mini-30 from years ago (early 90’s) had a .308 bore.

          • iksnilol

            How’d that work out? Only Minis I have experience with was a Mini-14 that patterned (a lousy pattern at that). Luckily it wasn’t mine.

          • FarmerB

            Let’s just say it wasn’t my most enjoyable firearms owning experience. It finished up with the govt crushing it – fitting end.

          • iksnilol

            Why’d I think of the scene in Office Space when they f*** up the printer? Seriously, reading your comment made “damn it feels good to be a gangsta” to play in my head.

            The accuracy was bad I assume? Rugers aren’t really known for accuracy in my part of the world.

          • FarmerB

            Everybody else at the time had SKK and a few SKS. I got the Ruger hoping for a bit better accuracy in the open country (chasing pigs). It turned out ‘spray and pray’ with an SKK was the better tactic than using the Ruger for precision aimed fire 🙂

          • iksnilol

            Had to google what an SKK was. Seems like my kind of SKS.

            Eh, I’m sure I could make a Ruger shoot well. But it wouldn’t be really worth it unless you had some sentimental attachment to the thing or something. Though I wouldn’t mind the work, distracts me from bad thoughts. Better bet would be to just get a bolt action AK or VZ 58. Put in a good barrel on it, remove the remains of the gas system (to reduce weight) and practice. Though, I am a good with a bolt action. Don’t know how you are down under with the bolt actions.

            FAUX EDIT: Oooh, a lever action could work. Something like a Browning BLR with extended mag (I am sure a 7 round mag wouldn’t get in the way of the lever on the 308 version).

          • FarmerB

            Yes, fantastic guess – I ended up with a BLR, although the earlier models were hard to fit with extended mags.
            Down under, pretty much all you are allowed now is bolt, lever or pump action (unless it’s a shotgun). Which is why I’m no longer there.
            Anything that looks like AK/VZ would also be no go – no matter what the action.

          • iksnilol

            Hmm, regarding the BLR: Any sources on extended mags? I can’t seem to find any in Europe or the US. Something like a detachable 10 round mag in 308 would be nice and would increase my interest in the BLR (not really interesting with the 4 shot mag).

          • FarmerB

            I think the problem is that the magazine cannot really protrude too much below the receiver, or the lever will strike it when extended – I’ll go down a take a picture today/tomorrow to confirm.

          • iksnilol

            That’s what I was thinking too. But from the pictures I have seen the magazine is a bit forward of the trigger area. And if you don’t have a too long mag it should work.

            Though I would appreciate you checking it out considering I don’t have access to a BLR.

          • FarmerB

            Might be feasible to get 10 – I took two photos, one with the Browning mag dropped down about 2 cm and another with a SIG 5.56 20 round magazine there – which clears but I suspect .308 magazine wouldn’t bend quite as far forward – my .308 mags don’t fit in the mag well so I used the SIG one.

          • iksnilol

            Thanks for the photos, that’s highly useful. From what I see, the .223 version could work with the extended mags. For the 308, you’d probably have to cut the trigger guard. Sounds a bit risky.

            Again, thanks for the pictures, they were really helpfull.

          • Beaumont

            Yeah, I’m wanting one like that myself. I thought Michael Bane was onto something with his “tactical” BLR, but AFAIK, he hasn’t done any mag modifications to it.

          • uisconfruzed

            Ruger out of the box aren’t known as great shooters, I am impressed with the Gunsight 308.
            Several of mine are tack drivers!

          • FarmerB

            Yeah, but in my experience, their mini’s aren’t in the club. Some of the No 1V are known to be good (if variable). Actually, my Ruger No 1H rabbit gun (in 375 H&H) shoots pretty well for what it is.

          • uisconfruzed

            More like a rabbit soup gun 🙂

          • OldOldLawyerl

            My mini-30 from 1991 had a .308 bore…with remington ammo it shoots 1 moa, factory from the box….my mini-14 never would shoot less than about 4 inches….

          • iksnilol

            The irony.

  • WFDT

    “Wildcat Cartride”: worst Disney attraction EVER.

    • M.M.D.C.

      Try bringing a laser pointer – it’s MUCH more fun.

    • uisconfruzed

      haha, I missed it myself

  • rogue14

    Be interesting to see a comparison between this and the 7.62 x 40 WT. You reviewed that awhile back but never shot it.

  • Mike G

    From an engineer’s prospective, you really stand to lose nothing and gain everything by going up to a case that is capable of higher pressure with more volume. Cost-wise, it has to be fairly comparable.

    Another great thing about it is that you don’t run near the risk of chambering a kaboom.

    • What do you mean? Your bolt thrust goes way up and stresses out the locking surfaces a lot more.

      • Cal S.

        Maybe accidentally loading a .300blk in your .223 upper?

        • Yeah, that’s a problem…

          • Blake

            I’ll never understand how. A 300 BLK round won’t even chamber in a 5.56 system unless you add some serious elbow grease. On top of that, if you are careless enough to even get to that point you shouldn’t be shooting guns.

          • 277Volt

            From what I understand certain .30 caliber bullet profiles allow it to happen.

          • sauerquint

            It does happen and has happened.

          • It’s easier than that. I bought some .300 BLK ammo to test this in my 5.56 AR. Removed the firing pin. They don’t just go right in, but it’s certainly something that can happen if you chamber a round that isn’t crimped on a cannelure.

  • James

    What magazine would this require?

    • Sianmink

      6.5 Grendel looks like it can use standard 5.56 magazines, but it doesn’t. So you’ll need Grendel specific mags and make sure to mark them because they don’t look any different.

  • Westood

    I do like the idea of not being able to chamber that in a 556 rifle unlike the 300blk, little safer

  • Darkpr0

    It seems to be a pretty roundabout way of getting a modernized 7.62x39mm… But I do like x39 and improving it for modern pressures and feeding. I don’t know that I would like it more than Grendel, but taking old ammo and developing it…. I can get behind this.

  • Joe Ker

    Great. Let me know when it’s SAAMI spec’d.

  • DetroitMan

    If I have to move up to 6.5 Grendel parts, just give me the 6.5 Grendel. The Grendel can approach the power of the 6.5mm cartridges in use during WWII, which is far more power than any .30 caliber that can be loaded in the AR-15. Along with that it can reach out to long range, something a cartridge with .30-30 ballistics can’t do easily. If you’re going to spend the money for Grendel parts, you might was well get the most out of it.

  • tsh77769

    I keep waiting for someone to make a grendel specific mag (the way it stacks inside) and a
    grendel specific receiver in the same way taht LWRC has the 6.8 specific mags from magpul and the 6.8 specific receiver. I even wonder if the LWRC 6.8 receiver might work for a 6.5 specifica mag.

    • mig1nc

      I was thinking along similar lines.

      I wonder if you could take a Six8 reciever and magazine set and neck it up to .308 to get a kind of improved .300Blk

      • Jetman

        Actually, this seems like a solution in search of a problem. There are other wildcat combinations based on the Grendel 6.5mm that replace the CARTRIDGE brass and use the 6.5mm bullet. One I’ve recently noticed is a 6.8SPC II cartridge brass combined with the 6.5mm bullet – AR15Performance calls it the Six5 (like LWRC’s Six8) and it uses the .30 Remington brass from the 6.8, allowing mags, bolt, and a new Six5 barrel (about $200) to benefit from the ballistic coefficency of the Grendel (long range is its forte) as well as the second most popular AR round, the 6.8.

        Speaking of the 6.8, isn’t a Grendel with a .30 bullet sort of substandard in flight performance to a .277 caliber 6.8 round? Skinnier means longer performance. I think they ARX guys got this one backwards by using the cartridge (isn’t it an AK 7.62 x 39 cartridge modified?) and ditching the most effective part of the Grendel. 70,000 psi is nothing to sneeze at but you’re still pushing a boxcar of a bullet, and if I recall, the 30-06 and .270 faced off back in 1928-ish and the .270 kicked it’s a** in performance.

        So… ARX allows the .30 caliber bullet to be pushed to 30-06 velocity. A 6.8SPC II barrel allows better performance beyond 300 meters than the 5.56 NATO round, effectively 80% of a 7.62 NATO without the heavier bullet and heavier rifle. The 300 BLK allows the reuse of 5.56 brass, making a .30 bullet work in suppressed capacity,
        but you have to really holdover high to reach 250 meters and beyond, requiring some mental math that gets tricky.

        Now we have a competitor for a niche that I can’t figure out – another .30 in AR but with a larger cartridge requiring – this is important – a bolt face for either a 7.62 Soviet or Grendel, plus different magazines that are proprietary? Logistically I don’t see a fit.

        I’ll stick with a tried and true 6.8SPC II if I want better performance all around. Saudi and Jordanian SF just picked up 40k rifles and put about $120M into development of the LWRCI Six8, and ammo now sells for $0.60/cartridge online. Sounds like 6.8 hit the mainstream and the only thing holding it back from a full competition with the US Army is 500k rounds they tossed onto the RFP a few years back to only limit the field to 5.56 or 7.62.

        • mig1nc

          Good post. I think the UK also helped further it with the UCIW.

  • BrandonAKsALot

    I like the idea and I can see a lot of the logic. I just hate how this stuff have to get shoe-horned to fit the AR-15 pattern stuff. I know it won’t happen anytime soon/ever, but I’d love to see some minor alterations and improvements to the AR-15 receivers that would allow more progression. Ditch the overly long mag well and increase the force bearing areas in the upper and a slightly larger bolt. Obviously, you might as well redesign at that point, but it’s a fun thought. Also, the charging handle.

    • Dracon1201

      Don’t you mean short magwell? That hampers and shoehorns things more than anything else.

      Sidechargers>>> AR charging.

      • iksnilol

        Long magwell if you look at it vertically. Short if you look at it horizontally. Stoner was his namesake when he made that infernal contraption that he called a magwell.

      • BrandonAKsALot

        What iksnilol said. I meant vertical length, not the actual internal, front to back length. It’s just unnecessary how tall it is. I haven’t been all that impressed with the side chargers I’ve seen, personally, but I don’t pay a lot of attention to them.

        • Dracon1201

          Ah, I see.

          • BrandonAKsALot

            I suppose that’s probably it. It’s still an advanced design today in many respects and slotting the upper and threading in a handle is just kind of crude. I know there are a couple non-recip versions, but the cost is exorbitant. It would pretty much require a redesign. There will always to be drawbacks to any design though, so I guess it’s pick what’s important and in the grand scheme, the charging handle could be worse.

          • Dracon1201

            Agreed. I’m personally a fan for reciprocating charging handles, I feel the flexibility and simplicity of them makes for a really useful feature. Everybody has their own preferences.

  • 277Volt

    Quite similar performance to the .30 Remington AR isn’t it?

    • A bit less. The .30 RAR has a much wider case.

      • ostiariusalpha

        It was a real waste that Remington didn’t use that case volume for larger caliber subsonic bullets, it would’ve made for a sweet suppressor rig, instead of a wannabe .308 with reduced magazine capacity.

  • Christian Hedegaard-Schou

    So, a cartridge that uses a weaker bolt, gives up all the ballistic and precision advantages of its parent, loses the subsonic capability of .300BLK, requires proprietary mags…

    All so you can get a little more velocity and a little less precision?

    • Bobing

      This is what confused me about this article. It trades away many of the advantages of 6.5 Grendel (which is in itself a compromise round, since it has to be forced to fit in an AR magwell), simply for…easier reloading because of .30 cal pills?

      And I need a custom chamber for it and lord knows what sort of headspace gauges to check for safety?

    • iksnilol

      Why’d you lose the subsonic capability? Heavier bullet + fast burning powder (+ less of it) works in any cartridge. Weaker bolt? Sure, it is weaker if you use the 5.56 bolts that have been expanded. Using a proper 6.5 grendel bolt and it works fine. Same proprietary mags as regular 6.5 grendel. AKA no “need” for proprietary mags but they are nice.

      Also, why’d you get less precision? Quality components = quality ammo no matter the cartridge.

      • HSR47

        The critical dimensions of the barrel extension are the same either way, so making a bolt accept a larger rim diameter will ALWAYS structurally compromise the bolt.

        • iksnilol

          Well, one is just modified to be able to theoretically work. While the other was specifically made to work with the stuff.

          Sometimes, you can’t just bubba it (this coming from a person who doesn’t mind modifying guns with a hacksaw and a file).

          • HSR47

            A purpose-built bolt designed to allow a cartridge with a larger case head diameter to be shoehorned into the AR15 is likely to last longer than a 5.56 bolt that has been bubba’d. That shouldn’t exactly be surprising.

            Still, under ideal circumstances, the average 5.56 bolt should last longer than the average 6.5 bolt.

            That being said, that doesn’t always happen in practice, largely because most 5.56mm AR15-pattern rifles tend to be so heavily over-gassed.

        • ostiariusalpha

          Somebody should tell the Alexander Arms proof bolt all about how it’s structurally compromised, because it just keeps banging out 70,000 PSI test loads.

          • uisconfruzed

            That’s what my avatar is, my AA hard use bolt.

          • HSR47

            The plural of anecdote is not data.

          • ostiariusalpha

            Actually, yes it is. Anecdotes may not be the best, most rigorous data points, but they are used in statistical analysis studies.

          • HSR47

            Actually it isn’t.

            Even assuming that your claim is true, it is still a sample size of one. That’s not statistically significant.

          • ostiariusalpha

            Your words were that “The plural of anecdote is not data.” Now, most of the time plural implies more than one, more than one anecdote makes a data set in statistical science. The larger the data set (more numerous anecdotes) gives greater confidence for any hypothesis based on that data, with acknowledgement for the possibility of black swan data points. That said, proof tests are not anecdotes, they’re experimentally replicable events; you can set up a test rig with an AA Hard Use bolt and see for yourself. Moreover, you don’t always need more than one data point to draw reasonable conclusions; sinking a dozen Titanics isn’t required to say with no small measure of scientific confidence that the bulkhead design was inadequate.

          • HSR47

            What you offered was an anecdote: a short and amusing story without any real substance behind it. In this case, no substantive data regarding measurements taken of the alleged bolt or of the testing done to it. Also no mention of a sample size larger than one.

            You claim that a particular manufacturer has put a single test bolt through a rigorous testing procedure, but you provided no actual substantive evidence to back up that claim. If you have an actual article that lays out the testing methodology, then please cite it.

            Regarding the claim that you don’t always need more than one data point, your example is in the negative: To clarify this, the only case in which you only need one data point is when that data point proves that something doesn’t work. The classic anecdote to show this is cooking a potato in a microwave: cook the first one until it explodes, and then don’t leave subsequent potatoes in as long. Here, the explosion of the first potato serves as definitive evidence that you left it in too long. Still, it only gives you a definitive proof in one case: That you left that particular potato in the microwave too long. What it doesn’t do is tell you exactly how long you should have left it in the microwave, nor does it give you enough data to build a framework to judge the proper cooking time for any given potato in your particular microwave. Thus, with your example of the Titanic, it’s sinking proves that it’s design was flawed, but it doesn’t give substantive insight into how to perfect the concept of an “unsinkable” ship.

    • Marc

      Using the x39 case is not giving up “ballistic and precision advantages” whatsoever. Quite the opposite actually, see 6 mm PPC.

  • Anonymoose

    Whatever happened to the .30 HRT?

    • The parent 10mm iAi Magnum case is not always available from Starline.

  • No thanks. If you want a .30 cal AR, get a .300 Reaper. Case capacity of 42 h20 grains using a cut down .308 parent case. And I know most reloaders have a good idea how much cheaper .308 brass is compared to 6.5 Grendel brass is. The reaper also uses 6.5 grendel mags too at a 1 to 1 capacity.
    My 2¢.

    • iksnilol

      I thought 7.62×39 brass wasn’t that hard to get? And Grendel uses 7.62×39 brass (sure, you have to fireform it but it is available).

      • In the USA? No, its not hard to get. The issue is the cost increase. 6.5 Grendel brass is $0.70 per brass case. For Privi Partisan x39 it is $0.50 a case. BUT fireforming takes more time.

        Privi partizan .308 Win cost the same $0.50 as x39, and no fireforming is needed at all to make the Reaper case (also has 3 more grains of capacity than ARX).

        Because .308 is probably the second most popular round to .223 in the USA, you can pick it up on a gun range floor and have it for free.

  • Mikenz

    It’s a nice cartridge . Bascially a reinvention of the Czech 7.62x45mm . “plus ça change, plus c’est la même chose”

    • uisconfruzed

      Shouldn’t ‘the more things change, the more they stay the same’ be in Czech?

  • Whiskey 3 Precision Systems

    I like the cartridge and of course I’m a Wildcat fan and Kudos to the designer but since we developed the .300 Reaper ® cartridge and use the 308 parent case with a bolt we developed to hold it on the AR 15 platform with a bolt and barrel swap just as the .375 and 6.5 Reaper ® cartridges it made more sense and way outperforms this guys creation by far. When you can push 155 grain bullets magazine length at 2550+ fps that’s smoking hot and legit 300 Savage ballistics all day long from a 20 inch AR15. Put that on a bolt action with a 24 inch barrel and we’ve got a heck of a little lightweight bolt gun. Again Kudos to the developer……we at Whiskey 3 Precision Systems love to see new stuff and see the wildcat market coming alive these days

    • Somewhere at the great loading bench in the sky, the ghost of Frank C. Barnes is chuckling to himself. The .308×1.5″ Barnes was introduced back in 1961.

    • smartacus

      Hi John and Jeremy; i love the idea behind .375 Reaper. I would be very excited to see a .338 Reaper, just to have it out there as an option for someone looking for the potential of the 338 Federal with the AR-ability of the 338 Spectre.

      • Already did Already did it brother, just waiting for production to allow us to get it released. It will run the 160 grain Barnes TTSX at over 2800 fps.

  • Tom Currie

    If you want to shoot .308 diameter bullets out of an AR-style rifle, there is a MUCH simpler and FAR more robust solution.

    It is spelled AR-10

  • Goody

    Is that like a x39 improved?


    Geeeeeee Billy, I wish there was a rifle that shared 30-30 ballistics without breaking the bank……………… Oh wait

    • no

      I’m just a small step above complete idiot when it comes to reloading, wildcatting, etc., so pardon me for having to ask why that says 7.62×41?

  • Jess Johnson

    7.62×40 would have been a better comparision.

  • smartacus

    OMG this just begs the question, what about a 338 Grendel?

    • Got you covered with something way better. The .338 Reaper ® already done brother.

      • smartacus


  • Nathan, the title is spelled wrong, its “Cartridge”

  • El Duderino

    “Grabs gun mag next to toilet…printed in 2011…waxed ecstatic about the .30 AR Remington as the next big thing in ARs…” This cartridge is pretty much identical in performance to that failed cartridge.

    • ostiariusalpha

      Maybe, but it doesn’t have the .30 RAR proprietary receivers, extension, barrel nut, handguard, reduced magazine capacity, and especially doesn’t have Remington to deal with.

  • Matthew Groom

    So, you take a 7.62×39, neck it down and blow out the shoulder, and then neck it back up to .30? So….you…have…a…. .30 Soviet Improved? Uhhh…..

  • scaatylobo

    I am a fan of either the 300 Blk,or the 7.62 X 39.
    They are better in the M-4 / AR platform and very available.
    Wildcats are not my go to.

  • OldOldLawyer

    Not sure why this is an article…it does not even give one velocity comparision with the 300 blackout….those of us who jumped on the 300 aac bandwagon early did so because of the subsonic capability….shooting hogs, or deer, or coyotes at 100-150 yards with 200-220 grain slugs with little noise is no small feat……deer, coyotes, etc dont know where the sound comes from..hogs will often just stand and look around as their mates fall over….in those roles, there is zero reason for more case capacity to push a big bullet slowly……and you can play sniper without hearing protection….I love wildcats, but if the new round does not give say 15% more velocity then what is the use….and why not at least once velocity comparision?

    • ostiariusalpha

      The truth is .300 Blackout starts to lose velocity with bullets over 220gr, and you basically want to keep your muzzle velocity as near beneath transonic as possible. Otherwise, your wasting the potential inertia of the heavier bullets. If you’re fine with 220gr subsonic loads, .300 BO is going to fit your needs more than adequately. As for the velocity difference between this wildcat and the puny Blackout case, exact numbers comparing weight-for-weight would sure be nice, but simple logic says that a 50% jump in case capacity will substantially increase any projectile speed.

  • Jon Pitchford

    Or, you could just build a 7.62×39 upper and save a lot of money on ammo.

  • lowell houser

    Question, just how much trouble is everyone going to go to to reinvent 7.62×39?