Wildcat Cartridges

In Blog by Rich2 Comments

What are wildcat cartridges? In simple terms, it’s a way of creating a new cartridge or caliber, by tweaking some aspect of an existing caliber. An easy example are Ackley Improved calibers. The idea is by making a steeper shoulder angle, you can have more straight body in the case, and more capacity. So an Ackley Improved 243 Winchester can be formed by firing a 243 Winchester case in a barrel with the Ackley Improved chamber. As pressure in the case rises, and the brass heats up, it will expand to fill empty space. You may have heard of ‘blowing the shoulder out’ which is what you would be doing. Wildcats can be created by tweaking many aspects of the case. Some are worth the trouble, some aren’t, and we’re going to discuss some of the advantages and pitfalls!

Wildcat Cartridges

Another thing that can be changed is the neck diameter. Everybody these days has heard about the 6.5 Creedmoor. It’s a very popular long range cartridge based on the 308 Winchester cartridge. The 6mm Creedmoor has identical case dimensions, only it fires .243/6mm caliber projectiles, instead of the .264/6.5mm projectiles used on it’s big brother. The advantages should be readily apparent, more muzzle velocity, less recoil, even some extra case capacity since the tail of the bullet takes up less space in the case. So you can change shoulder angles, and neck diameters, body tapers, etc. and create wildcat cartridges…so at what point is a cartridge no longer a wildcat?

What Makes a Wildcat?

Essentially the difference between official cartridges and wildcat cartridges is industry support and a set of written specifications. There are two larger organizations you may or may not have heard of depending how much reading you’ve done. One organization is SAAMI (Sporting Arms and Ammunition Manufacturers’ Institute) and CIP (Commission internationale permanente pour l’épreuve des armes à feu portatives) which translates to something like the Permanent International Commission for the Proof of Small Arms. Essentially, these are the organizations that sit down and decide how long a 308 Winchester case is, what shoulder angle it has, what neck diameter, etc. The idea is consistency, by spelling out the specifications for a cartridge, you ensure that it’s being manufactured consistently regardless of who’s loading the ammunition.

The next logical question most people ask is: How much does SAAMI or CIP ‘acknowledgement’ of a caliber really matter? It doesn’t really matter at all, depending on how much work you’re willing to do on the cases yourself. Official cartridge support is really all about factory ammunition and consistency. Factory ammunition needs to be manufactured to a safe standard, so there aren’t dangerous pressures and issues when people use the same ammunition in different firearms of the same caliber. However, hand loaders will almost always custom tailor their ammunition so that it’s “tuned” as perfectly as possible to an individual rifle. Most hand loaded ammunition, even in common calibers with official support, are not loaded to official caliber specifications.

Hand Loaded Ammunition

I have a 308 Winchester rifle in my safe, and I’ll tell you right now, I load my 308 ammunition really long. In fact, as a matter of common practice, I typically load the projectiles as long as I can. This usually means the Cartridge Overall Length or COAL, is longer than specification. There are a number of reasons that this makes sense. For one, it’s safer. If ammunition is custom loaded for my rifle, it’s unlikely to even chamber in another person’s rifle even if they both share the same caliber. Another advantage is case capacity and pressure. By loading the projectile further out of the case, this increases space within the case for powder.

Generally, you can expect lower pressure for the same speed when the cartridges are loaded long. Take care in doing so, however, loading long is one thing…if the bullet contacts the rifling, that’s another. If the bullet contacts the rifling, pressure rises quickly and dramatically, so make sure you’ve read a few manuals and you know what you’re doing before you start getting creative. I recommend you load some ammunition to specification for a while till you get the hang of things and you have purchased the tools that will allow you to push the limits a bit.


Two 6.5×47 Lapua cases, one left as original, the other necked down to 6×47 Lapua

The reason I’m talking about any of this is to make a point. If you hand load ammunition, it’s already likely out of spec to begin with. So why fret over the idea of a caliber that doesn’t have official support, yet? Unless you only purchase and shoot factory ammunition, official cartridge support is irrelevant. You’re going to be doing the work anyway, the only question you have to ask yourself is: Am I willing to do the extra work required to “form” my own cases for the wildcat cartridge I’ve chosen.


6.5×47 Lapua on the left, with a 123gr Lapua Scenar loaded, and on the right is a 6×47 Lapua with a 105gr Berger Hybrid, you can see the cases look identical. No annealing marks on the 6x47L case because it was stainless tumbled after its first firing

Disadvantages to Wildcat Cartridges

The downsides to wildcat cartridges are generally twofold, workload in creating the brass, and lack of information regarding loads. Since there’s no official cartridge support for wildcat cartridges, you usually can’t open a reloading manual and find starting and maximum powder charges for different powders. That isn’t always the case, though, and as a caliber gets more popular, it will start popping up in the manuals. Luckily, we live in the internet age, and somebody out there has likely tried it before you and posted their findings. That doesn’t mean you can take it for gospel. Just like any other facet of hand loading, you have to start low and work up slowly in order to be safe. Don’t just take what some guy on the Internet says as the word of the guy upstairs!

The bigger disadvantage, in my mind, is the workload required to actually form the cases. Since most wildcat cartridges require altering an existing case to the new chamber dimensions, it adds steps to your reloading process. A common modification when dealing with wildcat cartridges is an altered shoulder angle. Sometimes all you need to do is run the brass through a reloading die and fire it with a reduced load to “blow out” the shoulder. This is often referred to as “fire forming”  and it means the pressure within the case pushes the brass outward against the case wall of the chamber to “fire form” the new case. Sometimes the opposite is true, and the die will do most of the work for you.

The downside to fire forming, in my mind, is the wear you put on the barrel and the materials you burn up (primers, powder, bullets) in forming the cases. Now that’s not to say it’s all bad. In fact, I’ve heard several wildcat guys say their fireforming loads are every bit as accurate as the fully formed loads. I believe the muzzle velocity is probably a touch lower, but again, this is a point of contention for me. I don’t want to be burning barrel, powder, primer, etc. for a shot that’s somehow lower in performance than it should be. It’s one of the reasons I went with the 6×47 Lapua when I opted for a wildcat chambering for my match gun. All I have to do is “neck down” the brass from 6.5×47 Lapua cases, in one step, and trim the cases and I’ve got formed 6×47 Lapua brass. No fireforming required!

Wrapping Up

Wildcats are not a beginner’s toy. This is something you shouldn’t really embark upon before you’ve got some good experience hand loading, sizing, and reloading your own ammunition. Once you’ve got a solid grasp of how that all works, you can start looking into wildcat cartridges. Wildcat cartridges can offer a lot of benefits. In the case of the 6×47 Lapua you can get performance that rivals the mighty 243 Winchester and the barrels tend to last into the upper teens and low 2000’s before they start losing speed and accuracy.

Other wildcat cartridges have similar advantages, my advice, is read…read, and read some more! Learn everything there is to learn about the wildcat you’re considering. What brass is needed, how much brass prep needs to be done, and more importantly, what the advantages over traditional chamberings are. Then decide if it’s a worthwhile undertaking for you! As always guys, we love questions, so if you have one or something else to add, please do so below!

Owner and Proprietor of AccuracyTech, LLC. Rich is a Firearms Enthusiast, Precision Rifle Competitor, and Writer. He is committed to bringing readers quality reviews and articles related to the Precision Shooting Sports. If you have any questions for him, please use the contact form on the site.


  1. I have heard about the 6.5 Creedmoor being developed from the .308 many times, by many that should know better. Not picking on you. Here are the facts. “The 6.5mm Creedmoor, also called the 6.5 Creedmoor or 6.5 CM for short, is a centerfire rifle cartridge introduced by Hornady in 2007 as a modification of the .30 TC, which was based on the .300 Savage. It was designed specifically for rifle target shooting, although it is also achieving success in game hunting.”

    1. Author

      I don’t know if quoting Wikipedia and presenting it as a fact is wise. While a good source of information it’s only as good as those who write it. Just food for thought.

      If you click the link for the .30 TC in the same Wikipedia article you got your quote from…we find this:

      “The .30 TC is a cartridge developed by Thompson Center Arms.[1] It was released for sale in 2007. It was initially offered in the Icon series of bolt-action rifles, which were released at the same time. It was an attempt by Thompson Center to make a 308 Winchester length round with 30-06 Springfield performance.”

      So I stand by what’s written, the 6.5 Creedmoor has a 308 Winchester basis. I never said it was originally formed from 308. The 30 TC had some 308 influence so why is what I wrote such a leap?

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