.300 Karcher Wildcat
The .300 Karcher is a project that has been decades in the making. Essentially a .30-6.5 RPM, it results in .300 Win Mag performance in any modern .30-06 rifle simply by reaming a new chamber.
At it’s core, the .300 Karcher (KAR) has been an idea of mine for a long while, and with the advent of Weatherby’s RPM cartridges, I now had a starting point as far as developing it. The 6.5 and .338 RPM are based on a lenghtened .284 Winchester cartridge, designed by Weatherby to maximize performance in their 6-lug MK V action. The .284 Win and RPM cases have rebated rims, measuring .473” and using a bolt face the same as any .30-06 or any of the derivatives based on it, yet the case body measures at .500” which results in the case capacity being greater than a typical “standard” cartridge. At this stage, it is only a wildcat, but in my opinion, would be one of the simplest and cheapest ways to get into wildcatting.
All one needs is to ream a chamber in any modern .30-06, and the rifle is ready to go. Manson Reamers is who made the reamer I used for this project. The .300 KAR feeds out of the original magazine with no issues, the bolt face does not need to be modified. As far as loading the cartridges, all one needs is a collet style sizing die with the appropriate collet for a .30 caliber, and a .30 caliber decapping pin that has been slightly modified to reduce the taper so as not to crush the cases while necking up. A universal bullet seating die will need to be used to seat the bullets, but there is no need to invest in expensive custom loading dies if budget needs to be considered.
With the .300 KAR, while the case is dimensionally a .30-6.5 RPM, there is one major difference in my wildcat; the .300 KAR uses the same freebore as the .300 Weatherby Magnum. So far, in the couple months I’ve been playing with load development, I’m seeing consistently higher velocities than what Hodgdon reloading lists for their .300 Winchester load data. While they don’t list all of the exact bullets I used, here are a few examples of the same weight bullets and the same powder (maximum charge weight listed, 24” barrel):
168 GR. Hornady BTHP:
Hodgdon H4350 70.0 gr 3,034 ft/sec
180 GR. Nosler E-TIP:
Hodgdon H4350 66.8 gr 2,916 ft/sec
200 GR. Nosler AB:
Hodgdon H4350 64.4 gr 2,723 ft/sec
For comparison, here are my recorded results using the same weight bullets and the same powder charges during my ladder test (24” barrel):
168 GR. Sierra Match King:
Hodgdon H4350 70 gr 3206 ft/sec (+172 ft/sec)
180 GR. Hornady Interlock SP:
Hodgdon H4350 66.5 gr 2982 ft/sec (+66 ft/sec)
200 GR. Nosler AB:
Hodgdon H4350 64.5 gr 2877 ft/sec (+154 ft/sec)
While the 168 and 180 grain bullets are different, the 200 grain Nosler Accubond between both cartridges with the same bullet and the same powder charge (within 0.1 grain) and EVERY bullet fired resulted in consistently higher velocities in the .300 KAR over the .300 WIN, and there aren’t many who would describe the Winchester cartridge as underpowered. It’s been the magnum .30 caliber that all other .30 calibers are compared against since it was developed in 1963. By simply using .300 WIN load data, the .300 KAR handily surpasses the older cartridge. Of course, both cartridges can be pushed a fair bit, but the .300 KAR seems to consistently be a step ahead. I think this will be the case for all but the heaviest .300 Winchester loads, and due to sheer case volume (appx 83gr vs 94gr) I do think the Winchester case allows for more potential energy. That being said, I haven’t had an opportunity to really push the .300 Karcher until hitting pressure signs. None of my loads so far have resulted in sticky bolt lift, flattened or cratered primers, or anything else of the sort with the exception of one of the Sierra 168 MK loads, which jumped up 200 ft/second with a half grain increase. There were no other signs.
Now, consider the fact that there is no belt to contend with when reloading, it can easily be chambered and fed in any modern .30-06 simply by reaming the chamber, and (subjectively) isn’t punishing to the shooter. The case dimensions aren’t as wide as the WSM cartridges, which means less lost ammo capacity, and the longer case feeds smoothly into the chamber. The .300 KAR also doesn’t have a short case neck like the Winchester does. Of course, every rifle is different, and care should be taken when reloading, and what is safe in my rifle may not be safe in yours, so work up to max loads when rolling your own. I take zero responsibility for someone blowing up their rifle if they decide to play with this cartridge.
As far as anyone asking “why”, consider that the process of converting a rifle is the same as any other .30-06 based wildcat (Ackley Improved, Serengeti, .30 Gibbs, .300 Sherman). Ream the chamber, form the brass, and work up loads until you find something you like. For the .300 Karcher, the only real downside is brass cost and availability. Outside of that, the performance exceeds the Gibbs and Sherman cartridges without pushing towards extreme pressures or ridiculously long barrels, and forming the brass is much easier to do and can be done in a single step. I’m confident that anything the Gibbs or Sherman can do, this will do it better and easier. I think the same can almost be said of the .300 Winchester and .300 WSM, with the exception of the WSM being able to be used in a short action, whereas the .300 KAR requires a long action.
As far as rifles to use for this conversion, I’d say that just about any modern long action can be used, though the MK V Weatherby seems to be on the short side. In fact, I believe that that action introduces the same issue to the RPM case as do typical short actions and the original .284 Winchester cartridge. Long/heavy for caliber bullets must be seated deeply into the case in order to properly feed. I have chosen a Winchester XPR for a variety of reasons, and while not perfect, it is fantastic testbed for the wildcat. Though I do recognize the virtues of bullets with high ballistic coefficients, I’m also not one to choose or not choose a bullet solely on it’s BC. I’m a hunter, not a match shooter, and my interest lies in bullets with appropriate sectional density for the class of game I’m after. With that said, there is a fair bit of overlap and correlation between the two, and a high BC bullet tends to penetrate better, all other things equal. I started this project with the intent to use a 200 grain bonded bullet, for North American game up to elk, and for African plains game up to anything that legally requires a .375 caliber. I have my doubts that a MK V action would be sufficiently long enough to use that bullet and still get the results I’ve gotten due to bullet seating depth. I’ve had discussions with others regarding this, and I’m not convinced that seating a bullet long in the case and maximizing case capacity and powder fill isn’t better than seating deep and raising pressure that way. The numbers I was shown to make that argument were in a ballistic calculator, but I think to really convince me I need to see real-world testing. Regardless, the best part about this cartridge is that you can cut a new chamber in something most people already own, which is definitely going to save a few bucks when getting into a wildcat (no new/custom barrel needed).
In conclusion, I think this wildcat fills a very distinct but important niche. Not everyone wants to use a short action rifle, and not everyone needs something as powerful (and hard kicking) as a .300 Weatherby or a .300 PRC, and not everyone wants to jump on the WSM bandwagon. I’m proud to have finally built this as it’s been so long in the making, and as far as I know, while I’m not the first to have necked up the 6.5 RPM to a .30 caliber, nobody else has cut in a Weatherby style freebore.
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.300 Karcher - a 6.5 Weatherby RPM Wildcat
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