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Building a Switch Barrel Mini-Mauser

Building a Switch Barrel Mini-Mauser
Brownells, a Good Gunsmith and a Little Imagination
By Joe D'Alessandro

 

 

Awhile back, I had the opportunity to work with a Remington RX15 rifle and the 30 Remington AR cartridge. AR rifle intended, and with 44 grains of capacity and an operating pressure of only 55,000 PSI, the little cartridge put up impressive velocity numbers and the combination was very accurate.

It seemed that if a suitable bolt action could be found, and the cartridge's operating pressure could be bumped to a level mode consistent with modern bolt action rifle cartridges, in the 62,000 PSI range, there was the potential for a heck of a lightweight deer and varmint rifle. The addition of a 7mm wildcat version of the Remington cartridge could yield a compact and even flatter shooting rifle (30 Remington AR left, 7mm 30 RG right).

The project was interesting enough for me to see if I could locate a bolt action that would be proportional to the stubby cartridges with enough capacity to feed them reliably. An action that might even lend itself to easy barrel swapping. The challenge associated with finding an action for these 30 AR based cartridges would be in locating one that would handle the round's large 0.501" body diameter and 0.492" rim. The 30 Remington AR has the 2.260" length of a .223 Remington, but most small bolt action rifles chambered for the .223 Remington are almost at maximum case diameter capacity when handling the .223 Remington's 0.377" diameter body and 0.378" rim

If a CZ Micro and Remington M799 had a race...

 

After a lot of really boring and tedious rifle disassembly and dimension checking... I have a very short span of attention and virtually no patience, the choices came down to the CZ 527 Micro action and the Remington Model 799 Mini Mauser as candidates.

My personal preference was the CZ Model 527. The action's envelope is downright tiny, but the bolt, lugs and extractor are larger than those found on other similar size actions, (near right.) The CZ would provide greater strength to handle increased pressure and bolt thrust. The stronger extractor could pull spent cases that were exposed to dirt and debris as a result of me tripping and rolling down a hill while hunting. It happens.

Perhaps of peer importance, the CZ 527's bolt support extends beyond the rear action ring, which makes for a really slick feel when cycled. Illustrated below, a standard Mini-Mauser action's bolt wags like a dog's tail when fully open while the CZ bolt feels like it is traveling on roller bearings. Functionally it may not matter, but it's a little ugly as firearm operations go, while the CZ 527 has the feel of precision machinery, which is always nice in a quality firearm.
All of this of course means the Remington Model 799 Mini-Mauser action was selected... Really. My infatuation with the CZ 527 action came to an abrupt halt when I realized it wasn't going to be able to feed the intended rounds as a repeater. The CZ 527 action's feed port, when chambered for the .223 Remington, could not be ground, milled or even dynamited to a large enough dimension to pass the stubby 30 Remington AR based rounds. A version of the CZ 527 chambered for the 7.62x39 Russian with a 0.447" body and rim had possibilities, but the gun is designed with a detachable magazine which suggested much more modification and fabrication than I was willing to undertake. The action selected needed to have bottom metal and either a blind or hinged floorplate magazine.

The Remington Model 799, above - lower action, is a traditional Mini-Mauser, but this particular one was chambered for the .22-250 Remington. Handling a round with 0.470"/0.473" case body/rim diameters, it came very close to working as is, with the stock feed port and magazine box and follower. The Mini Mauser has sufficient bolt travel to cycle a longer 2.350" round.
Finalizing cartridges...

The 30 Remington AR was perfect as the 0.308" cartridge for the gun. Setting aside my box of crayons, the 7mm version of the cartridges was worked up as illustrated. RCBS.Load is one of the simplest, inexpensive CAD tools on the market. Drawings can be worked in real time so that the effects of changes on other dimensions and case capacity are immediate.

The result of the effort was a stubby case that would function in any 2.260" length action that could handle its diameter. With 45 grains of water capacity, it would have enough steam to power up through 140 grain 0.284" bullets and the modest powder capacity suggested it could do so in concert with a relatively short barrel. The case would be formed in two steps. The first step would be cold forming with a sizer die we would make in the shop, which would reduce the case neck to accept a 7mm bullet. The second step would be to fire form the case to push the shoulder forward and to achieve the revised shoulder angle.

The drawing was sent to Pacific Tool & Gauge for review and tooling fabrication. Dave Kiff was kind enough to point out some of the finer points of trigonometry and I believe he may have applauded my knack for creating new and exciting angles. Maybe not. After some finesse work and related drawing revisions, the project had two reamers, finish and under size... sizer, and a headspace gauge. Neato.

Pictured right, and getting a little ahead of myself, are examples of the two cases for the rifle, the 30 Remington AR, left and the 7mm-30RG right. The 7mm version's major shoulder diameter is shifted forward 0.199" by increasing the shoulder angle from 25° to 37° which moves the shoulder's 0.400" headspace diameter approximately 0.028" farther forward. The minor shoulder diameter is relocated approximately 0.022" farther forward than the 30 Remington AR, the result of reducing neck length to 0.284" in an attempt at adequate bullet retention and maximized case capacity.

I'm not an expert at cartridge design. A statement which suggests I also have a knack for overstating the obvious. Subsequently, having access to good tech support from services and suppliers was essential to a successful project outcome. The good support resources, like those found at Pacific Tool & Gauge and Brownells, can turn a question into a positive learning experience and it is done without stepping all over original ideas and without embarrassing the person asking the questions. This is one of the dynamics that makes firearm projects so much fun.
I wasn't goofing off... I promise
While the reamers and headspace gauge were in process, I contacted Brownells to get a recommendation for a replacement synthetic stock that would help reduce the weight of the Remington rifle. I also asked for some insight into barrel types and profiles that would approximate the original barrel on the Remington Model 799 so that only minor finishing would be required fit the stock with altering the barrel channel or having to glass bed to fill voids.
Brownells offers a Bell & Carlson's Carbelite synthetic stock for the Mini Mauser, Brownells' # 137-000-004, which appears above under the gun's original Remington laminated stock. The Carbelite construction is a hand-layered composition of Kevlar, fiberglass, and graphite that is high strength, light weight and unaffected by changes in weather and humidity. The new stock installation took less than sixty minutes with only very minor fitting required with the use of common hand tools. The end result was the Mauser looked and handled much better and shed a full pound, for a total firearm weight of 5¾ lbs.
The 6 groove, 4140 alloy, match grade barrel blanks ordered from Brownells are both Douglas brand; 7mm 9" twist, 0.308" 10" twist, both #2 contour.# 237-284-092 and #237-308-102 respectively. Both barrels began life as 25½" long and 0.025" - 0.50" larger in diameter than their intended finished diameters. After working through some capacity to bore volume and length assessments, I got a headache, put down the calculator and decided 21" in length would be a good starting point. Who says there is no such thing as good science anymore?

If you need surgery, better find a surgeon...

At this point, I had reached my limit of project competency; asking questions, making drawings, buying things and bolting stuff on. Yes, I do make parts on the lathe and the milling machine, but my approach to machine shop tasks would make a skilled machinist cry. The last barrel I attempted to fit, ended up threaded from end to end. Hey it's not easy to stop a lathe that is moving along purposefully with a mind of its own. So with rifle, barrels and reamers on hand and in a box, I called Dennis Olson, a Plains Montana gunsmith. I picked Dennis because I think he exemplifies what a gunsmith should be; highly skilled including part fabrication, lots of firearm knowledgeable, flexible in approach, and there is never a surprise when the bill arrives. No, I don't get a discount for mentioning his name. In fact, he never remembers who I am, which may be more intentional than I'd like to believe. Good guy.

I gave Dennis what information I had; objectives, pertinent dimensions, part descriptions, etc. He asked a few questions for clarification, then said, "Sure. I can take care of that and also make up a small tool to support the action when you are changing barrels". No scoffing, no laughing and no move away from the things that were important to me. A few weeks later, the work was completed, right in line with his schedule commitments. The rifle looked good, shot straight and worked reliably. All of the work - from barrel fitting and chambering, to magazine and feed rework, to refinishing, tools and caliber stamping came in at about the same cost as a budget priced bolt action rifle. Value.

The finished product, bottom, pictured with the second barrel and removal tool. Because the stock must be removed to make the change, a barrel change probably shouldn't be attempted in the field, but it can certainly be done at a campsite or at the rifle range. To the completed rifle, I added a set of Weaver style Warne steel bases, Brownells #947-000-183. This is a listing for a Charles Daly Mini Mauser application, same gun. The Model 799 was Remington contracted Eastern European production and not based upon any of Remington's proprietary products. Medium height Warne rings completed the mounting system, giving just enough clearance for the bolt handle to slip under the scope's eyepiece, but not so high as to have the objective bell floating a foot off the barrel.
Dollar a foot prototype reloading dies...

Two things I have learned while experimenting with modified cartridges; things change after a reasonable wring out period and custom dies are expensive and come with a long lead time... two or three years. No, not really, but twelve to fourteen weeks is probably the routine lead time and the cost is often several hundred dollars. Subsequently, I have taken to ordering a second reamer, along with a chambering reamer, that is appropriately undersized for use in producing a sizer die. Now I make prototype die sets out of 7/8"x14 threaded steel rod stock priced at 78 cents per foot. Remember, for those of us that can't thread on a lathe to save their life, this is a good place to begin. The dies will not be hardened, but they will last for hundreds of rounds and making changes and new pieces isn't cause to float a second mortgage.

The sizer was reamed and checked with the go/no go gauge for approximation of correct depth. It is easier to cut them more shallow and adjust when setting up the reloading press than to cut too deep and have to keep facing the die until it is ½" long. The die was kept to about the same length as a production set to ensure the press handle will be at maximum leverage position when the case is being formed.

The decapping rod and expander were purchased as a 7mm piece from Redding. The back of the fabricated die was tapped with ½"x20 threads to accommodate the part. The two smaller dies are used for neck forming. I made several of varying diameter to refine neck sizing and neck tension. The seater die chores were handled by the seater die from a Hornady New Dimension die set made for the 7mm WSM. Their seater dies are not exactly cartridge specific, but rather caliber specific and the large diameter WSM seater had enough clearance, and was of short enough, to seat bullets in the 7mm 30 RG. Not exactly art, but I'd be lying if I said this wasn't one of the most satisfying parts of the project. I think, next time, I will switch to the RCBS method of anchoring the decapping rod because it allows for a little bit of float inside the die for the decapping pin to center. With the rigid Redding type, the hole through die must be dead on or the decapping pin will be out of alignment. For further elaboration, ask to see my collection of threaded paperweights and door stops.

Results...
Without the distractions of explosions or shrapnel, the 7mm-30 RG chronographed 2850 fps with 120 grain flat based bullets and just about 2750 with a 140 grain bullets. Switching the gun over to 30 Remington AR, 125 grain bullets clocked 2830 fps and 150 grain bullets at just about 2700. Again, the goal was to build a small, compact and lightweight rifle suitable for northeastern hills hunting and I believe that was achieved. At only 6 lbs with scope, there are still the bonuses of next to no recoil and only modest report.
Can the switch barrel Mini Mauser and 7mm-30RG cartridge do things other combinations can't? Probably not, but then that applies to 80% of the available cartridges. But the satisfaction of thumbing through the Brownells catalogue, selecting optimal parts and working through a collaborative project that includes a personal cartridge? This amounts to a whole lot of enjoyment and a sense of satisfaction that no factory gun could ever provide.