Rocky’s Little Rocket

The Vintage .240 Gibbs Wildcat Still Rates As A Top-Tier 6mm Screamer.

There’s an ego lift in sticking your name on a newborn cartridge. Why? Wildcatting is to shooting what race car driving is to automobiles. And although some wildcat kittens are as practical as carrying water in a sieve, some of our greatest cartridges were birthed from existing parents.

The summit of a wildcatter’s dream is, of course, when his creation leaves the handloading bench for the ammunition factory, such as was the case with the .22-250 Remington.

Founding Fathers

Parker Otto Ackley, known as P.O. to the shooting world, is the Crowned King of Wildcatters—not so much on the basis of prominent cartridges in today’s lineup, but for sheer numbers. Apparently, P.O. couldn’t stand the sight of a factory cartridge. So he necked it up or down, blew out a tapered configuration to straight wall, even truncated cases to create new designs.

When asked if his long career of turning one round into another was worth it, P.O. said, “Save your money.” Well, that’s how the tale goes anyway.

I once had the privilege of regular phone conversations with the great Roy Weatherby. We talked of cabbages and kings, the Cape buffalo of Africa—and cartridges.

Roy’s creations are still amazing, particularly when you consider the vintage of some of them. His favorite, the .257 Weatherby, was developed in 1944 and remains as hot a .25 as anyone could ask for.

And yes, the story of a Cape buffalo dropped with one shot from Roy’s .257 is true. He was returning to camp from a plains-game outing. His PH, backing Roy with a fat-bullet double, said, “Shoot.” Roy did. The buffalo dropped. But for dangerous game, Weatherby went big bore—consider his awesome .460.


The blown-out .240 Gibbs (inset, right) and its .30-06 parent (inset).
Rifle is a laminate-stock McGowen built on an Antonio Zoli action and
topped with a Swarovski 2-12X.

Rocky’s Less-Traveled Road

Then there’s Rocky Gibbs (1916-1973). His goal was different from other wildcatters. He insisted on straining every molecule of potency from the .30-06 Springfield case, which meant employing a reamer that cut a chamber with straight walls and a short neck. The result was a “blown-out case.” Although there is minor merit in sharp shoulders and straight walls, the real thrust behind Rocky’s creations is about a 10-grain increase in powder capacity. And more powder equals greater velocity equals more energy—until the law of diminishing returns sets in.
When I get that terrible urge to play with a cartridge I can’t buy off the shelf, especially when I’m living in Musina, South Africa, I try to push the idea so far back in my mind that it withers and dies.

This tactic usually works. For years I’ve harbored a desire for (but never fell prey to) the .226 Barnes QT. I craved that blown-out, sharp-shouldered case firing a 125-grain .226-inch diameter bullet at 2,700 feet per second. “QT” means “Quick Twist” (1:5.5-inches needed) to stabilize that long bullet on its axis, so it won’t tumble like a wobbly football.

The .226 QT is accompanied by the even more alluring 6.5mm Barnes QT with a 200-grain bullet at 2,700 fps (1:5.5 twist.) Stories of this 6.5 penetrating east to west on a northbound moose tickled my fancy. But I got over it.

However, the wildcat that really intrigued me was the .240 Gibbs, and this time I succumbed. I simply had to have that unmercifully blown-out straight walled .30-06 short-necked case securing a 6mm projectile.

I had both a goal and a purpose for the cartridge. The goal was higher velocity for a .243-inch bullet over any factory cartridge. The purpose was twofold: open country antelope and whitetails on the river, both opportunities existing near my Wyoming home.

My longest shot so far on antelope is (by rangefinder) 360 yards. For me, that’s far. I know hunters are hitting at double that range. But I try to get near regardless of a rifle’s long-range ability (it’s called stalking). However, when conditions deny a closer poke, I planned to apply the .240.

The whitetails I pursue are the smallest of the five Wyoming subgroups and (in my opinion) the greatest challenge. I secure a stand above the river, patiently searching far below with my Swarovski 12X’s, waiting for a buck to walk into a clear spot in the thicket.


Attempting to create .240 Gibbs cases from .25-06
brass without annealing destroyed the brass.

A Potential Trail of Tears

I did not plunge blindly forward with the .240 project. I knew what I was getting into. The words of Scottish poet Robert Burns burned in my ears:

“The best laid schemes o’ mice an’ men Gang aft agley, An’ lea’e us nought but grief an’ pain, For promis’d joy.”

My “Rocky’s Little Rocket” project was full of potential schemes that “gang aft agley” leaving naught but problems from forming cases to ballistics initially not vastly superior to ammunition sent by the carloads to dealer’s shelves. Would the .240 Gibbs be another mad dash into the wildcat wilderness, where hundreds have proved little more than misty dreams? Seemed so at first. Building a load that operated at safe pressures with reliable case extraction can be fraught with disappointment and frustration.


Caution! These blown primer pockets (left) from overpressure
were caused by the wrong powder. So were these examples of
case head separation (right).

The Right Rifle

There is no free lunch in achieving high velocity. Yes, there are examples of great balance between velocity and pressure. Hodgdon’s 21st century LeverEvolution propellant launches a 160-grain bullet at 2,400 fps in the common .30-30 with acceptable pressures, and is safe even in older rifles.

More often, however, pushing that pill to super speeds requires plenty of energy to force the missile screaming down the bore. That pressure, in turn, demands a rifle capable of handling it, especially during the experimental phase, when experimenting with various bullets and powders with no bonafide loading manual to consult.

Research brought me to McGowen Custom Barrels of Kalispell, Montana. They provided me with one of their interesting 26-inch dodecagon barrels, paired with an Antonio Zoli action, known for its strength. Geometrically, the barrel is a polygon with 12 sides and 12 angles—very stiff. I wanted a full service stock—cosmetics of no value—and got it with a tough laminate.

I mounted a Swarovski 2X-12X scope with lighted reticle to take full advantage of the rifle and cartridge. I’d have supreme scope definition at 12X. The wide 2X field of view would serve if I jumped a buck while hiking to my vantage point high on the riverbank.

The stoutness of the rifle action proved valuable because loads that “should have been OK” proved too hot with totally unacceptable case extraction. Rocky Gibbs had to have known that the smallest caliber for his vastly blown-out cases could perform only under high pressure.


This is how to prepare annealed .25-06 cases with false shoulder
and 70-grain bullets seated out far enough to touch the rifling lands.

Rocky Gibbs

A bit of background on Rocky Gibbs may explain his burning desire to build his own line of cartridges. He was born Mannolis Aamoen Gibbs. He felt that this handle was a definite roadblock to making a mark in the world. The story goes that he and his mother—evacuees of the Texas Dust Bowl—traveled to California, not in a broken down old flivver as in John Steinbeck’s Grapes of Wrath, but by train. On the way, the boy asked his mother what that huge range of mountains was called. She replied, “The Rockies.” Be that a tale told at the cracker-barrel or fact, Mannolis Aamoen became Rocky.

We are told that hunting led to a love of shooting. His biography reveals the loss of sight in his right eye from typhoid fever that turned him into a lefty, but that apparently did nothing to deter his desire or ability.

A .270 Winchester came his way. Intrigued by the idea of increasing its performance, Rocky had it rechambered to .270 Ackley Improved, which simply required firing the standard .270 in the new chamber. Brass being so malleable, the fired cartridge took the new form of the chamber, sort of like pouring Jell-O into a mold. The “improved” .270 did not improve enough in Rocky’s assessment. Of course, the straighter walls increased powder capacity along with a mild influence on better handling of bolt thrust. But if the standard .270 could achieve a new shape by firing in an “enlarged” chamber, surely more could be done to further improve the cartridge.

So Rocky did more than fire-form a case into a new shape by simply blowing it out with straighter walls. His idea was to thrust the shoulder well forward—as far as possible—with deference only to bullet retention. The result was an absolutely new cartridge.

Even the .30 Gibbs—which is the closet offspring to the .30-06—is an entirely new round, requiring case forming and careful loading. The .240 is even more persnickety. That small bore allows very little volume for pressure dispersion. Therefore, as with the .17 Remington and other wee bores, a trickle more powder can result in significantly higher pressure.


The Zoli action is very strong and the Swarovski 2X-12X—featuring a
battery-operated illuminated reticle (inset) allows you to take
full advantage of the .240 Gibbs’ potential.

Problems and Dreams Deferred

Case forming was no walk in the park. Split necks and cracked cases persisted until I realized that H4831 and similar powders in the .240 Gibbs was like a charge of pistol powder in a .300 Magnum.

Finally 60-grains H1000 in a .25-06 case necked down just enough to create a false shoulder with 70-grain bullet seated well out provided 50 cases out of 50. But only by annealing the brass. I also discovered “factory brass” at Midway USA Company. These fine cases are properly stamped .240 Gibbs. If you have a 50-dollar bill in your pocket, kiss it goodbye for 20 of them. But the McGowen rifle was never intended for multi-shot varminting, so case cost was a non-issue. Now I use Midway .240 cases for serious hunting and use case of formed .25-06 brass for anything else.

Tale of the Tape

My ballistic wish list for the .240 Gibbs included a 90-grain bullet at 3,800 feet per second, an 85-grain bullet at 3,850 and an 80-grain bullet at 3,900. My “hoped for” loads were preceded by numerous failures with a list of powders gleaned from a variety of literature. Properly slow burning fuels in some cartridges were too fast in the .240 Gibbs—blown primers, split necks, cracked cases, lousy extraction.

But these gremlins preceded partial victory. Eventually I realized that no charge short of 100 percent load density would do the job. Finally, a 66.5-grain charge of Reloder 25 pushed an 80-grain bullet at 3,900 fps with slick extraction, insignificant head expansion, and tight-after-shooting primer pockets.

Even more pleasing was an 85-grain bullet at 3,850 fps (67.0-grains of Retumbo). While my 80- and 85-grain dreams came true, the 90-grain bullet proved disappointing. Reloder 25 and Retumbo were too fast, while H50BMG and US869 were too slow to achieve 3,800 fps.

Would I do it again? I might consider Fred Zeglin’s .240 Hawk instead, with its 4 percent case capacity increase over the .240 Weatherby. Reloder 15 and H4895 in the Hawk were listed at over 4,000 fps with an 80-grain bullet. This sounds like an expensive “free lunch” pressure-wise. Those two powders were useless in my .240 Gibbs, as were IMR4831, IMR4007SSC, W760, IMR4064, Ramshot Hunter and IMR4320. H1000 was fine for forming cases, but not for top loads.

The Hawk was also shown with a 90-grain bullet at 3,800 fps with H4831SC, another powder that the Gibbs didn’t like. While further testing may provide a great 90-grain bullet load in the .240 Gibbs, so far the highest velocity load with good manners proved to be 3,700 fps with Rl 25. Better to have the 85-grain missile at 3,850, I thought.

The goal of surpassing the velocity potential of the .243 Winchester and 6mm Remington was easy with the .240’s large case capacity. An 85-grain bullet in the .243 at 3,280 fps is my best so far. I’ve gotten 3,312 fps in the 6mm Remington with an 85 grainer. Data shows the .243 Winchester Super Short Magnum dashing at a cat whisker over 3,400 fps with an 85-grain bullet, while the .240 Weatherby achieves the same performance. As for the 6mm/284? I suspect that my efforts so far require further labor, because it earned nothing over the .240 Weatherby for me.


The McGowen rifle features a dodecagon barrel—a
polygon with 12 sides and 12 angles.

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One thought on “Rocky’s Little Rocket

  1. Daniel Norman

    I am purchasing a vintage P.O. Ackley build .240 Gibbs. I am not new to the Wildcat game, but I’d like to reach out to the author of this article, as none is listed, I’m not sure who to contact. Please help me to contact him. Thanks


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