The Barrel and Porting

rifling

The Heart of any rifle is the barrel. The twist determines the weight of the bullet that it will stabilize, the quality of its dimensions (both Chamber and the consistency of the lands and grooves, depth of freebore and perfection of its crown) determines the dispersion of the bullets thrown from it and the efficiency of its seal and the barrels length in relation to the quantity and burn rate of the powder determines the amount of energy the bullet will extract from the working “fluid” or gunpowder gas pushing the bullet down the barrel. As the 10mm is a pistol cartridge, most of the energy is extracted in the first few inches of the barrel. Indeed, projects such as Ballistics by the Inch (Dot) Com have shown that nearly 100% of the Energy is extracted in the first 10 inches. This is completely different than a rifle cartridge where shortening the barrel much below 16 inches causes an unacceptable loss of power and range. However, we can use that short barrel efficiency to our advantage. How? By incorporating a suppressor around the barrel and bleeding off gas starting at the 10 inch mark so that by the time the bullet is released by the rifling, the pressure pushing it has dropped to nearly zero.

 

The .223 rifle cartridge in a nine inch equivalent barrel (M4) produces a velocity of 2400fps with a 55 grain bullet (Army Standard), even less with heavier bullets. While this seems to be fast, it only represents an energy level of 703 ftlbs at the muzzle and bleeds energy so fast it is below a lethal level of energy (>350 ftlbs) by the time it reaches 250 yards.

The 10mm, by contrast has reached 95% of its energy in a nine inch barrel, leaves the barrel with nearly 1000 ftlbs of energy and carries lethal energy well beyond 350 yards in a package already at the size of an expanded 223 bullet and retains the energy to expand even more for an increased wounding effect at range (Hague Convention rules and the desirability of light armorpiercing qualities of ammunition are a lessor consideration in the civilian world than in military conflicts).

All this is a package that is lighter, simpler and more compact than an AR-15 with less blast and flash and a cartridge that is compatible with the civilians sidearm.

Attach a suppressor you still have a lighter, simpler carbine but now it is quiet!

For those who want the look of a suppressor and can’t have one because of cost or laws of your State, you can use a 16 inch barrel and cover the barrel with a mock Suppressor, that way you get the advantage of the additional velocity of the full length barrel.

crbinebarrel

The barrel should have rifling that will allow the greatest stability with the least friction and bullet distortion. Hart’s “R5” (actually a copy of a Russian invention) or the original Bren Ten’s “Power Seal” rifling would be extraordinary, though perhaps not cost effective. Any high quality barrel should work well as long as attention is paid to interior finish and the barrel exhibits a high level of precision of manufacture. As the pistol bullet takes a longer time to get to the target, it is important to realize that barrel quality, bullet stability and general accuracy of barrel and chamber dimensions are critical. The bullet must face the land and grooves (if not polygonal rifling) squarely and be properly centered and fully seated against the bolt with a high level of chamber finish to enhance extraction and feeding. The bolt must support the cartridge completely so that when the primer fires, the case does not tilt or “rock” in the chamber. The barrel can be threaded, but it should have an 11 degree muzzle crown so the bullet is released cleanly.

MAX10-45 9 inch

Perfection: A Prototype 12″ 10mm MAX-10 barrel for testing. (Note: this design was rejected and the 16 inch barrel made standard.) Notice the threading for both holding the barrel in the weapon and the barrel end threading. Abutment at threading end eliminated. Barrel end threading was set at 5/8-24 for ease of finding a Suppressor adapter. Later prototypes are all 16″ with the threading.

There is a long running debate about the need for a long chamber throat. While a long throat does seem to soften the peak pressure of pistol cartridge loaded to the edge, its need is debatable. The angle of bullet entry to the rifling is also critical. Benchresters like a 1/2 to 1 1/2 degree lead to the rifling. This area serves to align the bullet to the rifling and is critical for accuracy. It is how deep that is the question. The best Makers say a throat of 0.25 is perfect, so that will be specified for the Carbine with a low angle of 1/2 degree and a tight fit from just forward of the chamber to the beginning of the rifling. This will allow ejecting of misfires or loaded cartridges, but maximum accuracy and bullets will not be stuck in the rifling. Chrome-Moly barrels with chrome plated bore, Salt-Nitrited chrome-moly steel or Stainless material should all perform superbly and have a life of 10’s of thousands of rounds. High pressure throat erosion simply does not become a factor at semi-auto pistol cycling speeds and pistol pressures. For weight reasons it should be no larger in diameter than 3/4’s and threaded at the end at 5/8 x 24 right hand thread with a thread protector, but so as to be compatible with a brake or a flash hider or Suppressor device. Max efficiency with the 10mm appears to be 8-12 inches, but remember we are enclosing the barrel with a suppressor as one unit so we will bleed off gas at the 10 inch mark. This will extract the maximum amount of energy without making the weapon cumbersome. A 10 inch barrel with a rifle cartridge would be a waste, with deficits such as a huge flash, low efficiency, extreme muzzle blast and high torque recoil. Even with a suppressor, such a weapon would have a very high noise level, problems with barrel and suppressor baffle erosion and VERY high temperatures. As to balance, in a pistol a 10 inch barrel would be awkward and off balance. In a subgun carbine, it is perfect; extracting maximum energy with minimal flash, minimal recoil, and simplicity of mechanism.

barrel interior

Barrel pressure area. Notice that the pressure falls to near zero by the 4 or 5 inches. A barrel that is too long in a pistol carbine can actually slow a bullet to below peak velocity through friction. Barrel length should be selected for extracting max energy without braking the bullet or being so long as to make the weapon cumbersome, thus defeating the purpose.

10mm Efficiency
10mm

Notice that most (if not all) of the useful energy is extracted by 12 inches.  Perfect! Any longer and the barrel actually brakes the bullet, leading to negative efficiency! It appears that the best overall length is 10 inches, which is what I will specify for BattleAxe. The LAGE comes with a 9 inch barrel in 45 ACP or 9mm,  so a custom barrel will have to be purchased.  Their are several barrel makers that can turn a blank if you send them a stock blank.

Porting

The barrel is straight from the receiver abutment forward to the 8 1/2 mark where steps down sharply (at 90 degrees) to 16mm. This allows escaping gas from the suppressor to be diverted from the rear of the suppressor sideways. The gas comes from a series of cuts in the barrel that allow the gas to expand radially outward and then flow back where the exit out the rear of the suppressor. When the suppressor is off, act as a muzzle brake. one possible configuration is below.

porting

Barrel porting for weapon. Note that the suppressor slides over the porting holes, so that the suppressor with an O-ring at the back and screws to the barrel at the muzzle end traps the gas. With the Suppressor removed, the ports are part of an integral muzzle brake. With the suppressor on, the barrel vents gas into the suppressor.

The point is to vent gasses off as much as possible before the bullet leaves the barrel, thus getting maximum use out of the suppressor. The suppressor threads onto the end of the barrel 5/8×24 . The threading is not shown in this picture.

ALL ITEMS DESCRIBED and their sources will be placed on a “Sources” page so that “PDW’s” can be duplicated.

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