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Powder Choice and Carbine Performance
How much velocity will I gain with a longer barrel is one of the common question asked by shooters interested in ballistics. As a rule of thumb, longer barrels equal higher velocity, but there are many variables that make even that simple rule confusing.
Many of our customers are looking for a linear answer, something on the order of: You will gain 50 fps per-inch of barrel longer than 5 inches. That answer is impossible to give for a number of reasons, the two most prominent of which are available gas volume and that old theory killer – friction.
The rapid change from a solid to a gas is the chemical magic that makes gun powder a nearly perfect propellant. Faster powders, like shotgun and pistol powders, come to pressure more quickly and with smaller charges than slower burning products. To shooters interested in economy, the smaller charges equate to more shots-per-pound of powder. The problem with this practice is that the smaller charges also produce less of the gas that actually drives the bullet even as it generates the same amount of pressure.
We decided to look at the effect of powder burn rate on velocity, especially as it related to barrel length from a pistol to a carbine. We tested using our load guide maximum 9mm Parabellum charges at both standard and +P pressures. Fast and slow burning powders were used to compare the velocity shift from pistol to carbine length barrels. The pistol used in this testing was our often abused Ruger SR9 with a 4.14 inch barrel. The carbine was a 16-inch-barreled Uzi Type B. When the smoke cleared at least we knew that in all cases the velocity was higher with the carbine, but sometimes not by as much as shooters might expect.
We chose to use Ramshot Competition for the fast powder portion of our testing because +P loads had already been developed and published. Using a 3.6 grain charge behind a Nosler 115 grain JHP our test barrels produced 992 fps at an average pressure of 34,700 psi. The Plus-Pressure load used 3.9 grains, increasing the velocity and pressure to 1046 fps and 37,800 psi. Already a high pressure, low velocity loading because of the powder burn rate, we had low expectations for its performance in the UZI. It was worse than expected.
The Ruger SR9 cycled without a glitch with an average velocity of 961 fps. Using the +P loads, the velocity went to 971 fps. The Uzi hated these things and never fired two shots in a row. There is a lesson here.
The little Ruger’s locked breach system doesn’t need as much recoil energy to open its action compared to the Uzi. Unlike the Ruger, the big direct blow back Uzi uses the weight of its bolt and springs to manage the cartridge’s recoil energy and cycle the weapon. Direct blow back firearms are very simple to manufacture compared to lock-breach systems and are extremely common among the older submachine gun designs. They can be finicky, however when it comes to cycling.
The downside of direct blowback is that it works within a relatively narrow performance window. With too much energy, the bolt opens hard eventually damaging the bolt and springs. With too little energy from the round, it will short stroke; failing to open far enough to eject the spent cartridge and pick a new round off the magazine. With the low gas volume generated by small charges of fast powder, even operating at maximum SAAMI pressures, there was not enough energy to cycle the Uzi’s action.
Feeding the Uzi singly, it produced 1077 fps with the standard pressure loads and 1142 fps with the +P’s. The 16-inch barrel generated 115 fps and 171 fps more velocity respectively over the handgun. The lighter loading gained 9.7 fps per-inch of barrel and heavier one added 14 fps per-inch of barrel over the Ruger. Not one of these rounds cycled the Uzi successfully.
On the other end of the spectrum were the loads using Accurate #7, a much slower pistol powder often associated with high performance 9mm Parabellum loads. Our published maximum load using 7.5 grains developed 1,127 fps at 34,000 psi using Nosler’s 115 grain JHP. The +P data at 38,200 psi produced a velocity of 1,217 in our test barrel using 8.3 grains.
The SR9 again ran like a champ and averaged 1129 fps with the standard pressures loads and 1217 with the +P ammunition. Frankly, both seemed like good loads in the Ruger. Using these loads the Uzi was transformed from an ugly, unreliable paperweight into a weapon with obvious advantages over its smaller rival.
With the standard pressure loads, the Uzi averaged 1376 fps, a 22% increase over the 4 –inch barreled SR9. Using the slower powder increased the velocity per-inch of barrel significantly; raising velocity an average of 21 fps per-inch of barrel. Muzzle energy increased 49%, to an impressive 483 ft/lbs.
The +P loads produced higher velocity and energy, but were extremely linear in comparison to the standard pressure rounds. The +P loads left the muzzle at 1481 fps, still showing a 22% speed advantage over the Ruger using the same loads. Muzzle energy was 560 ft/lbs, a 48% increase over the pistol.
With these cartridges, the Uzi ran flawlessly and accurately. If you haven’t held one of these prolific but little seen (because they were banned in the late 80’s) carbines, they are heavy as sin. The folding stock model weighs in at arm-bulging 8 ½ pounds loaded. The weight and good sights make them a pleasure to shoot. Recoil was non-existent.
Our testing confirmed a few commonly held beliefs by people who load for 9mm Parabellum carbines. It is no surprise that increasing the barrel length increased velocity. What was surprising was how much difference powder choice made in terms of the increase in velocity and reliability in the carbine.
From our testing it is clear that Accurate #5 (which was also tested, producing lower velocities than #7 but perfect cycling) or Accurate #7 are the best choices for carbine performance. As a corollary, loads that will cycle your locked breech handgun may not cycle a direct blowback carbine. Make test batches of ammunition and test them before going into full-blown production. Don’t bet your life on unproven loads just because they work in your handgun. They will go faster, but they may not cycle at all.