[/QUOTE]And no, i don't have specific data for each inch of barrel length and it's affect on velocity with supers. 300 blk was designed to be efficient out of short barrels, so it's a game of diminishing returns. With a .30 caliber bullet the barrel length at which you get diminishing returns is much shorter than most other rifle cartridges. I would guess somewhere between 9" & 12" Even at 16", even with slow burning powders I would expect all the powder to be burned up so any additional barrel length causes additional drag resulting in less velocity. Terminal performance testing on over 2000 Texas feral hogs and various game animals as large as 800 pounds has proven it to kill much more effectively than the 6.5 Grendel, 6.8 SPC, 7.62×39 or 300 Blackout. Due to the optimization of bullet weight to barrel twist, accuracy has proven to be outstanding.
Sub one-inch groups are the norm with multiple bullet choices for varying applications such as hunting, target shooting and tactical use. I confess that I was compelled to reassess my prejudices against the .300 blackout last Fourth of July. Everyone brought some guns for some informal target shooting. I was able to shoot some AR-15s chambered in .300 Blackout. Big yawn, until someone put a suppressor on their SBR loaded with subsonic ammunition. As pathetic as the external ballistics are, I could still hit targets floating in the reservoir about 200 meters away once I had a sense of the probable bullet drop.
Of course the CZ Scorpion with a short barrel and suppressor was equally impressive. Do you want the ideal cartridge to use with a suppressor or in rifles with short barrels? The .300 Blackout has a big advantage over the 7.62×39 in those areas.
Because the .300 Blackout is designed for use in a standard AR style rifle, it will still cycle the bolt reliably when using subsonic ammunition and with a barrel length shorter than 16″. Additionally, it doesn't suffer nearly as big of a drop off in performance as the 7.62×39 when using a shorter barrel either. This demonstrates a distinct advantage in favor of .300 Blackout ballistics when fired from two of the most common barrel lengths for the modern M4 carbine – 14.5" and 9". Due to greater kinetic energy stored in the heavier .308" caliber projectile, the reduction in velocity does not affect hit probability to the degree it affects the 5.56x45mm cartridge.
Most factory ammunition, both supersonic and subsonic, contains a powder charge that completely burns in a barrel between 9-10" long. There is no need for a .300 Blackout firearm to have a barrel longer than 10.5". Longer barrels provide minimal muzzle velocity, but they add weight.
Some weight can be beneficial to combat recoil, but a rifle that is too heavy is cumbersome. The 300 Blackout was designed by Remington/AAC to create a reliable, compact .30 caliber round for the AR platform that uses a standard bolt and magazine. 300 Blackout is optimized for suppressed fire with heavy bullet subsonic loads but can also be used with supersonic ammunition when extra range is needed.
Super low recoil, lightweight, close range, deer and boar gun. Normally, when you cut barrel length and weight down on a hunting rifle, you lose precious velocity and increase recoil. A nice attribute of 300 Blackout is that it works pretty dang well with short barrels. So you can use a short action, short barrel, lightweight rifle, and not get beat up by recoil at all. I'd imagine this is the direction that Ruger was thinking with their 16″ barrel, under 6 lb, American Ranch rifles chambered in 300 Blackout. This tells me that AAC, the company that has fine-tuned their M4/AR-15 platforms to work with high velocity supersonic ammo and low velocity subsonic ammo, deems this the most versatile twist rate.
If, however, you are building your rifle around a specific bullet weight and load, then twist rate is a valid consideration for optimizing that load. If you're using lighter bullets a slower twist rate can give better performance. They aren't subsonic because they're heavy, they're subsonic because they are 'powder limited' to keep the muzzle velocity at about 1050 fps so they will be subsonic. By that I mean the bullet, if you could get enough powder in the case, could be pushed faster than 1050 fps into the supersonic range and not surpass the energy level of the supersonic rounds. To show how the data set changes with an increase in sample size, I made a table with the data from both 30 shot strings. The "shot" column represents the shot number in the respective string.
"28" barrel ft/sec" and "16.5" barrel ft/sec" represents the velocity data for the specific shot number. "AVG 28″ ft/sec" and "AVG 16.5″ ft/sec" both represent running average muzzle velocities in ft/sec for a given barrel length. "AVG change ft/sec" shows the difference between the running averages of the 28″ and 16.5″ barrels. "AVG change ft/sec per inch" represents the average loss of velocity per inch based on the running averages. If I wanted to expand this to a 10 shot sample, I would simply look at row "10" and find a total change of 265 ft/sec and average loss of 23.0 ft/sec per inch of barrel.
First off, there is no "explosion" with smokeless powder, it burns at a controlled rate. The hot flaming gasses are traveling at the same speed or slightly faster than the projectile, when they hit the still air at the muzzle, the collision causes the bang. A suppressor does not "handle the explosion", it gives the gasses a place to expand and slow down to subsonic speeds.
The next point is if you can't make an "ethical" shot on a game animal with subsonic ammunition, then why would you consider it for combat? All of this is basic physics, there is no magic when it comes to how gasses or projectiles act. No projectile/case option is ideal, they all have pluses and minuses and are a compromise in some way. If you need to shoot a .30 cal then an AR-10 in .308 is the way to go with the trade off of more recoil and heavier ammunition.
Wow, so little drop in velocity, but I guess that's what happens when it's poking along so slowly that wind resistance is minimal. The tougher part to account for on these heavier bullets is the rainbow trajectory. Making an accurate hit at 200 yards with a subsonic round is a bit of fantasy, because you're looking at 30+ inches of drop even if you sighted at 100 yards. It's also far lower in energy than I'm comfortable using on a deer. For boar at less than 100 yards, it'd work a treat and they wouldn't know what hit them if you ran the subsonic rounds with a suppressor.
Check out the gel test Black Hills did of their ammunition featuring the 110-grain Barnes TTSX bullet fired out of a 5.5-inch barreled SIG Rattler. There is no shorter 300 Blackout gun on the market, and out of the SIG's short tube this load provided 1,879 fps. Remember, with slower velocities bullets tend to expand less, and penetrate more deeply.
At this speed the Barnes copper solid bullet expanded to .59 of an inch, retaining all of its weight and penetrating 22-inches into the gel block. The .300 AAC Blackout, on the other hand, in supersonic form, is a completely different animal, and it was designed that way. The traditional powder inside the supersonic load is Hodgdon HI 10 or Win 296 (they're the same powder). Which is why pretty much all Blackout barrels shorter than 16 inches have pistol length (four-inch) gas systems. With the 300 BLK you don't see carbine-length gas systems until you get to a 16-inch barrel.
I chose a variety of loads for this little experiment with performance ranging from light bullet, supersonic velocity to slow and heavy subsonic rounds. Some are off-the-shelf factory loads while others are handloads using common .300 Blackout ballistics. I shot them to measure velocity and through Clear Ballistics gelatin blocks to see what we might expect from different combinations fired from the CMMG pistol. I will chronograph supersonic loads for the 110 & 125 grain using H110.
I have not developed my subsonic loads but will use a 225 gr. After getting velocity w/ a 16" barrel, I am going to cut down the barrel by 1" & get updated velocity data. Loads will stay constant, but I will adjust the subsonic loads as necessary to maintain 1050 fps. For Supersonic, my objective is to see how the velocity drops off with barrel length. With subsonic, the objective will be to maintain reliable function.
I will not want to give up too much supersonic performance and expect that I will end up with a barrel length of somewhere between 10" to 12". Testing would be done on a pistol lower so that I would not be creating an SBR in the process. A google search used to bring up a certain blackout "development history" on the first page. And having more muzzle energy than a 223, especially for shorter barrels.
The tradeoff for using subsonic ammo is that since the projectiles are heavier and travel at a lower velocity, they drop faster. A typical 190-grain subsonic bullet drops 4" at 100 yards, approximately 30" (2.5') at 200 yards, and about 96" (8') at 300 yards. This significant bullet drop means your subsonic .300 Blackout's effective range should remain within 100 yards. Just like barrel length, there is almost never a "best" twist rate for any caliber - unless you shoot one, single load within that caliber and you never chamber anything else. That's because calibers utilize different bullet weights for different applications, and the weight of the bullet in question is what determines an optimal twist rate. For example, 5.56 NATO can be loaded with bullets weighing less than 50 grains, and up to 90 grains.
No single twist rate will be universally optimal for all those different 5.56 variants. Barrel length directly affects the velocity of your cartridge. If your barrel's too short, the gunpowder in the shell casing won't have enough "burn time" to accelerate the round downrange, and your round won't reach optimal speed.
This will result in the round dropping too quickly and/or being affected by wind drift more than it should. Total effective range will be dramatically reduced, and you'll have less energy striking your target. A lack of energy is of particular concern in self-defense and hunting situations.
For velocity and accuracy testing, I used a combination of everyday ammo like American Eagle 55-grain FMJ and M855 Steel Core, and Norma 77-grain Match ammo.So why does this happen? It's almost certainly not because the "powder didn't have enough room to burn." Generally speaking, all the powder THAT IS GOING TO BURN does, in fact, burn in the first few inches of barrel length. What it does tell us is that the gas cloud is still expanding faster than the volume of the barrel is increasing when the bullet exits a 16-inch barrel. In other words, pressure is still building when we run out of barrel in the 16-inch rifle, so some of that pressure is wasted.
There is one other variable that probably contributes to this difference. And no, i don't have specific data for each inch of barrel length and it's affect on velocity with supers. When choosing between 300 Blackout 1-8 or 1-7 twist rates, consider over-stabilization. This usually happens when you shoot a bullet through a gun barrel with a faster twist rate than what's required for the particular bullet weight. Based on the chart, if you plan to use a heavier bullet, choose a faster twist rate to ensure stability.
Generally, you can still shoot through 300 Blackout barrels with a twist rate that's at the end of another fence, but it won't ideally give you the maximum ammunition performance. I vote for AR15 in 7.62 x39, only a bolt, (not b.c.g.) is needed, you might need an extended firing pin, of course 7.62×39 barrel and magazines are required. When you shoot this combination, there is no doubt of the increased power over 5.56x 45.
I have built 4 AR15s in this caliber, and while a bit more difficulty is involved the extra effort is well worth it. Most loadings for this cartridge are designed for 16″ barrels, so not much velocity is lost with shorter barrels. Depending on barrel mfg. gas port may need to be opened up some, unlike 5.56×45 chamberings which are typically overgased with carbine length gas systems. It will be a multiuse can , but my initial focus will be on suppressing my 300 Blackout. I currently have a Sig MCX Gen2 rifle - 16" barrel. To me a 300 BLK does not need anything close to a 16" barrel.
Then one day I was corresponding with Ron Reiber of Hodgdon Powder Company and he strongly suggested that we try their new CFEBLK powder. To make this long story shorter, this ended up being a game changer and the final piece to the puzzle. It gave us improved accuracy and more velocity with less pressure allowing me to exceed my original velocity goals.
Once I decided on a 1-15 twist rate for the barrels and we were good on ballistic performance and accuracy, all that was left was to decide on a name and test real world terminal performance. The same can be said for virtually all bottle neck rifle cartridges -- from 223 to the biggest magnums. And yet we have long barrels for many applications, such as long distance shooting.
And applications where one wants to minimize bullet drop and wind deflection. And applications where one wants to maximize down range velocity to increase energy on target. Subsonic ammunition uses long, heavy bullets and a reduced powder charge. The muzzle velocity remains below the speed of sound, eliminating the loud crack that usually occurs with supersonic ammo. Sticking with .308s for a moment, let's look at a cartridge whose performance and use depends heavily on barrel length and twist rate.
Since 300 BLK is made to be fired supersonic or subsonic, suppressed or unsuppressed, it comes with a huge spread of grain weights. Picking a twist rate will depend exclusively on what type of 300 BLK you chamber most often. In fact, depending on what twist rate you select, you may be able to shoot only supersorsubs, but not both. I've got two gel tests of this bullet from Black Hills, first from a 6.5-inch barrel and another from a 12-inch barrel.
From the 6.5-inch barrel this load did 957 fps, and that only increased to 994 fps out of the 12-inch tube. Multiple gel tests were done with each gun, with similar results--at some point the bullet yawed and exited the block, sometimes 20 inches in, sometimes 25. All the recovered bullets look almost good enough to reload, except for the rifling marks.
Even out of a .300 WinMag at two-anda-half times the speed these bullets are not designed to expand, so don't consider this a failure. The same is true of the 208-grain A-Max from Hornady, their original heavy bullet subsonic load. Cartridges loaded with these bullets are meant to be very quiet when suppressed, and they are. It was a select-fire weapon that sported a six-inch barrel, an AAC suppressor, and a proprietary collapsible stock. Not coincidentally, the final product was roughly the same weight and length as the famed MP5SD while having better ergonomics and increased terminal performance with both sub- and supersonic ammunition. The SST bullet is not supposed to expand at lower velocities.
