7.62×39 vs .300 Blackout Subsonic — The Rematch
In our first 7.62×39 vs. .300 Blackout comparison, we tried to level the playing field as much as possible. Both cartridges were fired from quality SIG Sauer platforms with 16-inch barrels, using similar-weight bullets and the same powder family in our handloads.
That first test covered supersonic performance from 0 to 300 yards, and the 7.62×39 made a strong case for itself. With the 123gr Nosler Ballistic Tip averaging 2,426 fps, compared with 2,033 fps from the 125gr .300 Blackout load, the Russian cartridge delivered more velocity, more retained energy, and a considerably flatter trajectory.
But I said at the end of that test:
That wasn’t the final verdict.
Because we hadn’t yet entered the territory where .300 Blackout built much of its reputation:
Heavy bullets. Subsonic velocities. Suppressors.
So the SIG rifles are back on the bench.
This time everything happens at 100 yards, everything is suppressed, and we’re going to compare factory ammunition before seeing whether my handloads can improve the accuracy.
And as you’ll see, this test answered one question while creating another.
The Rifles Return: SIG vs. SIG
I’m sticking with the same basic philosophy as the original comparison.
I didn’t want rifle quality muddying the results.
On the .300 Blackout side is the SIG MCX, and representing 7.62×39 is the SIG SPEAR LT.
Both are quality modern platforms, so we’re not comparing a bargain rifle against a premium rifle and then blaming the cartridge for the results.
This time, however, suppressor behavior would become a much bigger part of the story than I anticipated.
The Test
Everything was fired at 100 yards.
We tested:
| Cartridge | Ammunition | Bullet |
|---|---|---|
| .300 Blackout | S&B Tactical Factory | 200gr |
| .300 Blackout | DayAtTheRange Handload | 220gr Sierra MatchKing |
| 7.62×39 | PPU Factory Subsonic | 182gr |
| 7.62×39 | DayAtTheRange Handload | 174gr Sierra MatchKing .311 |
The objective was more than finding the smallest group.
I wanted to look at velocity, consistency, accuracy and—very importantly—whether the rifles would actually function with these loads while suppressed.
Round One: .300 Blackout S&B 200gr Factory Ammunition
We started with the known quantity: factory Sellier & Bellot 200gr .300 Blackout ammunition.
Five rounds across the chronograph produced:
| Shot | Velocity | Energy |
|---|---|---|
| 1 | 1,053.4 fps | 492.7 ft-lbs |
| 2 | 1,115.5 fps | 552.5 ft-lbs |
| 3 | 1,106.9 fps | 544.0 ft-lbs |
| 4 | 1,123.7 fps | 560.7 ft-lbs |
| 5 | 1,100.4 fps | 537.7 ft-lbs |
| Average | 1,100.0 fps | — |
The factory S&B produced an SD of 24.6 fps and an extreme spread of 70.3 fps.
That’s our baseline.
It also highlights something worth remembering when discussing “subsonic” ammunition: the speed of sound isn’t a universal fixed velocity. Atmospheric conditions matter, and ammunition operating around 1,100 fps can be very close to the transonic boundary depending upon conditions.
But now I wanted to see what carefully assembled handloads would do.
.300 Blackout Gets a 220gr Sierra MatchKing
For my .300 Blackout handload, I moved to the 220gr Sierra MatchKing.
The load used in this particular test was assembled with 12 grains of Reloder 7. This is simply a record of the ammunition fired in this test, not published load-development guidance; handloaders should independently verify appropriate data for their own components and firearms.
The chronograph immediately showed something interesting:
| Shot | Velocity | Energy |
|---|---|---|
| 1 | 1,027.9 fps | 516.0 ft-lbs |
| 2 | 1,045.9 fps | 534.3 ft-lbs |
| 3 | 1,057.2 fps | 545.9 ft-lbs |
| 4 | 1,057.4 fps | 546.1 ft-lbs |
| 5 | 1,051.8 fps | 540.3 ft-lbs |
| Average | 1,048.0 fps | — |
More important to me was the consistency:
SD: 10.9 fps
Extreme Spread: 29.5 fps
Compare that with the S&B factory ammunition:
| .300 BLK | Avg. Velocity | SD | ES |
|---|---|---|---|
| S&B 200gr | 1,100 fps | 24.6 | 70.3 |
| 220gr SMK Handload | 1,048 fps | 10.9 | 29.5 |
That’s a substantial reduction in velocity variation.
And on target, the .300 Blackout averaged approximately 3.2 inches at 100 yards across the groups fired.
But perhaps more important for this particular test:
The SIG MCX worked.
Suppressed subsonic ammunition and the .300 Blackout were doing exactly what that combination is supposed to do.
Now it was time for 7.62×39.
PPU 182gr Subsonic 7.62×39
The first 7.62×39 ammunition was factory PPU 182gr Subsonic.
Its five-shot chronograph string was:
| Shot | Velocity | Energy |
|---|---|---|
| 1 | 954.2 fps | 367.9 ft-lbs |
| 2 | 975.6 fps | 384.6 ft-lbs |
| 3 | 992.1 fps | 397.7 ft-lbs |
| 4 | 987.4 fps | 394.0 ft-lbs |
| 5 | 1,051.1 fps | 446.4 ft-lbs |
| Average | 992.1 fps | — |
The SD was 32.3 fps, with a fairly large 96.9 fps extreme spread.
So the factory PPU was substantially slower than either of the .300 Blackout loads.
And that’s when we encountered our first major problem.
The SIG SPEAR LT Wouldn’t Cycle
The PPU ammunition fired, but the rifle would not reliably cycle the subsonic ammunition in this suppressed configuration.
That matters.
We can sit at the bench and compare group sizes all day, but if we’re discussing a semiautomatic rifle and its intended suppressed performance, function has to be part of the equation.
This wasn’t necessarily an indictment of 7.62×39 as a subsonic cartridge, either.
The suppressor setup became an important variable.
The 7.62×39 SIG was being tested with newer low-back-pressure/flow-through suppressor technology. That can be highly desirable with full-power ammunition because reducing gas sent back toward the shooter and action has real advantages.
But with subsonic ammunition, we were already working with dramatically less gas energy.
That gave me a new question:
Would this same rifle behave differently with a traditional higher-back-pressure suppressor?
Hold that thought.
Can a 174gr MatchKing Improve the 7.62×39?
Factory ammunition wasn’t giving me everything I wanted, so it was time to see what the handloading bench could accomplish.
For 7.62×39, I selected a 174gr Sierra MatchKing .311 and Accurate 1680.
The ammunition fired in this specific experiment used 12 grains of A1680. Again, these numbers document what was tested rather than serving as a load recipe.
This time I fired 10 rounds across the chronograph.
| Shot | Velocity |
|---|---|
| 1 | 1,125.6 fps |
| 2 | 1,077.5 fps |
| 3 | 1,066.3 fps |
| 4 | 1,083.8 fps |
| 5 | 1,026.1 fps |
| 6 | 1,162.8 fps |
| 7 | 1,216.3 fps |
| 8 | 1,108.5 fps |
| 9 | 1,112.9 fps |
| 10 | 1,057.5 fps |
| Average | 1,103.7 fps |
And here we found another issue.
The SD climbed to 52.3 fps, while the extreme spread opened to a whopping 190.2 fps.
That’s not what I was looking for.
Several rounds also reached velocities that shouldn’t simply be described as “subsonic.” Depending on atmospheric conditions, this string crosses into the transonic/supersonic region. That’s an important distinction and something that needs more work before I consider this load developed.
The target wasn’t particularly forgiving either.
The 7.62×39 averaged approximately 4.9 inches at 100 yards in this phase of testing.
And the handloads still didn’t solve the cycling problem.
So instead of ending the test, we found the subject of Part Three.
The Numbers Side by Side
Here’s where everything stands after this round of testing:
| Load | Avg. Velocity | SD | ES |
|---|---|---|---|
| .300 BLK S&B 200gr | 1,100.0 | 24.6 | 70.3 |
| .300 BLK 220gr SMK | 1,048.0 | 10.9 | 29.5 |
| 7.62×39 PPU 182gr | 992.1 | 32.3 | 96.9 |
| 7.62×39 174gr SMK | 1,103.7 | 52.3 | 190.2 |
The handloaded 220gr .300 Blackout was the standout of the chronograph testing.
Its 10.9 fps SD was less than half that of the S&B factory load and roughly one-fifth the SD of my 7.62×39 MatchKing load.
There’s more work to do on that 7.62×39 load.
A lot more.
And that’s exactly why we test instead of assuming.
So Is .300 Blackout the Subsonic King?
For this test and these two rifle/suppressor configurations, absolutely.
Our first comparison belonged to the 7.62×39.
When both cartridges were running supersonic from 16-inch barrels, the 7.62×39’s velocity advantage produced more energy and a flatter trajectory all the way to 300 yards.
But this time we moved the fight onto .300 Blackout’s home field.
The results flipped.
At 100 yards, the .300 Blackout averaged approximately 3.2 inches compared with approximately 4.9 inches for the 7.62×39, and the MCX provided the reliability I was looking for.
My 220gr MatchKing .300 Blackout handload was also the most consistent ammunition of the test, producing an SD of only 10.9 fps.
Most importantly, the SIG MCX functioned with the heavy subsonic ammunition while the 7.62×39 SIG SPEAR LT did not reliably cycle either the factory PPU or my handloads in the tested suppressor configuration.
For a semiautomatic subsonic rifle, that’s difficult to overlook.
So after two tests, we have a fascinating split:
Supersonic 0–300 yards: 7.62×39 showed the ballistic advantage.
Suppressed/subsonic 100 yards: .300 Blackout showed the accuracy and reliability advantage in this setup.
But I’m still not ready to close the book.
7.62×39 Gets One More Shot — Part Three
There’s one variable bothering me.
The suppressor.
The SIG SPEAR LT was using newer low-back-pressure/flow-through suppressor technology.
That’s excellent technology for many applications, but we’re asking a semiautomatic 7.62×39 to operate on dramatically reduced-energy subsonic ammunition.
What happens if we change the equation?
For Part Three, I want to bring the 7.62×39 back with a more traditional, higher-back-pressure suppressor and see whether additional gas available to the operating system changes the reliability story.
Then we can work on the ammunition.
Can we tighten that enormous 190.2 fps extreme spread?
Can we keep the entire string consistently below the transonic region?
Can we bring those 4.9-inch groups down?
And most importantly:
Can we make the SIG SPEAR LT reliably cycle subsonic 7.62×39?
The .300 Blackout has won Round Two.
But the experiment isn’t over.
The next variable is back pressure.
And that’s exactly what we’re going to test next on DayAtTheRange — Always On Target.
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