I recently learned of a new-to-me subsonic load for 308 Winchester by Beck Ammunition. I had previously tried other factory subsonic 308 Win ammunition, but with very poor precision results. Based on a couple of favorable—if vague—reports about the Beck loads, I decided to try again.

Beck offers more than one 308 subsonic load, but because the test rifle has a rifling twist rate of 1/10" I wanted to keep the bullet weights (lengths, actually) not too great. The specific load I purchased is the “Silent Operator™” using the Hornady 168 grain A-MAX bullet. I believe the A-MAX line has been superseded by Hornady’s ELD Match bullets, so it’s possible Beck has just not updated their bullet description. For short range use, though, I believe there is no practical difference between the two Hornady projectiles.

The claimed muzzle velocity for the Beck load is 990 feet per second which is below the speed of sound under most atmospheric conditions.

The test rifle was a Tikka T3x Compact Tactical Rifle with 20 inch barrel and equipped with a Leupold 5HD 2-10×30mm scope.

Testing was conducted with the rifle supported by a bipod and rear stock bag on a sturdy bench rest. The range elevation is about 10,000 feet above sea level and the temperature was about 29 degrees. Shooting at high elevation means the barrel rifling twist rate can be slower to ensure a particular bullet’s stability, but colder temperatures require faster twist rates. More about bullet stability below.

Bullet stability is usually considered to be only based on the length of the projectile and the barrel rifling twist rate, but although it’s not usually mentioned, bullet velocity also has an effect on what twist rate is required for proper stability. Because this subsonic load’s velocity was much less than common 308 Win’s loads, I wanted to make sure that the bullets would be stabilized properly before risking shooting them through my suppressor, a Thunder Beast model 30P-1.

To do that I fired three shots into witness paper at 50 yards without the can on the gun. Those results were concerning because it appeared that two of the holes were somewhat elongated, indicating that the bullets were yawing excessively in flight. But because bullet stability can change in flight, I next fired five shots at a range of about 10 yards. All five holes were nice and round as they should have been, so I decided the load was stable enough for safe suppressor use.

The two groups below were fired from 50 yards with the suppressor attached. All shots during the session were fired with ear muff protection, so I can’t say if those fired with the can were hearing safe, but the discharge noise with the can was far less, as was expected.





The picture may not make it clear, but many of the bullet holes showed obvious elongation, demonstrating that once again at 50 yards the bullets were yawing excessively due to insufficient stabilization. Both groups measured just over 1 inch center to center, or about 2 minutes of angle. Is that good enough?

The answer obviously depends on what we need and expect. For example, precision of 2 MOA is actually pretty good for an average AR-15 and M193 ammunition. The other factor in this case, though, is the evident lack of bullet stability at 50 yards, and the question of what might be true at 100 yards or farther. Very often bullets become more stable at longer ranges in flight. Today wasn’t the time to test this ammunition at longer distances, but I’ll do that sometime.

That 2 MOA level of precision is much poorer than what I expect from the Tikka CTR. With factory Hornady 168 grain A-MAX ammunition from the CTR I regularly get groups in the 0.6 to 0.7 MOA range at 100 yards. I suspect I’d do even better with the CTR at longer distances if I had a scope with higher magnification mounted, but I’m perfectly content with the current setup for the rifle’s intended use.

The ~2 MOA results of the Beck ammunition are poor in comparison with regular ammunition, but much better that what I got with another subsonic load years ago. There were other variables involved then, though, so if I still have some of that I’ll get out and try it under the same conditions as this test at some point.

I fired a total of 20 shots during the session and measured their velocities with a Garmin Xero C1 Pro chronograph; the results: High - 958 feet per second; low - 814 fps; average - 887 fps; extreme spread - 144 fps; standard deviation - 42.5 fps. The average was about 100 fps slower than the factory claim, but that may have been due to a shorter test barrel. The significant part of the data is the widely inconsistent velocities. For precision rifle ammunition a standard deviation of 10-12 fps or even less is desirable; an SD of over 40 fps for the low velocities of this load is very high.

The last result to mention was the ammunition’s point of impact due to its much lower velocity than conventional loads’. At 50 yards it was necessary to dial up 3.0 milliradians, or about 5 inches, to get the lower seven-shot group pictured above, and which was still slightly low and right of the number 1 diamond point of aim. With how the sight is currently zeroed at 100 yards, it’s necessary to dial up 0.3 ₥ at 50 yards using Hornady factory 168 grain A-MAX ammunition.