300BLK Test

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Improve the accuracy of 300BLK subsonic ammunition.

The standard subsonic 300BLK load exhibits significant variation in muzzle velocity, which results in excessive vertical dispersion.

We will test various products and hypotheses to determine whether improving internal ballistics can statistically increase the precision of subsonic 300BLK.


The objective of subsonic ammunition is to avoid the sonic crack that occurs as bullet velocities approach the speed of sound, which is typically about 1100fps. Given this upper bound on velocity the only way to improve external and terminal ballistics is to increase the mass of the projectile. The upper limit on mass of standard .30" jacketed lead-core bullets that fit the AR-15 platform with a standard bolt and magazine is 225gr, which in the BTHP profile is a bullet 1.57" long, with a bearing length of 0.70-0.75".

The standard Remington factory subsonic load, designed to cycle reliably in the greatest variety of guns, consists of:

  • 220gr BTHP bullet
  • 10.4gr A1680
  • 2.12" COAL

According to QuickLOAD this generates MAP of 33kpsi and only 81% propellant burnt within a 16" barrel.


  1. Reducing friction between the bullet and bore should reduce the variation of muzzle velocity.
  2. Increasing peak muzzle pressure and/or burning efficiency should also reduce the effects of friction on muzzle velocity.



  1. Barrel alloy
  2. Barrel condition
  3. Bullet coating

hBN has a lower coefficient of friction and higher stability than established but messier friction-proofing compounds like MoS2 ("moly") or WS2 ("Danzac"). However it is unclear whether common practices with hBN actually reduce friction. In general we expect reduced friction to result in markedly lower muzzle velocities — a well-known phenomenon with MoS2. But no hBN users consulted to date have observed significantly reduced muzzle velocities. Rather, the most frequently cited benefit of the proper use of hBN is reduced bore fouling. Experts like David Tubb and Swiss Products claim that shooting only hBN-plated bullets and bores result in bores that can be cleaned with a single dry patch.

Here we will test four variations of hBN:

  1. Tubb et. al. advocate impact-plating bullets with HCPL-grade hBN, which averages 10 microns and is among the most lubricious formulations. For our tests we use a vibratory tumbler and steel shot to produce what we will call plated bullets.
  2. Tubb et. al. also advocate coating a completely clean bore with sub-micron hBN (we use AC6111-grade) suspended in isopropyl alcohol, and then firing a plated bullet through it to solidify the hBN. We refer to this as a coated bore.
  3. Rydol addresses a common concern with plated bullets which is that the impact-plating process results in a non-uniform and potentially excessive build-up of hBN. Rydol advocates bullet lubrication via ultrasonic embedment of sub-micron hBN and PTFE. We will test bullets treated by their method as Polysonic bullets.
  4. Rydol also sells a bore conditioner designed to embed sub-micron hBN and PTFE particles in bore irregularities, creating a smoother and more lubricious surface. We will test this as a Rydol bore.

There seems to be a consensus that hBN is easily stripped from both coated and Rydol bores by most cleaning chemicals.

We already performed one test of plated bullets: We compared 50 rounds handloaded with the standard specifications to 50 rounds using the same specifications but with hBN-plated bullets. From a 16" stainless barrel there was no significant difference in muzzle velocity variance between the plated and unplated bullets:

  • Uncoated average = 923fps, stdev = 25fps
  • hBN coated average = 941fps, stdev = 27fps

Subsequently we have found suggestions that hBN does not embed or interact with stainless steel as effectively as it does with other standard barrel alloys. Therefore this test will be repeated with 4140 chrome-moly nitride barrels.

Pressure and Accuracy

The standard subsonic 300BLK load puts a bullet optimized for supersonic flight down a bore at little more than half the peak pressure for which modern ballistic components are designed. This may result in higher interior ballistic variances, which would also result in higher muzzle velocity variance.

We will test loads that produce higher peak pressures and higher interior burn ratios than the 33kpsi and 80% of the standard load through a 16" barrel.

We will also test Lapua's 200gr subsonic bullets. These have a reduced bearing surface, which should reduce bore friction and any variance associated with that. They also have a profile optimized for subsonic flight, which may improve the exterior ballistic precision.

Test outline

Our objective is to find statistically significant improvements over the standard load and untreated barrels:

  1. Reduced variation of muzzle velocity.
  2. Reduced mean radius (i.e., increased on-target precision).
  3. Reduced velocities or bore temperatures as evidence that any variable has reduced bullet-bore friction.

Therefore we will run the following test strings:

Friction Sequence

  1. Using standard load and 16" bbl:
    1. Normal bullet
    2. Polysonic bullet
    3. (We already know that plated bullets show no improvement)
  2. Using standard load and 12" coated bbl (because it's easiest to coat a new barrel)
    1. Plated bullet
  3. Using standard load and 12" clean bbl:
    1. Normal bullet
    2. Polysonic bullet
    3. Plated bullet
  4. Using standard load and 12" Rydol bbl:
    1. Normal bullet

Pressure/Accuracy Sequence

Note that bullets are seated to max magazine length to minimize jump to lands.

  1. 220gr load with MAP > 45kpsi and > 99% interior burn: 10.1gr VV N110, 2.20" COAL.
    1. 16" clean bbl
    2. 12" Rydol bbl
  2. 200gr Lapua load with MAP > 45kpsi and > 99% interior burn: 10.2gr VV N110, 2.10" COAL.
    1. 16" clean bbl
    2. 12" Rydol bbl



All guns have pistol-length gas systems and will run the same NP3-coated BCG.


  1. Noveske 16" 1:7 stainless. Chambers 220gr SMKs up to 2.27" COAL and 200gr Lapua subsonics up to 2.14" COAL.
  2. AAC 12.5" 1:7 4140 chorme-moly nitride. Chambers 220gr SMKs up to 2.31" COAL.
  3. CMMG 8" 1:7 4140 chorme-moly nitride. Chambers 220gr SMKs up to 2.27" COAL and 200gr Lapua subsonics up to 2.14" COAL.


All cases are fired Remington 300BLK brass. Cases are full-length sized, then case mouth is chamferred, deburred, brushed; then primer pocket is cleaned.

Primers are Remington #7.5.

Powder is dispensed by volume after calibration.

Bullets are seated with a Forster Benchrest Seater die.


Two chronographs will be positioned in line starting 15 feet from muzzle.

  1. Competition Electronics ProChono Digital
  2. Caldwell Ballistic Precision Chronograph

Thermocouple to check barrel temperature secured with electrical tape to top of barrel 2.5" forward of gas block.


All guns will be shot without suppressors, which increase dispersion of muzzle velocity.

Muzzles will be shot with bare threads, to avoid the risk that thread protectors or any other device might loosen or affect harmonics.

One sample of every bullet/bore condition will be fired into a water tank so bullet engraving can be examined.

Test firing shall be at 100-yard targets from a shaded bunker. If accuracy fixture is operational at time of test guns will be locked in fixture for firing. Otherwise firing will be from sandbags with a benchrest scope in LaRue QD mount. Trigger is Timney 3.5# single-stage.

Every attempt will be made to avoid firing when wind gusts appear to exceed 5mph.

Every sample string will start with a cold bore and we will attempt to fire them at the same rate so that the final barrel temperatures can be fairly compared.


For each string:

  1. Load
  2. Upper
  3. Bore condition


  1. String
  2. Start time, barrel temp, and ambient temp & humidity
  3. End time, barrel temp, and ambient temp & humidity


  1. String
  2. Shot #
  3. Chronograph #1 FPS
  4. Chronograph #2 FPS


  1. String
  2. Mean Radius with 90% confidence interval


Failures to feed, fire or cycle will be noted but corrected as quickly as possible to avoid interrupting the string.