300BLK Test

Objective

Determine whether precision of 300BLK subsonic ammunition can be improved.

With current standard loads variations in muzzle velocity have been significant which, especially for subsonic rounds, increases vertical dispersion.

Background

The objective of subsonic ammunition is to avoid the sound signature associated with the sonic crack that occurs as bullet velocities occur Mach 1, which is typically about 1100fps. Given this upper bound on velocity the only way to optimize external and terminal ballistics is to maximize the mass of the projectile. The upper limit on mass of standard .30" jacketed lead-core bullets that fits in the AR-15 platform with standard bolt and magazines is 225gr, which in the BTHP profile is a bullet almost 1.5" long, with a bearing length of <<>>

In order to cycle reliably in the greatest variety of guns Remington has chosen to use a slower powder, A1680. The standard factory subsonic load uses:

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

According to QuickLOAD this generates MAP of 33kpsi and only 81% propellant burnt during barrel time.

Prior tests

A 100-round test compared 50 rounds handloaded with the standard specifications to 50 rounds using the same specifications but with hBN-plated bullets. From a 16" 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

Hypotheses

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.

Friction

hBN should offer higher lubricity than MoS2 or WS2. However the initial test with hBN-plated bullets showed no effect on velocity variance. In fact there is no evidence that the hBN-plated bullets in that test reduced friction. In general we expect reduced friction to result in lower muzzle velocities -- a well-known phenomenon with MoS2. None of the users or advocates of hBN consulted to date have observed their implementations of hBN to reduce muzzle velocities. The most frequently cited benefit of the proper use of hBN is reduced fouling. Experts like David Tubb and Swiss Products claim that impact-plating bullets and coating the bore results in bores that can be cleaned with a single dry patch.

hBN may not be effective against stainless bores.

hBN bullets are only effective when used in conjunction with an hBN-coated bore.

Bullet friction

== Bore condition Rydol coating Rydol bore conditioning

Tubbs and Swiss Products hBN impact plating hBN bore conditioning

MAP

Bullet type: Lapua gives reduced bearing surface and

Barrel metal: Stainless steel may not react to

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 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:

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" hBN-coated bbl (because it's easiest to coat a new barrel)
   1. Plated bullet
3. Using standard load and clean 12" bbl:
   1. Normal bullet
   2. Polysonic bullet
   3. Plated bullet
4. Using standard load and Rydol-embedded 12" bbl:
   1. Normal bullet

o hBN on treated bore better than regular bullets on untreated bore? o hBN on 4140-nitride better than regular bullets? o Polysonic better than regular bullets on stainless or nitride barrel? o Faster powders (on shorter barrels) give lower dispersion? o Faster powders and subsonic bullets (Lapua 200gr) give lower dispersion? o Friction test hBN coating. Test Lapua bullets for lower stdev of muzzle velocity. Find good loads.

Equipment

Guns

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

Test chamber length with 220SMKs: <<>>

Uppers:

1. Noveske 16" 1:7 stainless
2. AAC 12" 4140 chorme-moly nitride
3. CMMG 8" 1:7 4140 chorme-moly nitride

Measurements

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 bottom of barrel <<>>" forward of gas block.

Procedures

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.

Data

For each string:

1. Load
2. Upper
3. Bore condition

Temperature

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

Chronograph

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

Precision

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

Failures

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