Difference between revisions of "300BLK Test"

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(Created page with "= Objective = Determine whether precision of 300BLK subsonic ammunition can be improved. With current standard loads variations in muzzle velocity have been significant which...")
 
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= Objective =
 
= Objective =
Determine whether precision of 300BLK subsonic ammunition can be improved.
+
Improve the accuracy of 300BLK subsonic ammunition.
  
With current standard loads variations in muzzle velocity have been significant which, especially for subsonic rounds, [http://www.300blktalk.com/forum/viewtopic.php?f=140&t=86927 increases vertical dispersion].
+
The standard subsonic 300BLK load exhibits significant variation in muzzle velocity, which results in [http://www.300blktalk.com/forum/viewtopic.php?f=140&t=86927 excessive vertical dispersion].
 +
 
 +
We will test various products and hypotheses to determine whether improving internal ballistics can statistically increase the precision of subsonic 300BLK.
  
 
= Background =
 
= Background =
Line 12: Line 14:
 
* 2.12" COAL
 
* 2.12" COAL
 
According to QuickLOAD this generates MAP of 33kpsi and only 81% propellant burnt during barrel time.
 
According to QuickLOAD this generates MAP of 33kpsi and only 81% propellant burnt during barrel time.
 
= Prior tests =
 
[http://www.300blktalk.com/forum/viewtopic.php?p=817836#p817836 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 =
 
= Hypotheses =
Line 23: Line 20:
  
 
== Friction ==
 
== 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 [http://theswissriflesdotcommessageboard.yuku.com/topic/9031/hBN-again-and-this-time-Ill-leave-it#.U6BsPPldXEV Swiss Products] claim that impact-plating bullets and coating the bore results in bores that can be cleaned with a single dry patch. 
+
Variables:
 +
# Barrel alloy
 +
# Barrel condition
 +
# 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 [http://theswissriflesdotcommessageboard.yuku.com/topic/9031/hBN-again-and-this-time-Ill-leave-it#.U6BsPPldXEV Swiss Products] claim that shooting only hBN-plated bullets and bores result in bores that can be cleaned with a single dry patch.
  
hBN may not be effective against stainless bores.
+
Here we will test four variations of hBN:
 +
# 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'''''.
 +
# 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'''''.
 +
# [http://rydol.com/ 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'''''.
 +
# 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'''''.
  
hBN bullets are only effective when used in conjunction with an hBN-coated bore.
+
There seems to be a consensus that hBN is easily stripped from both ''coated'' and ''Rydol'' bores by most cleaning chemicals.
  
== Pressure ==
+
We already performed one test of plated bullets: [http://www.300blktalk.com/forum/viewtopic.php?p=817836#p817836 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
  
== Bullet friction ==
+
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.
  
== Bore condition
+
== Pressure and Accuracy ==
Rydol coating
+
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.
Rydol bore conditioning
 
  
Tubbs and [http://theswissriflesdotcommessageboard.yuku.com/topic/9031/hBN-again-and-this-time-Ill-leave-it#.U6BsPPldXEV Swiss Products] hBN impact plating
+
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.
hBN bore conditioning
 
 
 
MAP
 
 
 
Bullet type: Lapua gives reduced bearing surface and
 
 
 
Barrel metal: Stainless steel may not react to
 
  
 +
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 =
 
= Test outline =
Line 52: Line 52:
 
Our objective is to find statistically significant improvements over the standard load and untreated barrels:
 
Our objective is to find statistically significant improvements over the standard load and untreated barrels:
 
# Reduced variation of muzzle velocity.
 
# Reduced variation of muzzle velocity.
# Reduced mean radius (i.e., increased precision).
+
# Reduced mean radius (i.e., increased on-target precision).
 
# Reduced velocities or bore temperatures as evidence that any variable has reduced bullet-bore friction.
 
# Reduced velocities or bore temperatures as evidence that any variable has reduced bullet-bore friction.
  
 
Therefore we will run the following test strings:
 
Therefore we will run the following test strings:
 +
 +
== Friction Sequence ==
 
# Using standard load and 16" bbl:
 
# Using standard load and 16" bbl:
 
## Normal bullet
 
## Normal bullet
 
## Polysonic bullet
 
## Polysonic bullet
 
## (We already know that plated bullets show no improvement)
 
## (We already know that plated bullets show no improvement)
# Using standard load and 12" hBN-coated bbl (because it's easiest to coat a new barrel)
+
# Using standard load and 12" coated bbl (because it's easiest to coat a new barrel)
 
## Plated bullet
 
## Plated bullet
# Using standard load and clean 12" bbl:
+
# Using standard load and 12" clean bbl:
 
## Normal bullet
 
## Normal bullet
 
## Polysonic bullet
 
## Polysonic bullet
 
## Plated bullet
 
## Plated bullet
# Using standard load and Rydol-embedded 12" bbl:
+
# Using standard load and 12" Rydol bbl:
 
## Normal bullet
 
## Normal bullet
  
o hBN on treated bore better than regular bullets on untreated bore?
+
== Pressure/Accuracy Sequence ==
o hBN on 4140-nitride better than regular bullets?
+
# 220gr load with MAP > 45kpsi and > 99% interior burn
o Polysonic better than regular bullets on stainless or nitride barrel?
+
## 16" clean bbl
o Faster powders (on shorter barrels) give lower dispersion?
+
## 12" Rydol bbl
o Faster powders and subsonic bullets (Lapua 200gr) give lower dispersion?
+
# 200gr Lapua load with MAP > 45kpsi and > 99% interior burn
o Friction test hBN coating.  Test Lapua bullets for lower stdev of muzzle velocity.  Find good loads.
+
## 16" clean bbl
 +
## 12" Rydol bbl
  
 
== Equipment ==
 
== Equipment ==

Revision as of 16:54, 19 June 2014

Objective

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.

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.

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

Variables:

  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

  1. 220gr load with MAP > 45kpsi and > 99% interior burn
    1. 16" clean bbl
    2. 12" Rydol bbl
  2. 200gr Lapua load with MAP > 45kpsi and > 99% interior burn
    1. 16" clean bbl
    2. 12" Rydol bbl

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.