SBs’ barrels and completed uppers are not designed or intended for full automatic (machine gun) or similar rates of fire.  SB’s barrels are intended for match accuracy purposes.

 

Final Report: AR15 1000 Round Full Auto Test

 

Borescope Inspection Before 1000 Round Full Auto Test

 

Untreated barrel:  M4 style barrel unmarked appears to be new and unfired.  The gas port shows material build up as a result of a drill bit breaking though into the bore and material is in a raised condition around the hole.

 

Such will cause stripping of bullet jacket metal until such time as it is shot away.  Initial accuracy will be suspect with this barrel and was brushed five times with steel brush per the requirement of government AR barrels.

 

Hard Blue treated barrel:  M4 Style barrel obviously having been treated with a proprietary treatment is identical showing the same metal around the gas port and was brushed five times with steel brush as well.

 

Barrels are M4, lightweight profile, 4140 steel.  The chambering and throating of both barrels are consistent with best production processes.

 

Borescope Inspection After 1000 Round Full Auto Test

 

Introduction:

Two barrels previously examined in new unfired condition have been subjected to 1000 round extremely hot schedule barrel testing consisting of continuous full automatic fire as mounted on a legal M16 lower receiver.  One thousand rounds in 90 minutes with very high barrel temperatures recorded via instrumentation attached to the muzzle end the barrel.   One barrel was “ treated” and the other was standard production barrel without benefit of  performance enhancing process.

 

It was reported that test firers utilized firefighter turn out gear with gloves and helmets for protection of the heat generated by this combination.

 

Barrel Examination Untreated Barrel, 3/25/2008:
The first barrel examined was the untreated barrel which showed throat erosion as would normally be seen beyond the end of the useful life of such barrels.  Rifling is missing approximately one inch in front of the chamber.  The barrel shows signs of heat checking from hot schedule firing on the lands as well as the grooves for at least half the barrel.

 

The gas port erosion on the muzzle side of the port opening has progressed to an invert V however there is anomaly in the middle of that V heretofore unknown.  In the middle of the V there is a piece of metal that has not seen throat erosion much the same as if one observed a sudden onrush of water causing erosion in a low spot and leaving a big elongated boulder laying in middle of a washed out ravine.  The remaining projection is very shiny in appearance and as stated such has never been observed on any other AR gas ports.

 

It was reported that bullets showed a high degree of yaw at close range (15 yards), this being another indicator of a shot out barrel.  Photo below.

 

 

Barrel Examination of the Hard Blue treated barrel, 3/25/08:

The second barrel examined was the “treated” barrel known as Hard Blue.  The same hot-schedule firing rate was performed and a much higher barrel temperature was recorded.  This phenomenon is considered that the barrel did not show the rapid deterioration of the throat area, and the resulting velocities remained higher, thus causing the barrel temperature to rise at a higher rate.

 

The Hard Blue barrel still shows marks from the chamber reamer in the throat.  Normally such marks are shot away in 150 to 200 rounds of firing however this barrel still has them beyond 1000 rounds of an extremely hot firing schedule.  There is but a very light trace of heat checking which is judged to exhibit between 5% and 10% of what the untreated barrel exhibits for a very short distance forward of the chamber.    This barrel still produces excellent test groups after firing and appears quite capable of being installed on another rifle and used for many more thousands of rounds.

 

The gas port on this barrel also exhibits an erosion anomaly.  As stated above, the erosion progression is normally an inverted V.  However, this barrel shows tapering erosion, starting at the top.  The inverted V has a flat, and the erosion is estimated to be about half the depth of the untreated barrel and not exhibiting roughened edges to the degree normally seen.  Refer to diagram below.

 

 

Accuracy Before to 1000 Rounds Full Auto

 

Untreated Barrel

First 10-shot Group (inches)

Second 10-shot Group (inches)

Average (inches)

Hornady 53 grain, lot # 3070357

1.35

1.16

1.26

Federal Match 69 grain, lot # 129R407

1.51

1.32

1.42

Swiss GP90, lot # PS 524

2.41

1.95

2.18

Treated Barrel

First 10-shot Group (inches)

Second 10-shot Group (inches)

Average (inches)

Hornady 53 grain, lot # 3070357

1.20

0.72

0.96

Federal Match 69 grain, lot # 129R407

1.02

1.21

1.12

Swiss GP90, lot # PS 524

2.95

2.09

2.52

 


Accuracy after to 1000 Rounds Full Auto

 

Untreated Barrel

First 10-shot Group (inches)

Second 10-shot Group (inches)

Average (inches)

Hornady 53 grain, lot # 3070357

Barrel started to keyhole bullets, @ 15 yards: approximately 820 rounds.  Barrel lost all accuracy.

Federal Match 69 grain, lot # 129R407

Swiss GP90, lot # PS 524

Treated Barrel

First 10-shot Group (inches)

Second 10-shot Group (inches)

Average (inches)

Hornady 53 grain, lot # 3070357

1.23

0.94

1.09

Federal Match 69 grain, lot # 129R407

1.57

1.76

1.67

Swiss GP90, lot # PS524

2.10

1.86

1.98

 

 

Throat Erosion Readings: before and after

 

Untreated

 

Treated

Jake Gage

4.5

Before 1000 rounds full auto

 

Jake Gage

4.0

Before 1000 rounds full auto

T.E. Gage

2.5

 

T.E. Gage

2.0

Jake Gage

Damaged gage

After 1000 rounds full auto

 

Jake Gage

Damaged gage

After 1000 rounds full auto

T.E. Gage

Went in all the way to the handle: no resistance.

 

T.E. Gage

3.0

 

Overall throat erosion notes:

  • Jake Gage damaged from dropping onto concrete floor.
  • Untreated barrel’s throat grew larger than the gage could read.
  • Treated barrel’s throat grew by 0.100”, under extreme heat and duress.
  • Treated barrel’s temp was measured several times at 624F degrees at the muzzle.  Untreated barrel’s temp was measured several times at 572F degrees.

 

Professional bore scope evaluation completed by Mark Humphreville, a High Master, a Distinguished Rifleman, Member of the Presidents Hundred, and winner of Palma Individual Trophy in 1985 and 1986.

 

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