Test Report No 1

The penetration of bullets into sand and the implications for range design.

R.  S.  Taylor, S.Bryce, Aintree Shooting Services Ltd, Liverpool.

Abstract :
The penetration of bullets into sand is measured and a relationship sought between penetration and parameters such as velocity, energy and momentum.

Introduction :
The use of sand traps to stop and contain bullets is common practice on shooting ranges.   Sand however, exerts substantial vertical and horizontal loads on floors and walls, presenting structural problems for its use other than outdoors or in basements.  To lessen these problems a trap was designed in which sand was piled against a lightweight ramped surface and the tests described in the paper follow on from successful preliminary tests carried out on a prototype of this trap.

Apparatus and Experimental Method

The tests were carried out at the Aintree Pistol Club, Liverpool, in March 1993 A trapezoidal box, (Fig 1), made from chipboard and plywood was used.  The front and top were open, a steel 'goalpost' strap being used across A and B to prevent the sides being forced out by the sand.  The 30º backplate also served to restrain the sides. The horizontal length of sand opposing bullet penetration was 24", this figure being determined from the previous tests. The box was filled with sand which was struck level with the top and sloping front face using a straight edge. A 6mm plywood sheet was placed on top of the box and weighted with a 9lb weight to provide some restraint to any bullets that might leave the box via the top surface.

To contain the sand that was thrown up on impact, a blanket was draped over two arms that extended forward from this top cover.  A target template was also hung from the ends of these arms (Fig 2).
A string line was run along the A side of the box to act as a baseline for measuring the horizontal position of the muzzle.
A telescopic stand was used on which the firer could rest so that the height of the muzzle could be adjusted.
Each shot was fired through a chronograph which measured the bullet velocity.

Before each shot was fired, the horizontal distance of the muzzle from the string line, and its vertical position were checked to ensure that the bullet would be travelling horizontally and parallel to the sand box.

After each shot, the velocity of the bullet was noted from the chronograph, and the back plate and top surface of the sand were checked to ensure that the bullet had not left the box.  Disturbed sand was then replaced as needed and struck level with the top and sides of the box.

After all six bullets (four in the case of the 357magnum FMJ) had been fired, the x and y co-ordinates of the bullet holes in the target were measured, and the sand was carefully scraped away until the bullets were located.  The co-ordinates of the most forward part of each bullet were measured as follows :
x - horizontally from the A side of the box.
y - vertically from a straight edge laid across the top of the box.
z - horizontally from a straight edge laid across the front edges of the box.

The sand was then replaced, a sample being taken and weighed to obtain the density and moisture content.  The bullets were kept in separate containers for subsequent weighing and measuring.

Different bullet velocities were obtained by firing ammunition as follows :

	Lower Velocities	Higher Velocities
.22	Standard ammunition / pistol	High velocity ammunition / rifle
38sp	Standard ammunition / pistol	Same ammunition / rifle
9mm	Low loaded ammunition / pistol	Higher loaded ammunition / long barrelled pistol
357/44 mag	Standard ammunition / pistol	Same ammunition / rifle
7.62	Low loaded ammunition / short barreled rifle	Higher loaded ammunition / long barreled rifle

Summary of Results :
Graphs 1 and 2 show the horizontal distance from the sloping surface - penetration (z) - of the bullets plotted against velocity in damp and dry sand respectively.  No consistent pattern was noted.  Graphs were also plotted of penetration against energy, momentum, energy/sq in csa, momentum/sq in csa, area of frictional resistance, and displaced volume of sand, but again no pattern was noted.

An estimate was made of the actual distance travelled through the sand by the bullets, this being different from z when a bullet deviated from the line along which it was fired (Appendix B).  However graphs of these figures against the same parameters still showed no consistent pattern.
The data is given in full in the Appendix C, but Tables 1 and 2 in Appendix A give a summary of average penetrations (z).
As a point of interest, the speed of sound in air is about 1100 ft/sec
The absence of a consistent pattern in the data is probably due to the varied types of bullets being used.  However, some interesting information was obtained :

Depth of bullet below top surface of sand
Bullets were fired at 4" and 8" below the top surface of the sand to see if doubling the weight of sand above the line of fire made any material difference to the bullet penetration.  Prior to the experiment it was felt that firing only 4" below the surface was to risk having bullets leave the box, literally blowing away such a shallow layer of sand.  As it transpired, this did not occur even with the 7.62 rifle rounds.
It was found that while bullets fired 4" down generally tended to penetrate further than those fired 8" down, the difference was neither marked nor consistent.

Provisional conclusion :
The weight of sand above the line of fire is not a major factor in determining bullet penetration.

Damp sand / dry sand
The water in damp (not waterlogged) sand surrounds the individual grains and the resultant surface tension gives the whole a degree of cohesiveness, i.e. the ability to sustain a surface steeper (sometimes vertical) than that attainable by the dry sand (typically about 30º).  It also holds the grains apart, resulting in the sand occupying a greater volume, with a nett reduction in density.  This phenomenon is known as bulking.
In almost every case the bullet penetration in dry sand was less than in damp sand.
The sand used was an ordinary builders’soft sand with a predominant particle size in the 125-500m range.  With some of the higher velocity bullets fired into the damp sand, an area of lighter coloured sand was noted around the nose and in the wake of the bullet in its final position.  This was presumed to be drying out of the sand caused by the heat from the bullet and loss of kinetic energy.  However, the same effect was noted with almost all the bullets when fired into dry sand. In severely mushroomed bullets the light sand was also tightly packed into the concave area behind the mushroomed head.  A sample was taken and was seen to be substantially finer than the original.  The results of a particle size analysis are shown in Appendix D.

Provisional conclusion :
Bullets penetrated further into the damp sand due to the particles being both held apart and lubricated by the water.  In the dry sand, particles, being in direct contact with one another, offered greater frictional resistance to being displaced by the bullets, resulting in particles being ground down to the smaller sizes.

Mushrooming
Mushrooming is the term given to the characteristic distortion of the bullet nose after impact with materials such as sand.
Where a bullet was fired at two velocities, mushrooming was consistently greater for bullets fired at the higher velocity.  This result might reasonably have been expected.  Mushrooming was also consistently greater for bullets fired into the dry sand as opposed to the damp, (see provisional conclusion, damp/dry sand above).

.22LR
Only in the dry sand did the increased mushrooming materially reduce penetration.
Average increase in cross sectional area

damp sand	low velocity - 2.59x	high velocity - 3.49x
dry sand	low velocity - 3.06x	high velocity - 3.75x

38sp WC
Increased mushrooming did not materially affect penetration.
Average increase in cross sectional area

damp sand	low velocity - 1.56x	high velocity - 2.53x
dry sand	low velocity - 1.97x	high velocity - 2.63x

9mm FMJ
The higher velocity bullets were mushroomed while the lower velocity ones were undamaged.  A slightly greater penetration occurred for the high velocity bullets in the damp sand.  A wider deviation from the line of fire occurred with the higher velocity bullets in dry sand.
Average increase in cross sectional area

damp sand	low velocity - none	high velocity - 1.23x
dry sand	low velocity - none	high velocity - 1.59x

357mag JSP
In both damp and dry sand, the higher velocity bullets were very badly damaged and penetration was markedly reduced as a result.
Average increase in cross sectional area

damp sand	low velocity - 2.79x	high velocity - 5.37x
dry sand	low velocity - 2.86x	high velocity - too damaged to measure

357mag FMJ
The higher velocity bullets were more extensively mushroomed while the lower velocity ones were substantially unaffected.  Penetration was not markedly affected by the mushrooming.

Average increase in cross sectional area

damp sand	low velocity - none	high velocity - 4.09x
dry sand	low velocity - none	high velocity - 4.99x

44mag JSP
In damp sand the higher velocity bullets were extensively mushroomed while in dry sand they were badly damaged.  In both cases the penetration was reduced.

Average increase in cross sectional area

damp sand	low velocity - 2.93x	high velocity - 4.69x
dry sand	low velocity - 3.52x	high velocity - too damaged to measure

45ACP FMJ
Heavy and slow moving, this was the highest penetrating bullet in both damp and dry sand.  It suffered no mushrooming in either case.

7.62 FMJ
The figures for these bullets were not plotted on the graphs due to the high velocities distorting the scale and the fact that the results, particularly with dry sand were very erratic.
There was some evidence from the trail of light coloured (crushed) sand behind the bullets that they changed direction sharply towards the end of their travel.

7.62 in damp sand :
Maximum penetration 17", minimum 4.5".
Two of the higher velocity bullets and one of the lower velocity fired at 4" down, rose to the surface of the sand but with insuffient force to mark the plywood sheet.
All bullets were squashed and bent, the jackets being pushed smoothly back from the nose of the bullets.  The inside of the curve of each bullet was polished and the outside deeply scoured.
One bullet separated from its jacket - both being squashed and bent.  The jacket was found in the immediate vicinity of the final position of the bullet.

7.62 in dry sand :
Maximum penetration 14", minimum 2"
Higher velocity bullets - Except as described below, these were all too far from their original firing lines to be identified accurately.  Three were badly broken, three badly squashed and separated, jackets again being found near to the final bullet positions.  Various fragments of lead were found, each surrounded by crushed sand.  The one shot that was identifiable was fired at 8" down and came to rest 4" left of the line of fire, 3" down and 3" in from the sloping surface of the sand.
Lower velocity bullets - one fired at 4" down rose to the surface and lifted the plywood sheet sufficiently to drop on to the table immediately by the side of the box.  One fired at 8" down came to rest 9" left of the line of fire, 4" down and resting on the sloping surface of the sand.
With the exception of the bullet that left the box, all bullets and jackets were separated, jackets being found quite close to the final bullet positions.
All bullets and jackets were squashed and bent.

1oz 12 bore Solid Shot
Fired at 8" down.  Velocity 1350ft/sec.  Penetration (z) 8".  Packed in crushed sand.  Mushroomed, with increase in cross sectional area 2.44x

223 FMJ
Three shots were fired during preliminary safety tests at a velocity of about 3300ft/sec.  Nothing left the box but apart from a badly damaged jacket found 6" in, no substantial remains were found.

Summary of Provisional Conclusions :
The calibres used were typical of those used in most UK shooting clubs.  The maximum penetration obtained was just over 17" using a 45ACP bullet.  7.62 is not normally fired indoors, but even where the erratic penetration path of these high powered bullets brought them to the surface or the side of the box, they had insufficient energy even to mark the timber and chipboard.  A horizontal depth of sand of 24" would therefore seem to be more than adequate as a design figure for an bullet trap.

No clear relationship was found between penetration, or distance travelled through the sand, and velocity, energy, momentum, energy/sq in csa, momentum/sq in csa.  This was almost certainly due to the use of different bullet shapes and constructions.

The weight of sand above the line of fire did not appear to be a major factor in determing bullet penetration.

Bullets penetrated further into the damp sand due to the particles being both held apart and lubricated by the water.  In the dry sand, particles, being in direct contact with one another, offered greater frictional resistance to being displaced by the bullets, this resistance resulting in particles being ground down to the smaller sizes.

No conclusion was reached on the separation of the 7.62 jackets and bullets.  The intuitive expectation was that the stripped copper jackets would be some way behind the much heavier lead bullets.
Factors for consideration :
Thermal expansion effect due to warm bullet entering damp sand.
Thermal expansion effect due to transfer of kinetic energy to heat energy by friction.
High radial compressive loads on bullet nose.

Suggestions for future work :
Same test using a given bullet, say 9mm or 45ACP, loaded to give a wide range of velocities.
Same test using steel jacketed bullets.

APPENDIX A

TABLE 1 - DAMP SAND
average density 80 lb/cu ft moisture content 5%

FMJ - full metal jacket - round nosed lead cored bullet with full copper alloy jacket
RNL - round nosed lead bullet
JSP - jacketed soft point - flat nosed, tapered lead bullet with copper alloy half jacket
WC - wadcutter - full diameter flat nosed lead bullet
(m) - bullet mushroomed
* damaged, see text.

TABLE 2 - DRY SAND
average density 89 lb/cu ft moisture content 0.6%

FMJ - full metal jacket - round nosed lead cored bullet with full copper alloy jacket
RNL - round nosed lead bullet
JSP - jacketed soft point - flat nosed, tapered lead bullet with copper alloy half jacket
WC - wadcutter - full diameter flat nosed lead bullet
(m) - bullet mushroomed
* damaged, see text

APPENDIX B

ESTIMATE OF DISTANCE TRAVELLED

The distance travelled by the bullets is taken as the average of AB and AC+CB

CONVERSION FACTORS
1 grain = 0.0648 grams
1 ft/sec = 0.3048 metres/sec
1 inch = 25.4 mm

APPENDIX C

APPENDIX D