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
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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 builderssoft 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 |
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.
TABLE 1 - DAMP SAND
average density 80 lb/cu ft moisture content 5%
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(ft/sec) |
(in) |
357m)g 45ACP 38sp |
|
8 4 8 8 4 8 8 4 8 8 4 8 8 8 4 8 8 4 8 8 4 8 |
3 3 3 3 3 3 2 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 |
1383 1048 1035 967 1283 1295 1206 1609 1617 1202 1705 1701 780 776 1276 1551 1561 670 844 832 2760 2321 2307 |
14(m) 13 11 7(m) 7(m) 6(m) 10 12(m) 9(m) 11(m) 8(m) 8(m) 15 12(m) 11(m) 9(m) 9(m) 11(m) 10(m) 8* 8* 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.
TABLE 2 - DRY SAND
average density 89 lb/cu ft moisture content 0.6%
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(ft/sec) |
(in) |
22LR 357mag 357mag 45 ACP 44mag 38sp 7.62 |
38gr RNL |
8 4 8 8 4 8 8 4 8 8 4 8 8 8 4 8 8 4 8 8 |
3 3 3 3 3 3 2 2 2 3 3 3 3 3 3 3 3 3 3 3 3 |
1359 1035 1042 1006 1291 1313 1252 1629 1647 1157 1711 1687 773 1307 1622 1614 673 846 830 2321 |
12(m) 12 12 8(m) 7(m) 6(m) 5(m) 9 10(m) 8(m) 8(m) 8(m) 6(m) 15 14 8(m) 7(m) 5(m) 8(m) 9(m) 7(m) 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
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