A rifle is a firearm which has spiral grooves cut into the surface of its bore, for the purpose of communicating a rotary motion to a projectile around an axis coinciding with the direction of its flight.

The object of this rotation is to increase the range of a projectile, by causing it to move through the air in the direction of its least resistance, and to correct the cause of deviation by distributing it uniformly around the line of flight.

Various plans have been tried to secure the safest and surest means of causing the projectile to follow the spiral grooves as it passes along the bore of a rifled piece. Those projectiles, which promise to be the most successful for heavy guns, may be ranged under two heads, viz:

1st. Those which have flanges or projections on them to fit into the grooves of the gun in loading.

The flanges are made of softer metal than the body of the projectile.

2d. Those which are constructed on an expanding principle.

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The body is generally made of cast-iron; and the expanding portion is a band or cup of some softer metal, as pewter, copper, or wrought iron, which enters the bore of the piece freely when it is loaded, but which is forced into the grooves by the discharge.

The grooves are of different forms, determined by the angle made by the tangent line at any point with the corresponding element of the bore. If the angles be equal at all points, the groove is said to be uniform. If they increase from the breech to the muzzle, the grooves are called increasing; if the reverse, decreasing grooves. The practical method of cutting grooves consists in moving a rod armed with a cutter, back and forth in the bore, and at the same time revolving it around its axis. If the velocities of translation and rotation be both uniform, the grooves will have a uniform twist; if one of the velocities be variable, the grooves will be either increasing or decreasing, depending on the relative velocities in the two directions.

Twist is the term employed by gunmakers to express the inclination of a groove at any point, and is measured by the tangent of the angle which the groove makes with the axis of the bore; and this is always equal to the circumference of the bore divided by the length of a single revolution of the spiral measured in the direction of the axis.

The most suitable inclination of grooves for a rifle cannon has not yet been determined experimentally; and consequently a wide diversity of twists is employed by different experimenters.

The following table[4] presents a synopsis of the results in case of some rifle cannon tested at Fort Monroe, Va., in 1859, by a Board composed of ordnance and artillery officers.

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The following is extracted from the report of the Board:

“The method of obtaining rifle motion in these different guns is of two kinds:

“1. Flanged projectiles entering into the grooves of the gun.

“2. Expanding projectiles, which are forced into the grooves by the action of the charge. Although the flanged projectile, when made with great precision, has given good results, as shown by the tables of firing, the extreme nicety in its fabrication, and the care and trouble to load the gun, particularly when it becomes foul by firing, seems to render it not as suitable for service as the expanding projectile.

“From the results obtained, the conclusion is inevitable that the era of smooth-bore field artillery has passed away, and that the period of the adoption of rifle cannon for siege and garrison service cannot be remote. The superiority of elongated projectiles, whether solid or hollow, with the rifle rotation, as regards economy of ammunition, extent of range, and uniformity and accuracy of effect, over the present system, is decided and unquestionable.”

The Armstrong gun, of which so much has been said, belongs to the class of breech-loading rifle-cannon. Its projectile is made of cast-iron, surrounded by two leaden rings, placed at the extremities of the cylindrical part, for the purpose of engaging the grooves, when it is forced through the bore. The great range and accuracy claimed for this projectile, are probably derived from its great length compared with its diameter; but a gun of great strength would be required to project it.

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Target 40' by 20'.

Name. Calibre. Bore. Grooves. Twist.

Diameter. Length. No. Width. Depth.

in. in. in.

Sawyer, 24-pdr. 5.862 110 6 1.5 0.25 Uniform, one turn

in 34⅓ feet.

Dimick, 32-pdr 6.4 101 6 2.0 0.2 Increasing from 0

rectan. to one turn in 62½

feet at muzzle;

twist to the right

Dr. Reed, 12-pdr. Siege 4.854 109 7 ¹/₁₄th .03 to 0.8 Increasing from 0

circum. at commencem’t

to one turn in 50

feet at muzzle.

Do. 12-pr. Field 4.636 74 7 do. do. Do. do.

Do. 32-pr. 6.425 110 3 ¹/₆th .085 to .012 Uniform, one turn

circum. circular in 40 feet.

Do. 6-pr. 3.69 103.4 3 do. .077 t0 .111 Uniform, to the

circular right, one turn

in 25 feet.

Capt. Dyer, 3-pr. 2.9 44.5 8 0.4 .05 Uniform, one turn

in 16 feet.

Do. 6-pr. bronze 3.67 57.5 16 0.5 .025 Uniform, one turn

in 19 feet.

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Legend:

(A) = Weight of Projectile.

(B) = Weight of Charge.

(D) = No. of direct hits.

(E) = No. of ricochet hits.

(F) = Angle of Elevation.

(G) = Time of flight.

Name. Weight of Gun. (A) (B) 1000 Yards.

lbs. lbs. lbs. ° ' "

Sawyer, 8822 45 5½ 15 13 2 2

Dimick, 9300 51 6 7 5 1 2 15

Dr. Reed, 5000 22 3 26 14 9 2 15

Do. 1900 15 2 48 16 3 2 3

Do. 8500 50 6 10 8 2 2 15 3

Do. 1200 12 1½ 28 18 4 2 10

Capt. Dyer, 250 9 1 28 16 5 2 25

Do. 880 14 1½ 22 11 4 2 15

Legend:

(A) = Weight of Projectile.

(B) = Weight of Charge.

(D) = No. of direct hits.

(E) = No. of ricochet hits.

(F) = Angle of Elevation.

(G) = Time of flight.

(H) = Corresponding Elevation.

Name. Weight of Gun. (A) (B) 2000 Yards. Average Range. (H) (G)

lbs. lbs. lbs. ° ' " ° ' "

Sawyer, 8822 45 5½ 119 32 17 4 30 6 4359 13½

Dimick, 9300 51 6 58 21 6 5 6½

Dr. Reed, 5000 22 3 30 5 8 4 30

Do. 1900 15 2

Do. 8500 50 6 84 19 8 5 6⅓ 3665 11 30

Do. 1200 12 1½ 52 9 5 4 45

Capt. Dyer, 250 9 1 18 4 2 5½ 7 3270 13 30 15

Do. 880 14 1½

The following is a description of the several projectiles, viz:

Sawyer’s.—Flanged projectile; elongated; entire shell coated with an alloy chiefly of lead, and has a percussion cap on small end.

Dimick’s.—Expanding shell; elongated; cup of soft metal cast on rear end of projectile.

Reed’s.—The body is of cast-iron and the expanding portion is a cup of wrought iron, which is fastened to the body by inserting it in the mould and pouring the melted metal around it.

Dyer’s.—Description nearly the same as that of Dimick’s.

Notes in this chapter

Note 1 · See Table, pages 126, 127.