In rifling some of the guns an increasing twist was given, while in others the twist was uniform throughout the bore. The object of the increasing twist was to lessen the strain upon the gun, as the rotary motion was not started when the projectile was first put into motion, but developed as it moved down the bore. The projectiles were provided with studs which fitted into the grooves. The breech-loading guns on the polygonal system of rifling fired projectiles which were coated with lead fixed on with zinc, so that the bore of the gun was not injured by the rush of gas past the projectile as was the case in the rifled guns in which there was windage. Two systems of breech-loading were designed by Sir W. Armstrong, one being the screw system and the other known as the wedge. The projectiles invented by Major Palliser were specially designed to penetrate iron armour. Cast iron was found to be smashed against armour, wrought iron was too soft to do any damage, and steel in those days was too expensive to be of use. Major Palliser solved the difficulty by making his projectiles of chilled iron, and giving them a cylindrical shape with the pointed or ogival head. [Illustration: PROJECTILES AND CHARGES USED IN THE BRITISH NAVY. _Photograph by Stephen Cribb, Southsea._] 1. Projectile, 16·25 B.L., 1,800 lbs. 2. Charge for ” ” 960 lbs. powder. 3. Projectile, 13·5 B.L., 1,250 lbs. 4. Charge for ” ” 187½ lbs. cordite. 5. Projectile, 12-in. B.L., 850 lbs. 6. Charge for ” ” 211 lbs. cordite. 7. Projectile, 9·2 B.L., 380 lbs. 8. Charge for ” ” 103 lbs. cordite. 9. Projectile, 7·5 B.L., 200 lbs. 10. Charge for ” ” 77½ lbs. cordite. 11. Projectile, 6-in. B.L., 100 lbs. 12. Charge for ” ” 29 lbs. 13. Projectile, 5-in. B.L., 50 lbs. 14. Charge for ” ” 15. Projectile, 4·7, 45 lbs. 16. 4·7 Cylinder for Cartridge. 17. Projectile, 4-in., 25 lbs. 18. 4-in. Cylinder. 19. Projectile, 12-pounder. 20. 12-pounder Charge. 21. 12-pounder Case Shot. 22. 12- ” 8 cwt. Charge. 23. 6- ” Cartridge. 24. 3- ” ” 25. 1¼-lb. Cartridge. Experiments are often carried out to ascertain the resisting qualities of various combinations of armour offered to projectiles of varying weights and penetrative powers according to the distances at which they are fired. When iron was used for armour-plating, targets were built in duplication of those provided for the armoured ships. Both the Armstrong and Whitworth 70-pounders fired in the competition trials over 3,000 rounds, or three times the number assigned as the limit to the life of the old cast-iron smooth-bore guns. Of course, the bigger the gun the shorter the life, as a rule.