tively, is almost identical, in spite of the diversity of the conditions. Thus the thermal efficiency in the region of 25%, in other words, about one quarter of the heat energy of the fuel on the one hand, or explosive on the other, is converted into mechanical work.[1]
Now we know how formidable is the difficulty of air cooling in the case of the petrol motor; the difficulty in the machine gun is augmented by the fact that the surface of the barrel on which to attach the cooling fins or gills is only about half a square foot, as compared with say two or three square feet or more in the petrol motor of equivalent output. On the other hand the condition as to temperature is not so exacting in the case of the gun barrel, and the degree of durability demanded is incomparably less.
The approximate energy account of the service rifle and ammunition is as follows:—
| Kinetic energy of bullet | 28% | |
| Friction as heat imparted to bullet | 5% | |
| Friction„ as„heat„imparted„to„ barrel | 5% | Total barrel heat 30%. |
| Heat directly imparted to barrel | 25% | |
| Kinetic and heat energy of "exhaust" powder gases | 37% | |
| 100% |
Thus thirty per cent. of the total heat equivalent of the charge is imparted to the barrel and has to be dispersed by the cooling means employed. The actual heat equivalent of the service charge is approximately 7,000 ft. pounds, therefore the barrel heat per charge represents 2,100 ft, pounds, and taking the ordinary maximum rate of continuous fire as 600 per minute—10 per sec.—we have 21,000 ft. pounds per sec. as the
- ↑ Compare § 58.
207
