The gun should be frequently and critically examined inside and out for cracks or defects, especially about the interior orifice of the vent, of which impressions should be taken after every ten shotted rounds in practice, and at the close of an action.
The instrument described on p. 16 is convenient, but by no means indispensable—any small spar, such as a boat's mast, or even the rammer handle with a curved piece of wood seized to the end, will, in expert hands, take an impression of the vent or crack equally well.
62. As the best indication of the amount of firing to which any smooth-bored gun has been exposed, when it is not otherwise known, is given by the enlargement of the vent, particular attention will be paid, in the reinspection of the guns, to this point. The standard gauge will be used to ascertain the general enlargement, and the searcher to detect defects which may have been developed in firing. Impressions are to be taken of the lower orifice of the vent with softened wax, and if they show that the vent is corroded in furrows and enlarged considerably in diameter at its junction with the bore, a permanent impression is to be taken in lead to show the conical enlargement. (See mode of taking impressions, [Arts. 35 and 36.])
63. When the number of rounds fired is not known, an estimate may be made from an examination of the vent by cylindrical gauges differing from each other by .01 of an inch passed through it. If the number is estimated, it is to be entered in red ink.
64. In all the guns of the Dahlgren pattern the vents are (.2) two-tenths of an inch in diameter. In all other guns .22 of an inch exterior, .2 of an inch on the interior.
Observation of the wear of the vent in proof firing of smooth-bored guns gives the following as the average diameter of the vent, after the undermentioned number of fires:
| No. of rounds | 100 | 200 | 300 | 400 | 500 |
| No. of rounds | .24 | .26 | .30 | .35 | .40 |
These, combined with examination of the interior orifice, will enable a very correct judgment to be formed of the probable number of fires sustained and duration of the gun.
The larger the calibre and the heavier the charge the more promptly the wear is manifested on the interior and exterior.
This enlargement does not extend very far from the lower orifice until the enlargement on the exterior has reached a diameter of .3 of an inch.