Let me try this again.

Seems that this battalion served as infantry or artillery to suit the demands of the time. Here is correspondence from their compiled service records and other sources that show just that:

4/11/62 the 18th Bn was organized mostly from re-enlisted men of 1st Ga Vol Infy [Olmstead]
All 3 Capt’s carry date of rank as 4/2

5/18/64-1st Military District, Mount Pleasant, S. C., Gen Ripley to Capt. H. W. Feilden, AAG, Charleston, during my temporary absence last evening, an order was received by telegraph directing the 18th Ga Bn to be sent to the city for movement, & c. At the time the Bn was on duty, a good portion being on picket and the remainder at the guns and mortars of Btry Marion. It would have taken much time to have prepared the Bn for movement, and moreover the guns would in effect have been abandoned, unless it had been relieved by other troops, and the force at my disposal is stretched to such an extent that relief from this command is out of the question. Col Keitt telegraphed to me for instructions. I directed him to send the steamer back to the city and retain the troops. I must believe that some misapprehension exists with regard to these troops. They have been on duty as Arty nearly since the commencement of the war; have been at Btry Marion since last August, where they have been frequently engaged, and especially for the last few days, during three out of four in which there have been active engagements between the monitors and our Btrys, besides with the enemy’s Btrys on Morris Island. The withdrawal of this force is so near, rendering an important position in close proximity to the enemy’s laud and sea forces useless, that I have felt it my duty to retain the Bn until it is relieved by other and competent troops.

5/20/64 S Jones to Bragg: Your telegram of today received. The 12th, 410 men, left here on the 18th. The 18th Ga Bn, 225 men, left by passenger train yesterday. The 20th SC will leave tomorrow. Florida Bgde, 1,656 eff, expected here tomorrow, will be forwarded immediately.

S Jones to Bragg-You no doubt remember that when you were here I brought to your notice the fact that the city Btrys in Charleston were not manned, the outer Btrys and lines requiring all my force. I have not heretofore urged the Dept to send me reinforcements because I knew, to some extent at least, the pressing demand for troops in Va and North Ga, and appreciated the importance of successfully resisting the two chief armies of the enemy. But I have constantly felt and still feel the greatest anxiety for the safety of this place and Savannah. By the gallantry and good conduct of the officers, this place, under Providence, was successfully defended in the first ten days of July against an attack much more formidable than is generally supposed. The enemy’s plans were good, and if they had been carried out with more spirit and determination might well have resulted in serious disaster to us. The facilities for water transportation enabled the enemy, in a few hours, to concentrate his troops, without my knowledge, either to renew the attack on this place or attempt one on Savannah. I am, therefore, exceedingly anxious to have re-enforcements as soon as any can be sent. I desire, however, at present to bring to your especial attention the great need for instructed artillerists to man the city Btrys here. The recent success of the enemy’s navy in Mobile Bay may encourage them to attempt to run past our outer Btrys and take position in the Cooper or Ashley Rivers or both. They probably have information of the condition of our city Btrys; and, if so, it will of course encourage them to make the attempt. I have, therefore, to ask if Major Basinger’s Bn, the 18th Ga, now I believe stationed at Mattoax, on the Richmond and Danville Railroad, can be sent to this place. That Bn is well instructed in the use of heavy guns, and has had much experience in that service both here and at Savannah. With it here to man the city Btrys I do not think the enemy’s vessels could pass those Btrys. Can you not supply its place at Mattoax by a force capable of using the small guns there and guarding the bridge as well as the 18th Ga? When we have so few men well instructed in the use of heavy Arty it seems like an injudicious use of good and scarce material to keep that Bn where it is when it is so much needed here. I hope it may be found consistent with the public interest to send me Basinger’s Bn without delay, and if it cannot be sent now that it be sent as soon as it can be.

5/22/64 Arrived near Petersburg and ordered to this post in Amelia County relieving dtchmts from the 8th Va Infy

6/26/64 Mattoax, Maj Basinger to Sec of War: Your recent inquiry by telegraph in relation to the defense of Flat Creek induces me to explain myself more fully than it is usually convenient to do by that means of communication. The creek is easily fordable directly under the railroad bridge and, from what I can learn, in several other places nearby. On both sides of the bridge, and on this side of the creek, are dikes, but mainly below the bridge, which can serve as rifle-pits, and above the bridge there is a thick growth; but on the other side the ground rises somewhat steeply, so that it commands the rifle-pits. From the rifle-pits or dikes the ground rises with a gentle acclivity, affording not the least shelter for troops retiring before a superior force. It would not be easy, therefore, to hold this position against a vigorous attack made enemy under the circumstances most favorable to the defense. That it was held before was owing to the feebleness of the attack, and perhaps to the enemy’s indisposition to spare the time necessary to achieve success. As stated in my telegram, the position may be turned with great facility. Such a course would be too obvious to escape attention. It may be done on either flank, but more easily on the right. In that event a defense is impossible. Our force must at once retire or be captured or destroyed; and then there are two miles and a half to be traversed in retreat before they arrive under the guns of the works at this point. As I have no means of moving Arty this retreat would be more hazardous. For these reasons, it being conceded that the Flat Creek Bridge is scarcely less important than this, I conclude that the proper mode of defense there is the same as that adopted here, to wit, an enclosed fortification of sufficient strength to withstand, at least, a short siege. Indeed, if I may venture on an opinion, I respectfully suggest that as the best system for the whole line. The various posts so established would secure the most important points; they would have sufficient relation with each other to constitute properly one command; while in case of the approach of an enemy each would be in a degree self sustaining, at least until assistance could be sent or a diversion effected. The marauding parties of the enemy would then l)e limited to such injuries to the road as could be easily and quickly repaired. I have taken the liberty of offering these observations (discreetly, on account of the difficulty of communicating with the dept cdr, and notwithstanding the fact that I have not yet been placed formally and distinctly in command of the line, though notified that that will be done. The special order directing me to receive orders from Gen Kemper leaves me in command of this post only, while requiring me to perform such additional duties as he might impose. That additional duty consists in the command of the reserve forces along the line from the James to the Staunton, including the latter; but no command of the troops in general on the line for all purposes has yet been conferred on me, though Gen Kemper, I think, is of the contrary opinion. Certainly, no order I have yet received admits of that construction. I find this an embarrassing situation, and respectfully ask that my powers and responsibilities may be clearly defined. I need hardly say that my remarks on this head are founded entirely on what has reached me from authoritative sources in reference to my assignment to this command, and that I do not ask for anything not determined on in the proper quarter.
6/28/14 [Memorandum.] Answer. Major Basinger s views are appreciated, and will be acted on. The indefiniteness of his orders and the somewhat embarrassing nature of his position are likewise recognized, but have been allowed to continue, partly from the confidence reposed in him and partly because the President has in contemplation the assignment of a brigadier to command the whole line but has been unable yet to make a satisfactory selection. J. A. S.
6/28 [First endorsement] Engineer Bureau: The pertinence and force of Major Basinger’s views are appreciated, and it will be necessary that a small earth-work should be constructed at Flat Creek as at Mattoax bridge and a force adequate for a short siege stationed there. Take the proper measures for construction. J.A.S.

12/21/64 Major Basinger offering his Bn for consolidation into regiment if bill for consolidation of small units goes through congress. Says it would be best for the service. Says the Bn was mustered into service as infantry on 3/1/62 for the war-it consists of 3 Co’s

http://www2.lib.unc.edu/mss/inv/b/Basinger,William_Starr.html
Collection Number: 01266-z Collection Title: William Starr Basinger Papers, 1835-1932
2/23/65 trenches near Chafin’s Bluff- Extracted from Basinger’s letter to his Mother:
…There is a proposition on foot, indeed the arrangement is fully agreed on by both parties, to unite my Bn with the 60th Ga Regt. We are now waiting only for Gen Lee’s order to perfect it. When you next hear of me it may be as Lt Col of that Regt. The Bn is to go in entirely without disturbing anyone. Pearson will be Adjt. It is possible we may have to consolidate also, under the new act of Congress. If so, the arrangement will be consolidated to that extent and not more, I presume.
This is a thing I desire very much. The Regt is in Lawton’s old Bgde, the cdr of which is a very estimable man. I have taken a great liking to him…
…I forgot to tell you that we are in the division of Maj Gen Custis Lee. I have renewed my acquaintance with him and he is very kind and considerate…