For what it may be worth, here is some information [primarily his 1862 service with the Army of the West] taken from my personal use notes:

Frank Crawford Armstrong
[1862 notes]

Withdrawing from the U S service in 1861, he accompanied Col James McIntosh in the march of his force from Ark into the Indian country, & participated in the battle of Chustenahlah, in the Cherokee nation, Dec 26, 1861, in which the power of the Union chief Hopoeithleyohola was broken, serving as a volunteer aide on the stall of Col McIntosh. He next, with the rank of Lt, became AAG on the staff of Gen Ben McCulloch, his friend, Col D. H. Maury, being AG on the staff of Gen Van Dorn, commanding. In the famous battle of Elkhorn Tavern, he was with McCulloch until the latter was killed, & afterward, with Lomax & Bradfute & other fellow staff officers, went to the assistance of Gen Van Dorn, who warmly commended their services in his official report.

3/17 Van Dorn, in a communication to the War Dept, strongly urged the promotion of these experienced officers, declaring that if he could have substituted some of them for some of his highest cdrs, he could have put the enemy to utter rout. After serving a time as AG of Steen's Missouri Bgde, Armstrong was elected Col of the 3rd La Infy. Van Dorn renewed his recommendation that he be promoted to Brig-Gen, & after the army had crossed to the east of the Miss, Gen Price, having the same appreciation of the ability of the gallant young officer, appointed Armstrong to the command of all the Cav of the army of the West, giving him, with the consent of Gen Bragg, the rank of acting Brig-Gen. His energy & ability were soon manifest in the organization & increased efficiency of his command.

3/17 Hqs Trans Miss Dist Van Buren to Gen Cooper from Van Dorn-I have the honor to request that the President will be pleased to confer the rank of Col & Lt Col in the Provisional Army of the C.S., upon each of the following named officers
Capt W.R. Bradfute, Col
“ W.R. Beall “
“ Philip Stockton “
Lt L. L. Lomax Lt Col
“ M. M. (?) Kimmel “
“ F. C. Armstrong “
“ Ed. Dillon “
“ Charles Phifer “

3/22 S O #33 TM Dist-Lt Armstrong relieved from duty as IG of TM Dist

7/2 Hqs A of the W, Priceville, Gen Maury by D H Floweree to Gen Jordan-Maury requesting that Col F Armstrong of 3rd La be apptd B Gen so that I can place him in command of the Cav forces of this army & in charge of the outpost duty. There are 28 Co’s of Cav now on duty in this army. The Lt Col of the 3rd La is a most excellent officer & qualified to command it.
Gen Van Dorn applied to the President to promote Col Armstrong just before he went from here.
I see no other way of bringing up the Cav & of ensuring efficient performance of outpost duty.

7/6 Hqs 3rd La Infy, Camp near Priceville, G O #7- F C Armstrong, Brig-Gen-Having been promoted & assigned to the command of a Bgde of Cav, the undersigned, with feelings of regret, relinquishes the command of the 3rd La Infy. Well tried Veterans! distinguished not only for their daring gallantry on the battle-field, but for their soldierly & military bearing on all occasions, & the alacrity & willingness with which they have always borne the many privations & hardships they have had to undergo. Fare-well! Fellow soldiers! & remember that I will ever feel proud that I was chosen to command the Veterans of Oak Hill & Elk Horn! The pride of the Army of the West, the gallant 3rd La

7/6 Bragg orders-Col Armstrong appointed acting B Gen to report for duty w/Cav to report to Gen Price

7/7 Cav of A of the West assigned to Armstrong

8/2 Tupelo, Army of the West Hqs, Price to Jordan-I have the honor to forward to you the enclosed reports made by acting Gen Armstrong of two very successful engagements which he has had with the enemy in the vicinity of Courtland, Ala. I regret that the smallness of my Cav force & the consequent necessity of concentrating it all here have compelled me to order Armstrong to return with his command to this point. I beg leave to commend him to the favorable notice of the Gen cdg the dept & of the President.

8/2 Chattanooga, Bragg telegrams Price w/congratulations on the conduct of Gen Armstrong’s campaign-It shall be made the subject of a special dispatch to the Govt.

9/23 Armstrong tenders resignations [see next]

10/13 [over written to read 10/16]Coldwater-Armstrong to Gen Price’s AAG-Before leaving Baldwin [these letters were dated 9/23] I sent to your office my resignation as Col of 3rd La Infy-also a letter to the Sec of War requesting to be relieved from command of the Cav giving my reasons for so doing. The letters were not forwarded.
You will oblige me by forwarding them to Maj Gen Van Dorn as it is __ my desire to have them sent to the Dept & at once acted upon.

Note: 11/1 Resignation accepted-Asst Sec of War

10/16—Van Dorn issues S O #53-A of West Tenn-Holly Springs
I. Acting Gen’ls Cabell, Phifer & Armstrong are relieved from duty with their Bgdes. The senior officer present will assume command of their respective Bgdes.

10/17 Holly Springs, Hqs Army of W Tenn, Special Orders
III the appointments of Gen’ls Cabell, Phifer & Armstrong to the command of Bgdes, not having been confirmed, these officers are necessarily relieved.
The Gen cdg thanks these gentlemen for their skillful services & conspicuous gallantry on the battlefield of Corinth, & expresses his sorrow at being compelled to relieve them of their commands at this time, when the smoke of battle has barely unveiled that bloody but to them glorious field.

12/9 Grenada, Pemberton orders-Maj Armstrong announced as IG of the Army of this Dept

Post 1862 notes:
3/22/64 Hqs, Dept Ala, Miss, & E La, Demopolis, Ala -S O #82/IV- Orders Brig Gen Frank Armstrong to report to Maj Gen S D Lee. By command of Lt Gen Polk

6/4/64 Armstrong’s Cav Bgde Hqs near Dallas, Ga--# of mules req’d for ambulances [ref to Bgde organization]
1st Ms
2nd Ms
28th Ms
Ballentine’s Ms
Btry