Back to the original question...but first, please toss that book mentioned and locate a copy of "Cry Comanche". The one you referenced is not as good, mostly a plagiarizing of CC, and filled that is not historically correct. Sorry to be harsh (especially if the author reads this board...but that was how I percieved it).
I have focused on the US Army Texas in the 1850's extensively. It is a VERY underrepresented part of history and most of the information out there is incorrect. As cleared up by most, the 2nd Cavalry was formed in 1855, this is the unit R. E. Lee served in. They arrived in Dec. 1855 and served until they left in 1861, later they became the 5th Cavalry when the 2d Dragoons became the NEW 2nd Cavalry. Thus the current 2nd Cavalry (longest serving cavalry regiment) has NOTHING to do with R.E. Lee.
Next I want to clear up why in 1855 two cavalry regiments were added to the two dragoons and one mounted rifle regiments. For years people have asked why they just didn't make the 3rd and 4th Dragoons or the 3rd Dragoons and 2nd Mounted Rifles...its quite simple...Jeff Davis (the secretary of war) was a political genius! Prior to 1855 "cavalry" in US military terms was a generic term for horse soldiers, NOT A BRANCH OF SERVICE. It was applied to all horse mounted soldiers weather dragoons, hussars, uhlans, etc etc etc. If you look at all the US Manuals before the 1850's the M1840 Dragoon Saber is listed as the "Cavalry Saber", and the accouterments used by the Dragoons and the Mounted Rifles are for the "Cavalry Service". Is 1855 Congress conceded that the US needed two more mounted regiments. It passed a law authorizing the Jeff Davis and the War Department to raise two additional regiments of cavalry. The plan was that Davis could raise two more dragoon regiments, two more mounted rifle regiments, or one of each. However, because of the small old army rules of the period, he would have to appoint officers to the new regiments based on seniority, not merit. So if he raised the 3rd Dragoons, the senior LTCol. of the 1st or 2d Dragoons would have to be given command. Instead, Davis too the law and applied it literally and create a whole new branch of service with NO prior seniority and could promote ANYONE he wanted from any branch he wanted. Thus why the 2nd Cavalry had a Paymaster for CO and an Engineer for LTCol!
Next (and FINALLY to the point), arms: According to most books, they show the official order to organize the regiments and equip them. This was a dream and never really happened. The regiments organized and equipped so quickly that the arms companies could not follow suit and keep up. At least on 1855 order mentions breaking the Halls Carbine out of storage for issue to "the mounted arm" (BTW, I loved working at the US Cavalry museum)...which several companies of the 2d Dragoons took advantage of to turn in the M1847 Musketoon in exchange for older Halls! When the 1st and 2nd Cavalries were ready to take the field, only the M1847 Musketoon, the Hall's Carbine, and the new muzzle loading M1855 Rifled Carbine (not the pistol carbine) were available...as well as the M1841 Rifle know to have been carried by the Mounted Rifles and some companies of the 1st Dragoons in lieu of the hated M1847 Musketoon (come back to this in a second)
1856 inspection of Texas shows the companies of the 2nd Cavalry in Texas all caring "Rifled Carbines" or "New Rifled Musketoons". About half the regiment was inspected, the other half being in the field. Upon closer examination, the inspecting officer was probably using these terms interchangeably (since the M1855 Rifled Carbine WAS a muzzleloader) since ordnance records do not show any M1847 Musketoons rifled at US Arsenals until 1858. Also in Jeff Davis's Own, there is a reference to Fitz Lee carrying a rifled carbine with a swivel ramrod if I recall. So it would seem that the most appropriate carbine for the 2d Cavalry in 1855-56 is the M1855 Rifled Carbine (again this is NOT the pistol carbine).
Now in John Bell Hood's after action report of his battle at Devil's River, and the company inspection of his troop a year later. JBH mentions his men carrying M1851 Colt Navy Pistols and "Army Rifles", the inspection confirms his company was using M1841 Mississippi Rifles. Beginning in 1858, you start to see large issues on the M1852/3 Sharps to all mounted regiments.
FINALLY, I will share my latest information from the national archives and such. In 1855-1856 the Cavalry may NOT have carried M1851 Colt's as planned!!! Again it was specified in the original order...but as we see, that wasn't fully executed. While studying Army Belt Holster development I came across the 1855 specs and orders for the belt holster fro the cavalry regiments, worn on the LEFT side of the belt in front of the saber and drawn with the right hand. In 1857 we see the holster most commonly associated with the Civil War come into existence, this is the one worn on the right with the butt "forward" (BTW, this is NOT a left hand draw nor was is designed so you can draw your saber and pistol at the same time..it was worn behind the right hip, like a modern "hide out" pistol, drawn with your right hand between the flap and the butt). In 1858 there is a series of letters concerning holsters for the Colt Dragoons, especially those fitted with the stocks. Here is a synopsis. The arsenal says they made a holster in 1855 for the New Colts Belt Pistol and the two new cavalry regiments, in 1857 they were asked to make a new sized for the MIDDLE sized Colt (emphasis added), and now they were clarifying that they needed to make a larger holster for the older Dragoon/Army size. If there Cavalry got the M1851 Colt Navy in 1855, the what was a middle size colt in 1857? IN 1855 Colt debuted his newest creation and desperately wanted a contract for this new arm he and his partner Mr. Root had come up with...what he called his NEW MODEL BELT PISTOL (the same term used by the ordnance officer to described the first holsters). The long barreled M1855 .31 cal. Root revolver WAS smaller than the Navy (making the Navy the middle size)...and when approached by the government in 1855 for pistols, Sam Colt, an businessman, would have pushed his latest and greatest on them! Also archaeological evidence from 1st and 2nd Cavalry sites from 1855-1858 seem to have alot of .31 cal. bullets! Which means there were alot of M1849 and M1855 private purchase pistols, or the Army actually bought the Roots and by 1857 realized they were not fit for Army usage and replaced then with the M1851 and ordered new holsters!
Thus in conclusion. The 2nd U.S. Cavalry in its first year of Texas Service appears to have carried mostly M1855 Rifled Carbines and M1855 Root revolvers! (we also know from the 1856 inspection that some had the new Hardee Hat and some hat yellow trimmed M1854 Shakos...but that is a different story).
Sources include, but are not limited too:
"Firearms of the American West, 1803-1865" by Garavaglia and Worman
Numerous army and National Archives orders and documents including ordnance returns, letters from the ordnance and Quartermaster departments, and inspections of Texas posts and garrisons.
Chris Fischer
F-Troop