I've spent a couple of hours reading records and trying to figure these out. Specifically, "Most of the names of regular army soldiers who served during the War of 1812 appear in the fifteen volumes of enlistment registers that show the names of soldiers enlisting for the period 1798 - 1815.". "These registers were compiled in the late nineteenth century by the Adjutant General and are based on a variety of original records such as muster and payrolls, inspection and descriptive rolls, and other miscellaneous records in the Adjutant General's Office.As with the compiled service records, the information on these original records was transferred to the register, so no additional information is available from exarraning the original records.Fortunately for the researcher, these registers have been microfilmed on Registers of Enlistments in the U.S. Army, 1798 - 1914 (M233, 81 rolls)." They are now on Ancestry.com.
My example at the moment that got me into this is and what, as of this momemt I am thinking, is like this: Infy = Infantry Ens = Ensign RR or RR's = Roll of Recruits Rect'g = Recruiting MoRet = Monthly Return MR = Muster Roll DR = Descriptive Roll ?? (Duty Roster??)
(Someone asked about IR which may be Inspection Record or Roll.)
1657, Crockett, Greensbury F. H. Ens[ign] 32 US Inf[antr]y Col S[tephen] E. Fotterall [Born] Del[aware] [Column heading, Enlisted] May 17 1813 R[olls of] R[ecruits] Philadelphia Nov. 22, 29, Dec. 6, 11 & 18/13, Jany 1, 29 & Feby 17/14, Rec[rui]t[in]g at Lewiston, Pa - Ordered to New Utrecht L[ong] I[sland] June 3/14 - Mo[nthly] Ret[urn] June 13/14, Joined at Sandy Hook June 6/14 - Present as 3 Lieut in Capt Davis Co Aug 5/14 - Promoted 2 Liuet. July 22 1814 Transferred to Capt Goodman's Co, Jan[uar]y 5,  Mo[nthly] Rep[ort] Feb 1/15, Present - Mo[nthly] Rep[ort] March 1/15 - Furloughed Feb[ruar]y 14/15, for 3 weeks - returned March 11/15. M[uster] R[oll] March 31, D[escrptive] R[oll] Gov[ernor]'s Island, April 1,  Mo[nthly] Ret[urn] May 1/15, Present -
This is a particular series of records. I see that others have been caught in the same sandtrap.