Here's what I've found, hope it helps.
In the 1860 Marion County Census dated August 19,1860 Thomas Bullock age 23 (Born abt. 1837) is listed as a overseer living in the household of Jesse Warren, a 30 year old merchant.
T. J. Bullock (Age 24) enrolled July 16, 1861 at Corinth by Capt. Robert J. Bowen (Lawrence Rifles).
He is also found in this list of men from Marion County who enlisted in the Lawrence Rifles, 12th. Regt. Miss. Inf.
Nathan A. Lott
Dickerson R. Pope
Gabriel F. Blackburn
John W. Loe
William J. Forbes
Thomas J. Bullock *
Conveyance Book D, Marion County, Mississippi
Recorded this 22nd Day of February, 1862
J. P. Barnes, Clerk
If you compare the names in this list above with men of the same names from Company D, 7th. Regt. Miss. Infantry LWF Roster, I believe
you will find most "did not muster" or "apparently left with another company".