From Claude E. Fuller, The Rifled Musket, p. 3:

The model 1842 smooth-bore percussion musket, of which there were about 167,000 on hand at the beginning of hostilities [the US Civil War], was, with the xception of the change from flintlock to percussion, exactly the same as the Model 1840 flintlock, and it still used the .69-caliber round-ball charge. It occupies the unique position of being our last regulation smooth-bore and our first regulation percussion-lock, and also enjoys the distinction of being our first regulation arm to be produced in or National armories on the completely interchangeable plan. this is confirmed by the report of H. K. Craig, Colonel of Ornance, to Jefferson Davis, Secretary of War, dated March 17, 1854....

I won't get into it, as the information covers many pages, but I heartily recommend a copy of the Dixie Gun Works Catalog. The last sixty or so pages are a wealth of information, including a list of the proofmarks of all the inspectors of US military arms and when they were made.

Randal Marcy, in his book on crossing the plains (circa 1847 -- I don't have my copy at hand) stated that the Colt revolving long gun was the best possible weapon.

Since talking guns can be as acrimonious as talking religion or politics, I'll see what I can dig up on US military firearms of the Mexican War period. Keep in mind that this will be on the issued firearms, not those carried by militia units or as "private" arms carried by individuals.