Christy,

The crossed rifles usually designated an infantry unit . However, it was not too unusual to see some of the Texas Mounted ( Cavalry) Volunteers ( among others) wear the crossed rifles on their slouch and soft hats or kepis if they had them ( most did not have kepis in the this theater unless they were captured ones from the Yankees). In the manual of dress for the CSA forces , crossed sabers indicated a cavalry unit, crossed rifles indicated an infantry unit and crossed cannon barrels indicated an artillery unit. There were also supposed to be color designations ; yellow -cavalry; blue- infantry; red - artillery . Some Federal infantry units wore a curved horn as a symbol of their type of command. Sometimes, the crossed rifles, sabers or cannons would have a little letter or number attached just above where the rifles, sabers. etc crossed . This usually would designate a company ( letter) or a regiment ( number). Sometimes you would see a letter above the cross and a number below the crossing. I have one that has crossed rifles with an "E" above the rifles and a "6" below the rifles. As time went on , the boys probably weren't too particular about what type of designator they wore ( if any) . In fact , in some US military units ( now and then) if a fighting man could get another unit's symbol ( preferably an enemy unit that had been vanquished) like crossed rifles or sabers and stick it on their hat or on their uniform somewhere, this kind of told the world that you were "salty" or a combat experienced veteran or that you had "been there" (in combat) . In the modern era, you usually see more of this type of thing when there are not so many officers ( brass) around or you are out on the front( if there is one) or if you are in a remote location. Senior officers usually frown on this type of show of individuality. However, during the War Between the States , when CSA soldiers were often wearing captured Union uniforms , my guess is that officers and NCO's didn't really care too much what their men wore as long as they could shoot straight and they knew who to shoot at. In fact , when I served ( not in the Civil War) , I always felt like the morale of the unit I was with was lifted if there was some old salty Marine Gunnery Sgt that carried a captured NVA , Chinese , Russian or VC weapon ( preferably an AK -47 in mid 1960's) or some other "enemy" trophy. That was a reassurance to us that this guy was probably very experienced ( first hand)in dealing with the enemy and we were all better for it.
Sorry for the long answer ;however, your GGGrandfather may have taken that symbol off of a dead enemy soldier or maybe it was the one he was issued with his unit.
Regards, Craig