Metallic cartridges. The South could not produce them. The North did, but even if a fully automatic weapon was designed, the black powder fouling would limit its usefulness. BTW, if one were designed, it would have been steam powered like the proposed Confederate aero plane (the inventor wanted to carpet bomb the Union armies, sink their squadron of ships and rain Greek fire on Northern cities
As to the South winning the battles up to '63, they lost Fort Donelson to Grant in '62. This made holding Kentucky and Western Tennessee untenable. When Bragg and Kirby Smith attempted to recapture Kentucky, the campaign failed when Perryville was lost. Shiloh wasn't a Confederate victory and Beauregard withdrew the army to Corinth after that battle, leaving Grant in the field. Let's not forget New Orleans which was captured by Farragut in '62. All the way out west in New Mexico, Sibley's was losing to Canby at Glorietta Pass. Missouri was lost to the Confederacy at Pea Ridge (Elkhorn Tavern) in '62 too. Turning to Virginia and Lee, his invasion of Maryland didn't result in a victory at Antietam/Sharpsburg. Along the Atlantic coast, New Bern, NC was captured. Fort Pulaski in Georgia was captured.
Here are some Confederate Victories in 1862: Front Royal where Jackson beat Banks, Seven Days Battle, Fredericksburgh, Seccessionville (James Island near Charleston, SC).
Overall, I think the high water mark for the Confederacy was 1862. Had they won Perryville, Kentucky would have been theirs. From it they could threaten Indiana, Illinois, Ohio, Pennsylvania. The North would have to garrison those states (Think political demands on the Fed gubmint from the governors and Congress) as opposed to going on the offensive. Had Pea Ridge been won, the Trans-Mississippi's resources would be available to the Confederacy. Finally, had Lee pulled off his invasion of Maryland and returned to Virginia without battle, it would have demonstrated to France and England the Confederacy's viability (like Saratoga did during the Revolution). Foreign recognition, aid and assistance would have made Union victory less likely. By 1863, it was already off the table for the European powers. England wouldn't and Napoleon III would not act without England's concurrent consent.