It is Barry Burton's Samurai Edge. The Customized Samurai Edge is a 3-round-burst handgun unlocked by beating the game in under 5 hours, unlockable as Chris or Jill. If you mean Barry's magnum, then you must let him die. When you fight Lisa Trevor for the last time at the alter, you have a choice to give Barry back his 44. Colt Python magnum or not. If you choose not to, Lisa will come up to him and knock him down. You will get his family picture, and his Colt Python. It is a very powerful magnum, but I only suggest taking it if you are doing a speed run, because he will help you out during the Tyrant battle at the helipad. Although, if you do take it, I do advise you to save at least on .44 slug (as you cannot reload it, as 44. slugs are not found) for the Tyrant battle in the lab, because on shot will make him fall to the ground. He will still appear later at the helipad, but it shortens the battle in the lab incredibly. I would suggest using his magnum if you keep it on the Tyrant in the lab no matter what. With the remaining 5 shots, I suggest using them against Lisa in the alter, or the Tyrant battle on the helipad. Please note this is only available to Jill Valentine, as Barry only appears in Jill's story.
List of Samurai Edge models : http://residentevil.wikia.com/wiki/Beretta_92F_Custom_%27Samurai_Edge%27
Barry Burton had a heavily customized version of the 'Samurai Edge'. It fired .40 S&W caliber bullets in a 3 round burst fire mode and featured a longer barrel and compensator for increased accuracy and stability. Though never actually used by Barry, it is available to Chris and Jill as an unlockable weapon. It has unlimited ammunition. It's noteworthy that this Samurai Edge is not based on a 92FS like the other versions of the weapon, but rather a 96F model (hence the .40 caliber chambering) combined with a 93R-type firing mechanism (hence the 3-round burst).