Welcome admiral29, do post your introduction in the "Introductions & Welcomes" section.
To buy a .50 calibre pistol, you will first have to get a licence for Non Prohibited Bore Handgun (NPB - basically, those calibres not used by Indian governmet agencies, including the military). Refer
http://www.indiansforguns.com/viewtopic.php?f=4&t=504 and
http://www.indiansforguns.com/viewtopic.php?f=4&t=2780
Note that you will not be allowed to import a handgun, import of firearms is restricted since the 1980s. Almost all of the imported firearms currently available in India would be from that era or before. Approach one or as many licenced gun dealers as you want, and state your requirement to them. If they have a suitable firearm for sale, or if they know of someone who is willing to sell a suitable firearm, they will let you know. Then you should inspect it and after satisfying yourselves that it is a genuine item worth whatever price is being asked, pay the price and complete the paperwork. Note that in most cases, what you pay for your dream pistol will be enough to buy a good sized (imported!) SUV or a downpayment for a good sized home. Congratulations, now you are a proud owner of your dream pistol!
Now, you will have to hunt for ammunition, limited to the quantity mentioned in your licence. The .50 is not as easily available as the .32 and a few others, so there is a real possibility that you will have bought a very expensive and intimidating looking paper weight or conversation piece.
The "law about this" is actually three - Arms Act 1959, Arms Rules 1962 and whatever the babu says. There is a fourth one being considered by our lawmakers, Arms Ammendment Bill 2010.
Now that you have some gyan, why do you want to buy a .50 calibre pistol? Why not a pistol in one of the more popular calibres in India?
And finally, some un-asked-for, but important(ish) gyan: please give appropriate headings to your topics, please post them in the correct section, and atleast make an effort to go through other threads before asking questions - you will find that most of your questions have been already answered. The forum does have a search feature to help you browse.
