EDIT - Nexus user NasaNhak has updated and revised this mod to make it compatible with the latest changes to the game.

Please use this version of the mod instead:
http://www.nexusmods.com/metalgearsolidvtpp/mods/269/

The downloads HERE on this mod are for an older version of the game and not likely compatible so please use NasaNHaks updated one instead.

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This mod makes the game more challenging by tweaking various parameters of the gameplay. SP only - I HIGHLY recommend you do not use ANY mods when playing the MP aspect of the game. 

Be sure to read the README file to understand all of the specific changes.

In brief this mod does the following:

• Increases enemy hearing and vision ranges approx 50%
   
• Adds additional benefits to utilizing weather and camouflage during stealth.
   
• Decreases enemy knockout time by about 50%
   
• Increases bullet damage by roughly double default.
   
• Decrease the players health and regeneration significantly to make him only slightly better than a standard soldier.
   
• Decreases ammo and item capacities by around half default.
   
• Changes the empty magazine gadget from unlimited uses to 6 uses (can be restocked by calling in a resupply)
  

PLEASE NOTE: Before installing you may want to backup your save files (you should find a local copy of your save files under \Steam\userdata\[your user number]\287700 AND Steam\userdata\[your user number]\311340 - search for 287700 and 311340 if you can't find it and back up this folder somewhere) just in case. There have been no reported problems, but it never hurts to be safe.

*311340 is actually Ground Zeroes, but for some reason Steam saves the backups here even if you never owned nor played Ground Zeroes. 

You will also need to backup your 00.dat file found here:
\Steam\steamapps\common\MGS_TPP\master\0\00.dat

Full details are in the included README

I've tested it on my game and it seems to work fine, but please be aware that MGS V modding is still relatively new and we are learning new things all the time. If you have any issues or bugs please make a post and I'll see if I can figure out what the issue is. All feedback is encouraged and accepted. Not all of it will be taken (you can't please everyone) but all of it will be considered.

NOTE: I CANNOT yet adjust anything directly pertaining to the player (his speed, stamina, armour values, climbing ability, etc. etc.) 

Thanks so much and hope you find this mod useful and fun!

PS - You can keep updated on my Metal Gear Mods by checking out my facebook page here: http://www.facebook.com/MGSMods 
Also if you play Far Cry 3 or 4 please be sure to check out my other mods here on Nexus and at https://www.facebook.com/FarCryMods
or visit my forums at http://farcrymods.freeforums.net/