Required; Weapon Retexture Project:
http://newvegas.nexusmods.com/mods/38285/?
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Update 2.1
Added a fully automatic version of both weapons with tweaked stats to keep balance. No armor piercing on these to help differentiate them (and balance them too). Same location.
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Update 2.0
First off an apology: I had completely forgotten to put the mesh in the file. I only had the ESP. Whoops. Hopefully this will fix it. As always, if there are any bugs let me know.

-As of right now it looks like you may need to have Weapon Retexture Project installed to get the proper texture. (At least that's what testing between my laptop and desktop is showing). If anyone can confirm this it would be appreciated so I can try to find a fix.
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If you run into any errors or have a suggestion stop by the comments and I'll try to get right on it.
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UPDATE: 1.3
-I added much better looking meshes/textures.
PLANS FOR FUTURE UPDATES:
-See about hiding spots (these rifles are a little better than the All American); they deserve something other than a cheat chest. I'll keep the stash though, I like it.
-Add them to NPC's around the wasteland.
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UPDATE: 1.2
-I added a container for the weapons with some ammo. It's still in the same place (Sniper's Nest) in a hollowed out rock just behind the shack. There are pictures for those who need them.
PLANS FOR FUTURE UPDATES:
-I'll probably try out the other meshes/textures I have for the gun and if they work upload them.
-See about hiding spots (these rifles are a little better than the All American so they deserve something other than a cheat chest. I'll keep the stash though, I like it.
-Add them to NPC's around the wasteland.
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UPDATE: 1.1
->I added a .556x45mm version of this weapon. It has slightly modified stats to reflect the 'change in barrel'. (Lighter weight, slightly less damage but higher chance to hit in VATS and less AP cost, [possibly] slightly higher crit damage]. It's next to the original and you can look at the name to see the difference (hint: the caliber is in the name).
->I also got a different model for this weapon (modder's resources FTW). It appears to be higher definition as well. It should be added this weekend provided I don't get amnesia/emp'd.
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For those who wanted a marksman carbine with no scope here you go. I gave it higher damage and innate AP.
The original owner heard rumors of a treasure lost since the great war. He found an active landmine. His legacy continues with his weapon, designed to take down heavy targets from range.
(Side note: While the real SIG 556 fires 5.56mm rounds I wanted rarer ammunition to help balance the gun)
NON-REPLACER

Stats:
Damage: 28
Weight: 7 lbs (The real 556 is around 7 lbs empty)
Crit. Damage: 28
Crit. Chance X: 3.000
Fire Rate: 3.5 (The All-American fires @ 6 RPS)

The weapon is located at the Sniper\\\'s Nest. It\\\'s location is obvious. If you haven\\\'t unlocked the location yet type:
coc slsnipersnest

If you want me to add a version with the same stats as the original leave a comment.

Model Credits:
http://newvegas.nexusmods.com/mods/47486/?tab=1&navtag=%2Fajax%2Fmoddescription%2F%3Fid%3D47486%26preview%3D&pUp=1
http://newvegas.nexusmods.com/mods/38285/?
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[old files: pre v1.3]
(micalov)
http://newvegas.nexusmods.com/mods/47069/?tab=1&navtag=%2Fajax%2Fmoddescription%2F%3Fid%3D47069%26preview%3D&pUp=1