YOU NEED MATTHELZOR'S Montana 5.56 MOD AND ALL IT'S DEPENDENCIES INSTALLED BEFORE INSTALLING THIS, THIS IS ONLY A SOUND CHANGE, AND DOES NOT CONTAIN THE ACTUAL RIFLE MOD ITSELF.

Get it here -> https://modworkshop.net/mod/19076

Notes:
Personally, the rifle shot sfx from Matthelzor's Montana 5.56 mod wasn't my cup of tea
Not saying that it's not one of my favorite weapons to use, matter of fact I love it more than the actual "Little Friend" by OVERKILL.
In short; this changes the yayo_default.ogg file to a more modern rifle shot.

Goes inside your mod_overrides folder, but only after you've installed the original Montana 5.56 mod, I cannot stress this enough.
(C:\Program Files (x86)\Steam\steamapps\common\PAYDAY 2\assets\mod_overrides****\Montana 5.56\Assets\sounds****)
Preview video:

FAQ(?) (Who asks questions anymore?):
1.) "This doesn't sound like the sound an AR-15 makes when shooting."

• You're correct! This is actually the sound from an M4A1 Carbine, recorded from Insurgency: Sandstorm

2.) What is that music in the background of the preview video?

• That is "The Only Thing They Fear Is You" from DOOM Eternal, the mod for the background music I'm using is https://modworkshop.net/mod/28998

3.) Why do you have around 11+ grenades in your grenade launcher, and an insane amount of ammo?!

• I had messed with the code to compensate for my small weewee.

4.) Did you contact the owner(s) of said mod in question about making this?

• I have not. If a request to take the mod down should happen either via Private Message or through the public comments, I shall do so as that would only be fair. --UPDATE I don't think they care since it's a very subtle aesthetic change, neat

end.