First you of course need to improve the core gameplay before adding more variation for guns, but I think this would actually be a somewhat interesting concept. Also I don't know anything about which guns where used in Vietnam war or any historial facts.
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A gun which is a mixture of an assault rifle and a semiautomatic rifle.
The gun shoots in 3 round bursts, where the 3 bullets are fired with an extremely high speed, but there's a little break after the 3 bullets to shoot another 3.
The gun would have a good performance at longer ranges, but is outclassed by smg and shotgun in close combat.
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Damage:
Head: 50
Body: 20
Limbs: 15
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Advantages over smg:
+ Longer range
+ Less recoil, atleast when standing still
+ No need for crouch to get a tolerable recoil (This allows the traditional strategy of popping your head up behind a wall and shooting a little bit, the changing position and doing the same again)
+ More ammo concervative
Disadvantages over smg:
- Destroying blocks takes longer time
- Close combat is harder
- Reloading takes longer time (to add more flavour)
- Only shoots in bursts
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Advantages over semi:
+ More room for missing shots
+ Better block destruction
+ Faster RoF
Disadvantages over semi:
+ More tracers which makes stealth harder
+ No OHKO
+ Less ammo concervative
+ More recoil
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Here is an example of Type-95 in MW3 to give a little impression what kind of functionality it could have:
http://www.youtube.com/watch?v=z7wLselj0So