Now that the shotgun is here, animations could never be more fitting.
Why not seperate the components of a gun into individual .kv6 files and animate them with .ani files?
Here's my concept:

Any .kv6 starting with that weapon's name is considered a part of it, and the one with that exact name is the "centerpiece" of it. Any .kv6 starting with an underscore is ignored in third person. ie:
semi.kv6 (the above-mentioned center piece)
the below are optional for modders, but are named appropriately for easy reference:
semimag.kv6
semibolt.kv6
_semihands.kv6 (this is ignored in third person and will not show up)

semi.ani (semi.kv6's animation file; all of the parts below aside from parts with @ in their name are attached to this and thus affected by its movement)
semimag.ani
semibolt.ani
@semihands.ani (these three are supplied for the three optional .kv6 files, but there can be as many .kv6 and matching .ani files as you like)
semi3p.ani (this is the optional ani file for third person; this overwrites the default semi.ani in third person and thus looks different; this can be done with any and all .kv6 files)

An explanation of how the format of .ani files could be is: With .ini files as a reference ([name] to define an element, then specifications below), we define the animation's name and provide the data as 3D positions and rotations with delays (milliseconds? steps? frames?) at the end, along with a boolean for whether or not it tweens from the previous position.
Example:

[template for people who don't understand]
position x,position y,position z; rotation x,rotation y,rotation z; delay; tween;

[reload]
0,0,0; 0,0,0; 0; 0;
0,1,0; 0.5,0,0; 1; 1;
0,0,0; 1,0,0; 0.5; 1;
0,0,0; 0,0,0; 0.5; 1;

[fire]
0,0,0; 0,0,0; 0; 0;
0,0,-2; 0.5,0,0; 0.5; 1;
0,0,0; 0,0,0; 1; 1;

To add to this idea, these .ani files should normally be supplied for each .kv6 file (that is, every single one including sub-models like "semimag"), although if there are none, there should be default backups.

Answered questions

How will modders be able to continue modding if the animation is in .ani files?

This does not impact older models at all whatsoever. They will be treated as a single model (because they are) and will be animated with a fallback .ani file for the according weapon.

In other words, they will look the exact same and will not require porting.

OMG! OMG! OMG!
Your basicaly saying stop motion kv6 right?!

Nope. The only reason why multiple models would be needed for this idea is for different parts, each with their own animation file containing keyframe data, for the end-product model. In other words, one model for each piece of the gun, all with their own designated keyframe data files. Animated 3D sprite tables would be much harder to develop.