UPDATE for those hopeful for an M16 or AR15 in Fallout 3;
Within a day or two of making the OP, I had read many of the other requests and saw what most people were requesting, supporting, or modding; stuff like the M82 Barrett (OMG), M2 browning (Ma Deuce!), grenade launchers, lazer bazookas and other big boy super toys; you know, the exciting stuff. Unfortunately it seems that the M16, being more or less humdrum after years of FPS popularity and exploitation, is rather low on the list for professional attention.
I am not a professional, but I have been giving it my attention;
It's been a few years since I've modded anything (I cut my teeth on WOLF3D

), but I'm taking a crack at this one. I'm dead tired of every 5th raider and mutant ambushing me with what I feel should have been the American assault rifle, the AR15/M16 (which I'm pretty sure there are more of in the united states than any other assault rifle <civilian as well as military>).
That being said:
I'd like to tentatively announce that I have an M16/AR15 mod that is close to completion.
This is still a work in progress:
Here's what I've accomplished: Triumph!
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MESH -- I have successfully replaced the 3d mesh of the vanilla assault rifle with an imported 3d model of an M16 (M4 to be exact). Took longer than it should have due to problems with NifSkope and other tools writing to the newer .NIF versions and outdated plugins for my 3dmodelers.
A) It imports into the game and is playable.
B1) It fires nicely (projectiles do not exit projectilenode at strange angles from gun towards cross hair)
B2) it fires consistently (there is no sputtering when firing, animation link is done through GECK (ie: no work to be done on my end))
C) Minimal clipping errors (except for thumb on the heat-shield it's no worse than the standard models, could be corrected by scaling)
D) Magazine Reloading (magazine reloading syncs with character animation)
E) Ejecting empty shells (edited animation position to appear from correct location)
F) Charging handle animation after reload (Something I'm not likely to do anything about, would have to edit .kf

animation, whole new ball of wax).
*) Obviously any characteristic (spread, ROF, damage, ammo, ect) can be changed with the GECK easily to suit taste or balance if needed or desired. I've decided not to concern myself with this until the skin is imported completely and correctly, if at all.
Here is what I'm working on right now: Trials.
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TEXTURE - I've only been semi-successful in importing the texture. Nifskope, GECK, and F3 have some real problems viewing older (?) versions of .BMP and .TGA. The texture is there; 3dmax sees it, photoshop sees it, milkshape can see it with some modification to its settings.. MSPaint sees it! Attempts to simply open and resave to update file version have not been successful.
This stumpted me for two days, I tried .dds converters, I tried half a dozen compilers/decompilers, updated plugins, and more. I couldn't figure out why it wouldn't display in the first place. I burned out on trying over and over, and cooled the project for a day or two.
Tonight with a cooler head, finally, I think I've fixed this by copy/pasting the (outdated?) image I want .BMP/.TGA in photoshop onto/over an image file whose version is apparently visible. This sounds like a more obvious solution than it was, as I couldn't nail down why the file image was invisible or otherwise unviewable in GECK, NifSkope, and F3. Once I discovered that it appeared indeed to be some aspect of the file and not the image itself, it was simple to fix. Simple? well...hmm.. Sure. Simple. Further development could be more complicated as texture is broken into 4 or 5 pieces (as is/was the original mesh). It could also be super easy, or really complicated.
Late tonight, I'll start on tomorrow.
I have just recently been successful in applying the texture to the gun (or the part I was working with anyway). I hope to have a fully textured and imported model as early as dec 22rd, but nearly every step of this project has had every one of my time estimates wither on the vine as new complications arise. To be candid, when I first successfully imported the mesh without error I thought I had cleared the largest hurdle, then I had immediate problems importing textures which shut things (and me) down for a couple days.
The Immediate Future:
Some general information about the materials used, and;
What needs to happen before I can release this mod!:
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I cannot 3d model. If I drew a stick figure, you wouldn't recognize it, probably think I spelled 'OK' vertically.
I also cannot texture. I could rip photo images (hmmmm...), but I lack the skills (possibly tools as well) to create these.
Before I can release this mod, I will have to find another skin (mesh and texture) whose creator will permit me to use their artistic material in a non-profit mod which will be released to the nexus community with their full credit given. It's possible that the modeler of the one I'm using now would consent, but the model file has been floating around on my machine for over a year now. I wouldn't know how to contact them even if I could find out who made it. Now that I've ironed out the major kinks of the process, I can have a new skin in the game within minutes sans texture. Once I manage to workaround the texture import hiccup (which may not be needed for newer skins), I could potentially have it posted within a day.
If one of you is a modeler, are on good terms with a modeler, or have a model with skin that is released for public use for any AR style rifle, post a link or (even better) a letter of permission from the owner with their terms of distribution, I don't need to take credit for work that's done by other people.

If desired, I can post screenshots of the working M16 ingame I have right now.