If you hold Aim while the Reservist Rifle reloads, it gets stuck in front of you and you cannot activate iron sights (the gun lifts a bit and the crosshair fades if you try) until you get a shot off.
IIRC, you cannot holster your weapon, either.

The reservist rifle is not an item found in the game. It is in the item lists carried over from Fallout 3 I believe - but it is not an item used in the game and so it was never finalized to be used with iron sights. Adding in unfinished items included only for use by modders without first editing them yourself can cause issues with the game as they were not intended to be used in the game and can not be found without modding or using the debug console.

Do you mean another similarly sounding gun though?

If you mean the service rifle from this game though- do you have the rapid reload perk? Also do you have any mods that affect reloading animations (such as the cowboy repeater fix)? I don't really have any solutions if it is the service rifle - but it might help them to address the issue in a patch if you could clarify.

Right. I somehow always mix this up. I don't even know why...
Anyway, it's the Cowboy Repeater. Works with the Lever-Action Shotgun as well.
Haven't tried it with every weapon, but it takes specific ones.

Oh yeah that's the reload bug. It svcks and clicking "fire" is the only thing that fixes it without a mod. There is a mod on newvegasnexus that fixes it by changing the animation (not "correct" animation but a sort of generic one that doesn't freeze your controls). The reason firing (even single clicking without actually firing the shot should do) works is because that is the default way to cancel a reload in mid-sequence. For some reason the game seems to think it is still in the reloading sequence long after it has finished (for guns that are handloaded each bullet at a time.)

This should be adressed in the next patch as it is now very well documented to occur pretty much any time you perform any action while reloading one of those weapons other than firing. Until the newest patch comes out, you can remember to click once after reloading or you can this fix:

http://newvegasnexus.com/downloads/file.php?id=36317

Note that the "script" version does not work and only the animation change one does.