It be the sounds associated in the gun spec.
Its been a while since i delved into a gun spec, so, i can't remember all the correct terminology. However,
There is a section of text that has something similar to 'firesounds='
This will be followed by a pair of numbers. The first number is the animation frame in which the sound is triggered and the second number (after the comma) is the sound number.
So it will look something like,
fire=364,
1
reload=244,2
ironsights=400,
1
putaway=56,4
If you notice, 'fire' and 'ironsights' have the same number at the end. Change the number on ironsights to the same as your putaway sound (or another better suited sound) and you will have fixed your problem.
I hope this helps.
Unfamillia