It is kinda strange that when you switch to your Backup Weapon (aka Pistol) that your Model always puts its Main Weapon on the Back before unholstering it's Pistol.
Usually when in Real Life the Need for the Backup Weapon arises (a Situation of imminent Danger combined with a Major Weapon Malfunction or unsuitability of the MW -> firing from an APCs Drivers Hatch) a Soldier lets go of its Main Weapon (so it falls into the Weapon Strap or in Worst Case - to the Ground) and unholsters his Backup as fast as Possible.
In Game this exhausting Animation sent my virtual Soldier a good 100 Times to the Ground. There is mostly not much use to the Backup Weapon at all - the only time I use it in a regular mission is if the Setting dictates it or my MW is out of Ammunition. This is not what a Backup Weapon usually is for.
There should be an Additional Mod Node for all Weapons that represents a Weapon Strap / Sling. A Player could choose from a Variation of already available Weapon Straps today. There are not only the traditional 2-Point Straps (issued with the most GI Weapons) but also more sophisticated Variants like a 1 or 3 Point Strap / Sling.
The implication in Game would be that if a Player chooses to use a Weapon Strap and switches to the Backup Weapon the Model would not play the ~3 Second long Animation but would start the unholstering Part of the Animation immediately while the MW stays on the Front Torso of the Model.
On the Balance State of this new Item: It is mostly dependent on the Weight and Length of the Main Weapon in Use. But I know from my Experiences in the Federal Armed Forces that running with my StG-77 was rather uncomfortable when it hung simply from my shoulders or dangling between my feet.
If a Player used a Sling instead of a Strap it would generally limit the Models movement Speed as long as the Main Weapon is not equipped - because they are usually not built to Shoulder a Weapon. (I do not have much experience with Weapon Slings so this is an Assumption)
For Straps you have to Press the "Weapon to Back" Key first to shoulder your Main Weapon to run at normal Pace with your Backup, while not doing so will lead to not more as fast Walking (as the Weapon hinders your Movement because it dangles around in front of your torso)
Just to clarify - I do not want that Players can instantly switch between Main and Backup Weapon like it is commonplace in most FPS but the current state of Weapon Handling is antique (to say it friendly, since the same animation and handling is more or less unchange in place since OFP) and does not represent any common practices of real Soldiers.
The attached Picture is to demonstrate the Differences between a Strap and a Sling, for those who are wondering or if I used the false terms.