Weapon customisations may only be applied to a piece of equipment that is part of the Standard Regimental Kit or a favoured weapon of the soldier's Regiment . - OWCRB, page 190.

Since Armour, Artillery, and Mechaised Infantry (among others) start with vehicles in their Regimental Kit, can they apply weapon customisations to the weapons of the vehicles?

As examples, consider:

Apply Custom Grip to the pintile-mounted Heavy Stubber on a Leman Russ or Chimera.

Apply Fluid Action to the Autocannon of a Chimera.

Apply Extended Barrel to the Earthshaker Cannon of a Basilisk or the Battle Cannon of a Leman Russ or the Autocannon of a Chimera.

Anyone tried any such modifications? If so, how have they worked out?

Whilst the average guardsmen may modify his own gear in the field it is generally frowned upon. However as long as it doesn't decrease the fighting effectiveness of the regiment (in fact it often increases it!) then it left alone.

Vehicles are a rare commodity, unlike the countless trillions of guardsmen available. As such each armoured regiment maintains its vehicles meticulously with teams of techpriest enginseers and mechanics keeping everything operating in the field. I really can't see an armoured regiment allowing each tank crew to modify the vehicle itself, though things like the interior decorations (bobble-heads, fuzzy dice etc) and accessories (pintle mounted weapons, storage compartments etc) would be allowed. Modifying the vehicles main weapons and systems however would be severely frowned upon, and only the most veteran tank crews (that crew things like Vanquishers or super-heavies) will be overlooked when it comes to modification.

The only way i could see it being allowed is when an armour asset is individually assigned to a regiment (An infantry regiment with a single Leman Russ for example) at which point there won't be any oversight of the asset while it is attached. Of course when the fighting is done and it comes time to return it…

Kasatka said:

This is pretty much the exact opposite of the RAW. Typically only the most commonly used weapons can be modified, not the oddities picked up along the way.