Not sure if this glitch is well known or not but since I haven't been able to play capture the relay or extraction in weeks I forgot that this can happen.
Actually it wasn't even on a regular match of CTR or Extraction it was CTR on Cell vs Rebel.

I had expanded arsenal installed and had the Alpha Jackel as primary and the L-tag as my second so no pistols.

Every time I would grab the enemies relay no matter which weapon I had equipped, it would stay equipped and not auto swich to a pistol which is what it's supposed to go to even if you have expanded arsenal and two "primary" weapons equipped.

It sure made delivering the relay a lot easier since I could shotty or L-tag Nano's while carrying the relay lol. Also the relay instead of being held close to your "chest" in your left hand, it sort of would look like it was attached to the barrel of whatever "main" gun I was using since my left hand is holding the gun underbarrel and the relay at the same time. So there is some funny looking clipping going on between the relay and the gun.

I also had weapon pro and rapid fire, along with expanded arsenal, not sure if the first two modules along with expanded arsenal makes any difference as to triggering this glitch or not.

If some of you could test this out maybe in a private match with your buddies since well getting a public one for CTR or Extraction is nearly impossible! Let us know if it happens for you too.
I don't recall this glitch being possible pre-patches but I could be wrong.