[covert]

Im not familiar with that specific gun but we use the south african guns wich are notoriously hard to load, or so they say lol. i think its quite easy even with 20mm rubbers.

Here is how i do it.

hold the handle with your righ hand and grab the left rubber with your left hand.
Pull the rubber back a bit and then slid the handle against your chest while holding onto the rubber till its firmly stuck in place on your chest,

Now with your right hand free grab the right hand side rubber and you should now have both ends firmly in your grip.

Then just pull back and slide into place. As simple as that.

Remember never load a gun above water.