Leaderboard

Yes, that's where they're supposed to be. They should have already been installed in the forearm from the factory. BTW, they're steel washers, not rubber o-rings. There are two flat washers and one that looks bent. The 4th one is the retaining washer with the ears that fit into the channels on either side of the forearm. One flat washer goes in first, the "bent" one next, then another flat one. The one with ears goes in last and holds the other ones in the forearm. In that picture, you're already seeing damage to the forearm where the magazine cap is coming in contact with it. The magazine cap should not come in contact with the forearm at all. I would not shoot that gun until the proper pieces parts are installed. I would highly doubt that it came that way from the factory. You said it was new but I'd take a look at who you purchased it from. Just sayin'.

Before you do anything else, inspect the forearm. The spacers being referred to above are installed inside the forward opening of the forearm. They aren't seperately included in the package like the stock spacers/adjustment plates are. These spacers take up space between the rear of the barrel ring and the forward bearing surface of the forearm. They should have already been installed in the forearm. If those aren't present, when you install the barrel and forearm on the receiver, there will be an open space behind the barrel ring and in front of the bearing surface of the forearm. Putting that O-ring behind the magazine cap and in front of the barrel ring will not do anything to take up that space. The barrel ring will have no support at the rear and firing the gun with the barrel tightened in such a way could possibly damage the barrel ring, up to and including breaking it off the barrel. These are the spacers that should be installed in the forward opening of the forearm:

Is the barrel fully seated into the receiver? Are the metal spacers seen as part 16, 17, &18 on this schematic in place on the forearm? Benelli schematic

Could this help? https://freedomfightertactical.com/products/the-freedom-fighter-tactical-forend-washers-for-the-benelli-m2