I was looking at the CSMC RBL 20 gauge and I own a SKB 280E 20 gauge and noticed that these two shotguns look almost identical. Was the CSMC built on the SKB model?

Back when they released it there was a lot written on The Double Gun suggesting so.

I was looking at the CSMC RBL on Gunbroker and kept looking at my 280E thinking I was looking at the same shotgun except the RBL is over $5k.

I have both. Each has its pluses and minuses. The SKB was way cheaper though, and that has to count for something.

I use the SKB as a rainy day gun and beat the hell out of it. The stock has cracks in the cheeks where they usually show up on these and I epoxied the [bleep] out of them. I also cheaped out when I needed a little extra stock length and put a flimsy slip on rubber pad on it and then used electrical tape to keep it from sliding around. I do a fair amount of rabbit hunting and like the open chokes. I have some really nice guns in 16 and 20 gauge that hover around 6 lbs, the SKB feels a little clunky compared to them. For me, this gun is a rabbit killing tool.

I don't baby the RBL when I use it but I would rather not destroy it unnecessarily. That being said, the RBL feels lighter in the hands and is a much better upland gun. The extra flexibility of having choke tubes is nice too. I enjoy grouse and woodcock hunting with it and have had some great days afield with it.
I definitely does its job if I do mine.

If you look at both shotguns they look the same down to the placement of the screws and pin holes. I think the only difference in the two is that one has screw in chokes and the other does not. Oh and you can pick up a SKB for $1500 and not 4K+. I have the SKB 20 gauge 280E with 28" barrels choked M/F. Great gun for pheasants or sporting clay's.