.408 extreme big bore

I was thinking of the same. They have .408 barrels in stock, so the wait would be minimal. I have a .357 ordered, but there is delay in getting the .357 barrel. Just not sure about availability of ammo, might be making your own. Mr Hollowpoint is one supplier of quality big bore ammo as is Hikalbigbore but they don’t have.408 . Hunters supply and Neilson specialty ammo are couple more but it doesn’t seem to be a big selection of retail sellers of that size, maybe I am wrong and someone will enlighten us. 
I have a few different calibers bigger and smaller but no .408 yet. I am a fan of Extreme rifles but everyone has different need/wants/taste.

In Big bore rifles there is not a lot of players and Extreme seems to have the variety and availability compared to others. There may be better made but not at the same price point or level of customer service, at least in my experience. I put about 2500 rounds thru my .457 before it developed a small leak and it is being repaired and tested in a timely manner by Extreme. Within a day of my email to them they had a shipping label sent to my email. Just to put things in perspective, I am on one big bore manufacturers build list since November 2020 and that is for 2 guns. Another took my money for 3 months, couldn’t give me answers and then I found the same rifle at an overseas retailer. I had it shipped and still made it here before I could get an answer from Air Force. They refunded my money and I am not a preferred customer. They had another rifle of mine that was leaking right out of the box the day I received it. They had it over 2 months for repair. Not a company I would recommend from my experiences.