The only thing that bothers me (and it bothers me for all big bore repeaters) is whether or not you will be able to get enough flow into the chamber fast enough to have a decent ROF.
Yes, that is the biggest problem. Maybe with multiple air inlets?
Oh and thanks for the tip, I will have a look at the thread again.
(HAHA!! I found the part and guess what? It was the exact same one I was looking at myself

)
You would really rather hold a 3 to 4 inch diameter gun than an over/under.
Actually, If I would do this, I would probably make the chamber 6 inch pipe. It will be more expensive, but I can make the cannon shorter and still give enough chamber volume.
Also, if I were to make a large bore repeater, I wouldn't hold it at all. I would make a stand for it.
This design should work as long as everything is sealed, but now there is no way to service the bolt if something broke.
This was addressed on the other thread. The whole point of this is so that it is cheap and easy to make. You can make the whole breach part for around $15 USD (including spring). This means that multiple parts can be bought in case something doesn't work or you mess up.
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