Make a gun attachment point (the thing the gun is attached to, which allows it to elevate/depress, i do not really know if it has a name >_<). In the guide i will make a tank destroyer-like 40mm gun breech.



Next up add a mantlet. Make it small so it will not stand out.



Add your cannon breech and customize it.



Now you want to grab an addon structure and place it on the RECOILING part of the cannon.



Now delete the front and back side of the cube and move it backward and make sure it covers the breech without breaking the design.



Finally, customize it so it looks like the breech of your dreams.



Tip: when the gun is fired, the addon structure will recoil back leaving a gap between it and the mantlet if it does not cover it.

If you followed the guide correctly you should have a fully recoiling custom breech (that might or might not look too large).

(Notice the gap i talked about before)

I might or might now make a video tutorial for this (i am not really that great when it comes to filming, so do not have high hopes). If you need any help comment, since i (probably not) or other people might respond !