homemade boiler feedback

OK, Ive looked at alot of designs yesterday i looked at heat master boilers, not that inpressive. www.heatmastess.com
Now I know Garn is great , but I have a question doesnt it work a lot like a rocket stove the thing I dont get is when you build a rocket stove you use firebrick why doesnt it work in a gasser? another thing is could you build a baby garn alike and just have storage tanks circulate the water while firing?
Anouther design barrel in a barrel could I use a 275 gal fuel tank inside a 800 gallon tank as a smoke dragon ? It would have a large enough fire box that I could put a firetube from half way down the bacck and acroos the top towards the front and then out the chimmney..
What about therm. crack ,,, if you circulated the water to the hx constaly would this iliminate the chances being the water wouldnt be that off temp on the retrun side