I'd be interested in what the sponsored smiths here have to say but over the years I have never been asked for more than 50% up front and usually less has been the norm in my experience.
Either way, I'd be fine with paying the parts up front - did the parts cost $3,800? It's certainly possible when you include a custom action and optics which could easily be $3,800 without the stock, bbl, etc.
Now the delivery date is another matter entirely. What was the original agreement with regard to the delivery date?
*Added* On the other side of the coin; I had a custom rifle
built by Mike Bryant of Wheeler Texas. He shipped the rifle to me BEFORE he even sent out the invoice for the rifle. I called him thinking that he didn't realize that he hadn't received final payment but he laughed and said he knew it. He was just behind in his paperwork. Now that's trust!