Here is a link you might find useful. Dress up kits. Do it yourself plans, etc.
For the price of some of their kits you could come close to buying a mil sim marker.... They figured out the market for it long before those mil sim markers came out....
How about making a .50 cal replica? Or a gatling gun?
You can make your own barrels and dress them up with zilencers and ported shields like a 50 cal.