Yes, it can be legal to make a silencer. For everything Form-1, from silencer designs that are easily made, to filing forms with the BATF, to 3D modeling. Remember, you must have an approved BATF Form-1 to make a silencer. All NFA laws apply.
Im wanting to build a short 5.56 can that I will use occasionally on my 10.5" 5.56 SBRs. Not looking for quiet so much as blast reduction. I want to keep the can light so I wanna use Titanium where I can. My concern is the pressure in the blast chamber with such a short barrel. Tube is 1.5" OD 0.100" wall in the blast chamber area.
There are no "ex-Marines", just Marines who arent active anymore.
I would do the hoop strength math for confidence but you will be fine in fact its overkill by a large factor. The heat of a SBR is truly intense and technically Ti could be used for the entire project but inconel is the heat king. Its all about the duty use for the materials used.
The only reason after 243 years the government now wants to disarm you is they intend to do something you would shoot them for!
http://www.silencertalk.com/forum/viewtopic.php?f=10&t=79895
