materials

[mr fixit]

I have not been able to find a source for 17-4 tube in quantities any less than a mill run of several thousand pounds, and I've spent a great deal of time searching.

4140 isn't available in tubing either, but 4130 is. If it's annealed it's nearly as strong as titanium, 60ksi. If it's cold worked and not annealed it should be a bit stronger. At high temperatures it will be significantly stronger than titanium, but titanium will be lighter.

Heat treating 4130 roughly triples its strength. Without heat treat it can work just fine, but it's not a great choice and you'll have to end up with a heavier tube.

So, annealed 4130 is only slightly weaker than titanium. No one really argues that titanium is unfit for building cans, right?

[BinaryAndy]

So, annealed 4130 is only slightly weaker than titanium. No one really argues that titanium is unfit for building cans, right?

Compared to Grade 9 Ti that's correct, although of course it's significantly more dense than titanium so a can made from 4130 will necessarily be heavier. Grade 5 Ti is a lot stronger but it's not available in tubes.

If you can find 4130 cold drawn seamless tubes that were not annealed or normalized after being cold drawn, that's actually a good bit stronger. Nobody publishes data on freshly cold drawn tubes so I can't give you an accurate yield strength, but I am fairly certain it's north of 80ksi.

In short, yes, annealed 4130 can work quite well for a suppressor. If you want to make the lightest suppressor that you can, use titanium, 17-4 or heat treated 4000-series steel (or if you're reeeally serious, Aermet, maraging steel or 10-2-3 Ti). If you want to make the most durable suppressor that you can, use heat treated 4000-series steel or 17-4 (or if you're reeeally serious, stellite, inconel, hot-work tool steel, Greek ascoloy, etc.).

[ECCO Machine]

Maybe I'll bit the bullet and turn a 17-4 bar into tubing. Should be a lot of fun boring that one.

Man, it would be really tough to get a consistent diameter +/1 .002" or so through 8 or 9 inches of 17-4 even on my big lathe running a 1" dia. boring bar. To do 1.5" OD .030" wall, I'd probably start with 1.75" or even 2", crank the ball bearing fingers in my steady down hard and hope that keeps the chatter down going so deep. I'd run the steady for turning OD down afterward, too, leaving ~1" in the steady and dealing with that after parting off the rest by putting a slug in the ID and sinking the tube into my spindle bore, just the last bit needing turned sticking out.

In the end, though, there's just no need unless you're planning to put the thing on a belt fed MG. Thin gr. 9 tube holds up fine, even on full autos. The mechanical properties suggest that it's weaker than it proves to be in actual use, where it is less prone to deformation and tearing than other alloys of similar yield, compressive, shear, etc.

This is the 10 baffle 17-4 heat treated core plus end cap (.430" aggregate thickness) of a prototype after the thin walled 8620 steel muzzle brake failed and sent a bullet through it. Happened toward the end of the second magazine in my 800 RPM 13" .308 machine gun. The tube was .070" wall gr. 9 Ti, and it was starting to discolor from the heat, but still held together. In fact, you can see ~3/4" of the tube still on the core because I couldn't get it to unscrew with the damage to the baffles; I cut it off and re-used the shortened tube with a new, shorter baffle stack for another prototype. Some of the damage to the rear of the baffle stack happened knocking it out of the tube, but the carnage from the bullet was still something to behold:

https://imgur.com/Ov6EVsU

https://imgur.com/V2EGLrw

https://imgur.com/q826DTM

I had a similar baffle strike situation on the same MG with a 2" dia. .120" wall 304 SS tube and 17-4 end cap. The tube deformed badly and the end cap, threaded 24 pitch 3/8" deep, was blown off.