Considering 4130 Chrome Moly tube for 556

[John_Texas]

Hey guys, it's been a few years since I checked in here. Never could login with my old screen name of John_Tx so I made a new account. Anyway...

A bit of background - I have three Form 1s of which I've built two (.30 caliber and .22), next up is to make a can for 5.56. My .30 caliber can is 316 stainless steel - 11" long and 1.5" in diameter at the barrel end. I gradually reduced the diameter to the other end due to less pressure. Just 60 degree cones, I think I have nine of those. Subsonic 300 Blackout is incredibly quiet, really really quiet. I was amazed. My .22 can is the usual aluminum with K baffles, it's extremely quiet as well.

Stainless steel was a huge PITA to machine (Griz gunsmith lathe) so I was thinking about 4130 seamless tube. I've looked over the specs for 4130 tube and I think I'll be fine. Does the following pass the sanity check? Any reason to go 1.5" OD?

Tube: 1.625 OD, 1.495" ID - bore inside ~to 1.500" which leaves 0.65" wall
Cones and spacers: 1.5" round bar (bar tolerance is +/- 0.005") Clean up the bar and bore tube to make a slip fit of the round bar

I have some stainless round bar leftover so I could make the blast chamber baffle out of SS.

Since I like long suppressors I'm going to make this 9" long. Cones will be 60*, clipped and about 1" long, still settling on a final design.

Proud owner of a new Sig ModX9 suppressor - only 13 months to get the stamp

John, Texas Hill Country

[John_Texas]

Took me a while to go through all of the advice and comments

Had my professional machinist buddy review my plan, not for efficiency of the design but for material choice and dimensions. There was a glaring problem (to him fortunately) - the 0.065" wall is too thin to thread for the end caps. My .30 cal suppressor has 0.12" wall and that worked fine for threading.

So I could bump up the wall thickness at the increase of overall weight (and $$) or TIG the end caps and stay with 0.065" wall. I would prefer a removable cap for cleaning but for this one I'm more interested in keeping the weight down so I'll chamfer the end of the tube and the end cap to create a groove for welding. After welding I'll turn the weld down and make it pretty. Also welding the end caps will save me a ton of time not having to thread the tube and end cap.

Just ordered the steel from Online Metals - I ordered enough for mistakes so it's $400 out the wallet. (The 15% off coupon code is ERN22A) Online Metals was less expensive than Aircraft Spruce which was a surprise.

John, Texas Hill Country
--I'll be in the shop--

[John_Texas]

After another consultation with my professional machinist buddy, I'm going to TIG the end caps instead of threading. This will save some weight and more importantly a bunch of time as mentioned in the above post. All of the steel has arrived but I doubt if I can get started on this for another month. I'm getting a knee joint replaced so I'll be out of action for a while .

John, Texas Hill Country
Form 1 .30 cal and .22 cal suppressor. Form 4 Sig ModX9 (9mm) suppressor
Form 1 5.56 suppressor in the ready to machine phase

[mr fixit]

Here is a build I did a bit back very similar to what you are talking about:
viewtopic.php?f=10&t=156336

4130 and 4140 that are not heat treated are really crap to machine. Very gummy and hard to get a good surface finish. You might try very sharp carbide inserts made for aluminum and lighter cuts, not sure if that will work or not.

[genpar]

Idaho towns are limited to one bar per 1,500 citizens. Liquor licenses are so hard to get that some people acquire them with the express intent of leasing them out. The mission of the Idaho State Liquor Division is to provide control over the importation, distribution, sale, and consumption of distilled spirits. idaho liquor store hours

[John_Texas]

Here is a build I did a bit back very similar to what you are talking about:
viewtopic.php?f=10&t=156336

4130 and 4140 that are not heat treated are really crap to machine. Very gummy and hard to get a good surface finish. You might try very sharp carbide inserts made for aluminum and lighter cuts, not sure if that will work or not.

Thanks, much appreciated! I have enough material to play around with so I'll see how well it machines (or doesn't.) I bought a bunch of carbide inserts so I'll have sharp tooling to use.

John - Texas Hill Country

[YugoRPK]

Ive done several with .035 walls in 4130 threaded 28TPI . I dont know what you machinist is talking about. .060 is like sewer pipe thickness.

[John_Texas]

Progress report.
My experience with 4130: yes, it is a PITA to machine - sort of. It seems about the same difficulty (machinability) as 300 series stainless steel. I have been keeping the material lubed up with TapMagic but last night I ordered a mist coolant system from Little Machine Shop (and coolant.) This should greatly help in the machining process and I can easily move this to my vertical mill when necessary.

As far as the expertise of my machinist buddy - he isn't just a machinist but was a project manager for the old Cincinnati Millicron. They made the machines that stamped out parts, giant lathes, or were used in injection molding, etc. Most of the stuff they made was so large it had to travel by railroad car. He is an expert for anything to do with metal grades and machining. So enough of that.

I have the shell (or outer tube) ready to cut to length. Blast chamber spacer is complete, I made that 0.1" wall thickness or something like that. The wall thickness is too much but I would rather err on the side of overkill. Just finished seven spacers for the cones, these are 0.5" long and 0.05" wall thickness.

Was going to start on the cones which I was putting off because I hate doing these, especially with stainless steel or now with 4130. Another consultation with my material expert buddy and he recommended I use A36 alloy - easier to machine and strong enough for the cones. I might make the first cone from 4130 and then the rest from A36. Just ordered A36 yesterday so the cones will have to wait.

Also I can work on the end caps so I hope to get everything completed except for the cones by the end of week.

[John_Texas]

Got the stock for the end caps turned down (pix below) and I rethought about my blast chamber spacer - it was way too heavy so I machined it down to 0.075", much better.

I need to cut a piece of the tube/shell and and finish turning down a practice end cap so I can experiment with Tigging the cap to the tube/shell. I'm always afraid of blowing through thin material but at least with Tig I can modulate the heat.

Hmm.. I was going to attach a pix but apparently that's not available until I have xx number of posts?

[John_Texas]

Spacers and the end caps are made, just started the first cone yesterday. I don't think A36 alloy machines any easier than 4130 though. Hope to have the cones completed in another week or two. The weather turned lousy yesterday and the shop will be too cold to work in until mid-week.

Finally pulled the trigger on a coolant system - Kool Mist from Little Machine Shop. It uses compressed air to blow a coolant mist out of a gooseneck nozzle. Holy cow! I should have bought one years ago, my tooling stays cool to the touch as well as the work piece. One gallon of coolant concentrate makes about 40 gallons of coolant, at the very slow flow rate of the mist, that gallon should last me years.

Other bonus's are a much better finish and I can take a larger cut, I usually take a 0.050 cut but now I can increase that another ten or twenty thousands. Cleanup is significantly easier then using some kind of petroleum product.

Tigging the end caps on the tube will be an interesting event. I'm going to cut a section of tube and practice with a cutoff. I think the best way to avoid blowing through tube end is to preheat the cutoff (and endcap when it's time for that) - I'll have to experiment as to how much heat to apply to the slug. Probably a Mapp torch will heat the slug enough, if not I'll have to use my oxy-acetelyne torch.

[3strucking]

Adjust the pulse setting on your welder to avoid burning through thin material.