6061 vs Full-Auto .308

[Pillar]

Hey Guys,

Its been a while since I've been online, but I've come across something interesting that brought up some questions.
Not too recently, I shot a 9mm Uzi at full-auto. Afterwards, we had some coffee, and before we got back to it, I unscrewed the can and eyeballed it. It was so damn light! The guy said most 9mm auto cans are made from Al.

Now I understand a .308 is significantly more powerful than a 9mm pistol, but why cant 6061 or even 7-series be hard-coat anodized for .308 semi-auto use and the occasional burst-fire? A lot of the hunters around here make there own silencers, and those that don't want Al ones. Ok, given they don't fire off magazines a a time, their cans still last.... It's lighter, easier to machine, and quite cheaper than SS.

I'm still new to this and I appreciate any lessons.

[Fulmen]

Aluminum works fine for slow fire with rifle rounds, but unless it has steel inserts it will erode the baffles. And erosion increases with rate of fire. Where silencers are free this isn't a big deal, but if you have to wait 6 months and pay 200bucks for the stamp it quickly becomes an issue for most.
As for rate of fire it all boils down to accumulated heat. Aluminum looses strength pretty fast, so a can that will last for thousands of rounds on a bolt rifle could blow up from a single mag-dump. Even steel cans can do this, a friend of mine has blown apart at least two B&T Impulse II's with a semi AR. It took about 100-120 rounds in rapid succession (beta-mag).

[Capt. Link.]

Aluminum also suffers from metal fatigue.The more the metal expands and contracts the more fatigue sets in.It will in time just fail.Metal inserts may protect the baffles until erosion damages the metal but in the end you toss the suppressor.I have seen 50BMG's shot full auto with aluminum constructed suppressors so its not a strength issue it's a unpredictable lifespan issue.For some military groups this may be acceptable risk I'm not willing to risk a high pressure failure.If you must use it for baffles but not a tube!
Not a argument just a point of view.
-CL

[Fulmen]

All materials suffer from fatigue to some extent, not just aluminum. If they can make airplanes (the safest form of transport) from aluminum you can be pretty sure that fatigue isn't an insurmountable obstacle.

[Capt. Link.]

All materials suffer from fatigue to some extent, not just aluminum. If they can make airplanes (the safest form of transport) from aluminum you can be pretty sure that fatigue isn't an insurmountable obstacle.

Airplanes must be the number one worst choice to defend the safe usage of aluminum.
They are inspected often by professionals with imaging equipment and yet suffer catastrophic failure vs its alright because they make them in my country yet none are made in the USA the largest consumer base for suppressors in the world.Don't make me google up all the tragedies.
-CL

[Pillar]

well 6061 aircraft-grade aluminum seems to stand up just fine to the heat the turbines give off. Wonder how often they replace panels...

Hey, I'm just a noob. You guys have been doing this longer than I've been walking. I just want to build an integral can good for some semi-auto to burst fire for a .308 without having to pay that tax stamp's money in machining bits...

I guess reading your answers, I see now that I can make it work, but that's not the goal is it? A man's gotta have faith in the Can he built. So just stick to SS? Worth buying the carbide tips for 1 or 2 jobs? Or should I just go easy on the sugar and waste less ammo at the range?

[Fulmen]

none are made in the USA the largest consumer base for suppressors in the world

The US marked is shaped by the tax and paperwork, the use of high-capacity semi's and a fixation with tactical equipment, all which favors extremely rugged and long-lasting cans. Once you look past these factors everything changes. While the US might be the biggest marked I wouldn't be surprised if Scandinavia has far more suppressors per capita, they have become extremely popular with hunters in recent years. For this use the light-weight aluminum cans dominate, and while they do wear out eventually it's not a big deal as they are cheap and easy to replace. There has been a few incidents with cans failing, mainly due to mfg error, but to the best of my knowledge no significant injuries.

Fact remains that aluminum sees frequent use in many critical applications, if fatigue was such a major issue this simply wouldn't have been the case. ALL materials suffer from fatigue, no exceptions. The trick is to understand it and design accordingly, and we do understand how fatigue affects engineering materials quite well.

[silencer_kid]

you can use AL, but the dimensions will be larger than say a steel alloy can, etc.
i would make the blast chamber a steel alloy insert, the rest AL. if its a thread-on type then that end cap maybe a 7xxx AL.
when the internal AL parts die, you get new ones <-- you'll need to figure out this mystery on your own.

that aside, 6061-what? 6061-O, 6061-T6, 6061-T4, what 6061 exactly?

as for fatigue issue, it has to be accounted for. AL used in auto mode of high powered rounds, will likely get hot enough to cause alignment to be off quite a bit, so the internals have to address that. vs an iconel can which will remain closer to spec at those high temps, etc.

1) know the application/use requirements
2) know the service life expectancy
3) choose materials wisely
4) make design accordingly

and remember, if you have to make the internals "bigger" than a counterpart, the dbSPL will be affected, how much so dunno unless you test both, etc.

[Fulmen]

Just to be clear, I do not advocate the use of aluminum for high rates of fire. It's doable, but requires much more of the design and might not save any weight as the strength-to-weight-ratio drops below that of SS at appr. 500F.

[noisecatcher]

As a previously certified A&P mechanic I didn't seen much 6061 aluminum in a high heat zone you will see stainless, inconel, Titanium and the like.

I was wondering what everyone thought of say 2024-T351 strictly as spacers not baffles and you would almost have to anodize it to slow corrosion.

[silencer_kid]

for spacers only, i would choose the AL that had best machinability.

[Fulmen]

I don't think fatigue resistance is that important for spacers, the outer tube should take up the bulk of the radial stresses. So the major loading should be compressive which is less of a problem. That being said I don't think 2024 is a poor choice as long as it's for moderate use. For full auto center fire rifle I don't think aluminum has any advantages over stainless.

[noisecatcher]

I just have some 2024 sitting below the lathe vs. Buying other material.

[Pillar]

I don't think fatigue resistance is that important for spacers, the outer tube should take up the bulk of the radial stresses. So the major loading should be compressive which is less of a problem. That being said I don't think 2024 is a poor choice as long as it's for moderate use. For full auto center fire rifle I don't think aluminum has any advantages over stainless.

Not even 8" of spacer weight? Well, I'd probably mill out some slots in the spacers to save weight, if I did indeed use any. But I think for the added weight (In a can where the baffles are welded to the outer tube), a stainless can would give one a lot more reassurance, not to mention consistent performance, over and aluminum can that does have spacers.

I guess what it boils down to is: In case like yours, where silencers require tax, is it worth spending extra by buying the necessary cutting tools and inserts to work stainless, or would you just stick to 6/7-series and just anodize it yourself, knowing the you might have to make another in case you go bonkers at the range and empty a clip or seven?

[tmc4065]

6061 is a joke for anything needing to be strong in any direction . . . several alloys carrying 2-3x the strength, aren't much more expensive, and machine just as well if not leave a better surface finish.

2024, 7050, 7075, 7068 etc . . .


6061 is great for taking up space but under heat and compression it falls short unless you want to make the baffles/core with considerable thickness at the detriment to decreased volume inside of your can.

[Fulmen]

6061 is a joke for anything needing to be strong in any direction . . . several alloys carrying 2-3x the strength

Really? 2024T351 has only 15% higher yield strength than 6061T6. 70xx can indeed achieve twice the yield strength, but it is more expensive and harder to find for many. But 3x? I'd like to see some references for that claim.

While not perfect 60xx is a good, general purpose alloy. If I could get 70xx I would prefer that for critical parts, but I've made several cans from 6063 without any issues.

[fastfire]

viewtopic.php?f=10&t=136176

http://api.viglink.com/api/click?format ... t_calc.htm

[CMV]

6061 is best left for making fixtures & widgets. 2024 & 7075 are superior flavors.

7075-T651 vs 2024-T3 vs 6061-T651

Brinell hardness 150 vs 120 vs 95
Ultimate Tensile Strength 83,000 vs 70,000 vs 45,000 psi
Tensile Yield Strength 73,000 vs 50,000 vs 40,000 psi
Fatigue Strength 23,000 vs 20,000 vs 14,000 psi
Shear Strength 48,000 vs 41,000 vs 30,000 psi
Specific Strength/strength-to-weight 196 vs 177 vs 115 kN-m/kg

6061 is 50% machinability while 2024 & 7075 are 70% compared to 1212 steel. They definitely drill much, much better. 7075 & 2024 round bar is only slightly more expensive at online metals. By all means use whatever you want, but for me it would be hard to justify 6061 over other commonly available AL alloys. I do remember reading members (Enfield maybe?) only having 6061 available in his country.

[Rich V]

Yes 7075 is the better alloy to use but try finding 7075 seamless tubing.

[CMV]

First google result seems like a hit http://www.twmetals.com/7075-drawn-aluminum-tubing.html Never shopped for 7075 tube, but it is most likely very uncommon. Regardless, availability doesn't change the alloy's properties and there are many other good materials that are commonly available that are much better suited for outer tube applications than 6061. 2024 tube is available.

We buy a lot from TK (Thyssen owns online metals) at work and our rep is very helpful. I could ask him to quote some hard-to-find materials if enough people were interested in doing a group buy.

[Rich V]

First google result seems like a hit http://www.twmetals.com/7075-drawn-aluminum-tubing.html Never shopped for 7075 tube, but it is most likely very uncommon. Regardless, availability doesn't change the alloy's properties and there are many other good materials that are commonly available that are much better suited for outer tube applications than 6061. 2024 tube is available.

We buy a lot from TK (Thyssen owns online metals) at work and our rep is very helpful. I could ask him to quote some hard-to-find materials if enough people were interested in doing a group buy.

I would pay dearly to get some 17-4 seamless tubing, something around 1.5in OD x .065 wall