Material selection & wall thickness...

[MCKNBRD]

OK, I need to draw from the fountain of knowledge that is this forum. I’ve got 2 eF1s in the queue…one for a .30 cal can and one for a .22LR can. What I’m asking here is going to be important in the design of the .22LR can more than the .30, but the calculations work the same for both, and others might benefit from the discussion, too.

Per Mongo’s Suppressor Engineering 101 post, I pulled the calculation for hoop stress:
Wall Thickness = Pressure * Inside Radius / ((Allowable Stress * Joint Efficiency) – (Pressure * Safety Margin))

The pressure I used was a calculated ‘uncorking’ pressure, in a worst-case scenario. I used SAAMI Max pressure, adjusted for case and bore volume.
Muzzle pressure = (SAAMI Max * Case Volume) / (Barrel Volume + Case Volume)
(I seriously doubt that even a 2” barrel on a .22 would result in a SAAMI max pressure getting into the blast chamber, so that adds another safety factor.)

Anyway, using ‘Yield Strength’ for the allowable stress, 1.0 for joint efficiency (boring the tube out of solid stock), and a 40% safety margin (.6*pressure), I’m getting some surprisingly thin walls for an aluminum tube on the .22LR can.

Somebody check my math, mmkay? 2” barrel on a .22LR; inside tube diameter of .84” (radius=.42); 7075-T6 Al.
The calculated muzzle pressure is 5630.137psi
7075-T6 Al yield strength is 68,000psi
(68,000*.42) / (68000-(5630.137*.6)=.0366”

Now, that tells me that I can use a .84” ID tube with an OD of .920” (standard ‘heavy barrel’ OD for 10/22s) and it be MORE than adequate.

What I can’t figure out is how come 7075 has higher yield strength than 303, 304, 316, or 416SS…I know there are other factors (corrosion resistance, yield changes at different temps, etc.), but jeez…

What am I missing here? Based on ‘common sense’, a tube made from SS should be thinner than one from Aluminum; yet, running the calculations, that doesn’t seem to be the case.

Thoughts?
Byrdman

[57fairlane]

7075 is strong stuff . . . I've made wheel stud spacers for race cars out of it that have been going on 3-4 years with no visible wear.

The problem with the really thin stuff is you can no longer get a thread in there unless you want to do a ridiculously small thread pitch. I suppose if you made a monocore that was the threaded joint (a la AAC prodigy) you could get by with a really thin wall.

[JFettig]

You'll want to add the blast chamber volume to your uncorking pressure equation. At the speed things move it might not even be that high.

[twodollarbill]

For me wall thickness is really determined by TPI used on my endcaps.
I've pretty much used 24 TPI on my last builds and they need .028" depth.
I like the tube to have atleast double the thread depth, so I've been using atleast .065" tubes.
You may like to bore open the thread area so that your baffles don't get hung up on your threads.
If you want to have a final thinner tube....you can remove some material from the OD.
You can even "step" the outer tube and give it that AAC look

[MCKNBRD]

You'll want to add the blast chamber volume to your uncorking pressure equation. At the speed things move it might not even be that high.

I intentionally kept it out; the lower pressure from adding the blast chamber volume would (theoretically...) allow it to be even thinner.

$2Bill: I definitely hear what you're saying. I'm looking at monocore vs. baffle now (for my 'big' .22 can) and this is a factor.

Assuming...of course...that I keep the threads there. Not sure why I couldn't just turn the tube w/ the endcap in it, or weld them together.

I guess I'm not that far off with thinking about how strong 7075 is...are there any downfalls to using it?

Byrdman

[57fairlane]

I guess I'm not that far off with thinking about how strong 7075 is...are there any downfalls to using it?

Byrdman

Yes,

You will no longer be able to look at 6061 the same way again.

[twodollarbill]

You will no longer be able to look at 6061 the same way again.

How true.
I like how 7075 takes the final finish pass.

[Historian]

For me wall thickness is really determined by TPI used on my endcaps.
I've pretty much used 24 TPI on my last builds and they need .028" depth.
I like the tube to have atleast double the thread depth, so I've been using atleast .065" tubes.
You may like to bore open the thread area so that your baffles don't get hung up on your threads.
If you want to have a final thinner tube....you can remove some material from the OD.
You can even "step" the outer tube and give it that AAC look

As Lou Costello, of the Abbott and Costello ( AAC ? ) team used to say, you are a
baaad boy!

<< http://www.youtube.com/watch?v=f7pMYHn-1yA >>

Very good.

[whiterussian1974]

The pressure I used was a calculated ‘uncorking’ pressure, in a worst-case scenario. I used SAAMI Max pressure, adjusted for case and bore volume.
Muzzle pressure = (SAAMI Max * Case Volume) / (Barrel Volume + Case Volume)
(I seriously doubt that even a 2” barrel on a .22 would result in a SAAMI max pressure getting into the blast chamber, so that adds another safety factor.)
Thoughts?
Byrdman

The rest of your post made sense, but I think that the correct formula would be.
Muzzle pressure = (SAAMI Max) / (Barrel Volume / Case Volume)
I could be wrong because I'm almost asleep, but I think that's the correct calculation.
Ex: .308 NATO______55kpsi/(1.51cu-in/.33) = 55kpsi/4.57 = 12kpsi
Blast Chamber = 1.5"OD x 2"L = 2.827cu-in___12kpsi / 2.827 = 4244psi assuming no gas leaves the system.
Of course, some % (muzzle brakes average 40% leakage into next chamber) will escape through the Borehole into adjoining chambers trying to fill them.

Let me know if/where I screwed up.

[MCKNBRD]

Not saying you screwed up, but I used a pressure / volume ratio:
P1*V1=P2*V2
and solved for P2:
(P1*V1) / (V2)=P2

Where P1= SAAMI Max, V1=Case Volume, and V2=volume behind bullet at the muzzle (when it uncorks)

V2 should equal the bore volume plus the volume in the case; it isn't a ratio of the bore volume to the case volume.

I think my biggest issue was looking at muzzle pressure at uncorking vs. what the pressure would be in the blast chamber, after uncorking. I'm still running some numbers and playing around with it.

Haven't gotten my stamp yet, so I've got time.

Byrdman

[whiterussian1974]

The reason that I used "Muzzle pressure = (SAAMI Max) / (Barrel Volume / Case Volume)"
was to account for propellent continuing to burn as it travels down barrel.
Not accurate, but better than "Muzzle pressure = (SAAMI Max * Case Volume) / (Barrel Volume + Case Volume)"

"P1*V1=P2*V2 solved for P2: (P1*V1) / (V2)=P2" would be appropriate IF no exothermic chemical reaction was concurrent.

I was solving for a slower burning/higher pressure powder.
"Ex: .308 NATO______55kpsi/(1.51cu-in/.33) = 55kpsi/4.57 = 12kpsi
Blast Chamber = 1.5"OD x 2"L = 2.827cu-in___12kpsi / 2.827 = 4244psi assuming no gas leaves the system.
Of course, some % (muzzle brakes average 40% leakage into next chamber) will escape through the Borehole into adjoining chambers trying to fill them."

A very rough Rule of Thumb is Muzzle P=1/6(max chamber)
Quickload gives: 35514psi => 7302psi => 4.8/1 using 180gr flatbase @ 45.27gr powder

Just factor in that ALL of the propellant products don't exit the barrel prior to bullet, blowby and boreline gas jet passing beyond the blast chamber. Then use Safety Factor 2-2.5 Hoop Strength. The REAL issue comes with chemical changes to metals at increased temp and their effects on cummulative fatigue.

These are however, negligible for .22lr.
.22lr is so low pressure that you could use 7075 for the barrel itself; if a rifled bore liner of harder, more durable and microscopically smoother material is used.

[57fairlane]

.22lr is so low pressure that you could use 7075 for the barrel itself; if a rifled bore liner of harder, more durable and microscopically smoother material is used.

Or plastic a la AR-7

[whiterussian1974]

.22lr is so low pressure that you could use 7075 for the barrel itself; if a rifled bore liner of harder, more durable and microscopically smoother material is used.

Or plastic a la AR-7

Touche