Bolt thrust?

[A5BLASTER]

Been looking at the 6mm turbo 40 for a bit and doing my best too find intel.

On the website 6mmar. It says that the case taper has been reduced and a few post I found about it most say that the bolt thrust has been reduced because of the reduction in case taper.

Is there some data too I'm missing somewhere to support such thinking?

And if so if AI the grendel case reduces backthrust could this same technique be used to reduce the bolt thrust when chambered for 6.5mm and not 6mm?

Don't know a ton about AI a cartridge but seems intresting.

[HokeyPokeyKid]

It does have less backwards thrust because the case walls are gripping more because they are straighter. How much I don't know, or if it's even enough to allow higher pressures.

[lazyengineer]

It does have less backwards thrust because the case walls are gripping more because they are straighter. How much I don't know, or if it's even enough to allow higher pressures.

Interesting. Just by the seat of my pants, I used to not imagine it's much. The decrease in thrust is going to be limited by the tensile strength of the casing walls, which I din't think was all that much, but now I don't know. That, and whoever designed it, still has to design it for a worst-case scenario of a greased casing in there; which means almost no decrease in bolt-thrust. I've always been of the impression that case-grip decrease on bolt-thrust was overstated, but must admit I don't actually know and haven't run the calc's myself.

Well... now I'm curious. So here, let me take a shot at this, starting with .223:

Head diameter: 0.376" = 0.188" radius
Wall thickness: 0.012" (seems variable, likely thicker near the head, but we'll start with this).
Brass Tensile strength (yield): 19600 psi (i.e. it takes that many pounds of force per square inch of brass, to start flowing - I think that's what it means at least).
Max Pressure: 62,366 PSI (for a hot one)

So, starting with the force balance:
62,366 #/in^2 * 3.1415927*(0.188-0.012)^2 in^2 = 6069 # force on the inside of the casing trying to push it toward the bolt. (as an aside, I imagine this is why the 6.5 Grendel is a lower pressure rated round, as it has more square inches inside, which will mean more force going on the bolt lugs, requiring one to lower the pressure to compensate for it).

And how much force does the brass tensile strength yield.
19,600 #/in^2 *(3.1415927*(0.188)^2 - 3.1415927(0.188-0.012)^2) = 269 # force. Which is to say, up to 269# of bolt thrust, I suppose is being taken up by the tensile strength of the cartridge casing.

Interesting, if I did that math right (which I pretty much made-up and banged out just now, so maybe I didn't). But if that's right, there's actually about a 4% decrease in bolt thrust in .223, from the casing - if it's really sticking to the wall. Didn't really think it would be that much. That might indeed be enough to start to matter.

Hey somebody speak up if I blew this.

[A5BLASTER]

It does have less backwards thrust because the case walls are gripping more because they are straighter. How much I don't know, or if it's even enough to allow higher pressures.

Wasn't thinking so much for increasing pressures, mainly along the line of could it help with bolt fatigue at pressures typically of grendel handloads.

While I have never broken a bolt or extractor, I know some of my loads are past saami of 52k, not stupid past it like 56,57 or 58k psi but more probably in the 53 to 54k psi range.

But have been thinking of doing up a 6 turbo 40 in pistol fourm for my backwoods climbing stand gun and found it interesting that the inventor and some folks that have it say the AI treatment of the case has reduced bolt thrust.

And that got me to thinking could this help bolt fatigue life with my handloads and also keep me from having to add a second caliber to the reloading bench.

Doing up cases would be simple, 6mm Redding bushing full length sizing die and seater from 6mmar and Starline basic brass.

Change the seater out for a 6.5 and change the expander out to .264 and add a correct sized bushing and bang 6.5 turbo 40 case.

But not knowing much about AI type of cases wasn't to sure if the reduced bolt thrust was true or just talk.

Still not sure if I want too do something of such nature but found it interesting and wanted to discuss it more.

[grayfox]

From what I can recollect... internal case pressure during the shot (that pressure is felt on every internal wall/bottom of the cartridge) and bolt face cross sectional area will determine the rearward thrust, not the slope angle of the shoulder or the bullet diameter. As above, you'd have to assume virtually no friction on the side walls as a worst case assumption. The Grendel has more potential thrust so that's why they limited the max pressure, all this b/c of the greater cartridge and bolt face cross sectional diameter.

...And from some earlier studies of mine on the 2 most common alloys for ammo brass, C24000 (low brass, 80-20 Cu-Zn), and Cartridge brass (C26000, 70-30 Cu-Zn), the cold work tensile and yield strengths are more like 61,000 psi/50,000 psi (C24000) and 76,000 psi/63,000 psi (C26000). Of course the mfrs could have their own alloy specs. I mention the cold work b/c the walls, case web and bottom are more likely cold work not annealed.
So if the 6mm Turbo40 has a similar case bottom dimension as the Grendel it's probably limited to thrusts like the Grrr as well.

But that's just off the top of my head, no back of the envelope calcs to go with it... (not an engineer so I could be off on this a bit).

[kmon]

Not sure how much of Ackley's writing you all have read but here is a little old reading thanks to gun Digest https://gundigest.com/ackleydownload

iA little about case taper starts about 1/4 the way through on the right hand colum 1. Tapered case design

Now I am not even going to try to check Lazyengineer on the math. At one time that would have been no problem but my math brain ain't what it used to be. I do know expect for a 10% capacity gain expect about a 4% velocity gain. The minimum taper does one thing if nothing else the cases stretch at a much slower rate, not really an inssue for the Grendel case but still that part is a fact.

[kmon]

I had wondered about a 6.5 Grendel AI and if it would chamber and cycle well in an AR with that sharp of a shoulder and minimum case taper

[A5BLASTER]

I had wondered about a 6.5 Grendel AI and if it would chamber and cycle well in an AR with that sharp of a shoulder and minimum case taper

The owner of 6mmar says not a single problem with feeding and ejection and the grendel mags work just fine.

It is a grendel case just necked down to 6mm and has the AI treatment.

Me personally I'm not so much interested in it for the powder or velocity increase. It is more of thought that I could impart a lil more pucker factor to my bolt life with the pressure of my handloads.

Admitly I couldn't find a ton of data on this round but what I did find every owner said it ran perfect. And each post said the same thing bolt thrust was reduced because of the AI treatment.

Don't know if that is from collected data are just from hearing that being said since AI a case came about many many moons before I came along lol.

[grayfox]

It might be possible that an AI case allows more powder but burns it at a lower peak, actually the MV is a function not of the peak, but of the area under the pressure curve, so a lower peak but longer-flatter curve so to speak, could give more MV and lower peak P, and a lower bolt thrust since the peak is lower. But that's all a guess on my part.

[bj139]

Interesting. Just by the seat of my pants, I used to not imagine it's much. The decrease in thrust is going to be limited by the tensile strength of the casing walls, which I din't think was all that much, but now I don't know. That, and whoever designed it, still has to design it for a worst-case scenario of a greased casing in there; which means almost no decrease in bolt-thrust. I've always been of the impression that case-grip decrease on bolt-thrust was overstated, but must admit I don't actually know and haven't run the calc's myself.

Well... now I'm curious. So here, let me take a shot at this, starting with .223:

Head diameter: 0.376" = 0.188" radius
Wall thickness: 0.012" (seems variable, likely thicker near the head, but we'll start with this).
Brass Tensile strength (yield): 19600 psi (i.e. it takes that many pounds of force per square inch of brass, to start flowing - I think that's what it means at least).
Max Pressure: 62,366 PSI (for a hot one)

So, starting with the force balance:
62,366 #/in^2 * 3.1415927*(0.188-0.012)^2 in^2 = 6069 # force on the inside of the casing trying to push it toward the bolt. (as an aside, I imagine this is why the 6.5 Grendel is a lower pressure rated round, as it has more square inches inside, which will mean more force going on the bolt lugs, requiring one to lower the pressure to compensate for it).

And how much force does the brass tensile strength yield.
19,600 #/in^2 *(3.1415927*(0.188)^2 - 3.1415927(0.188-0.012)^2) = 269 # force. Which is to say, up to 269# of bolt thrust, I suppose is being taken up by the tensile strength of the cartridge casing.

Interesting, if I did that math right (which I pretty much made-up and banged out just now, so maybe I didn't). But if that's right, there's actually about a 4% decrease in bolt thrust in .223, from the casing - if it's really sticking to the wall. Didn't really think it would be that much. That might indeed be enough to start to matter.

Hey somebody speak up if I blew this.

I checked the reasoning and the math and it seems good as long as the elastic limit of the brass is not exceeded.

[LRRPF52]

From what I can recollect... internal case pressure during the shot (that pressure is felt on every internal wall/bottom of the cartridge) and bolt face cross sectional area will determine the rearward thrust, not the slope angle of the shoulder or the bullet diameter. As above, you'd have to assume virtually no friction on the side walls as a worst case assumption. The Grendel has more potential thrust so that's why they limited the max pressure, all this b/c of the greater cartridge and bolt face cross sectional diameter.

...And from some earlier studies of mine on the 2 most common alloys for ammo brass, C24000 (low brass, 80-20 Cu-Zn), and Cartridge brass (C26000, 70-30 Cu-Zn), the cold work tensile and yield strengths are more like 61,000 psi/50,000 psi (C24000) and 76,000 psi/63,000 psi (C26000). Of course the mfrs could have their own alloy specs. I mention the cold work b/c the walls, case web and bottom are more likely cold work not annealed.
So if the 6mm Turbo40 has a similar case bottom dimension as the Grendel it's probably limited to thrusts like the Grrr as well.

But that's just off the top of my head, no back of the envelope calcs to go with it... (not an engineer so I could be off on this a bit).

Chamber shape has a huge influence on bolt thrust. The more taper there is to the walls, the more thrust there is.

A tapered chamber wall acts like a conical shape charge pushing back towards the bolt face more.

When you straighten the case taper more, you have less bolt thrust, all other things equal.

If you push a 7.62x39 to the same pressure as a 6.5 Grendel, shooting the same bullet weight, even though they have the same head diameter, the 7.62x39 will have more bolt thrust.

There are several reasons why the Russians limit the 7.62x39 working pressure to 45,000psi or less, with factory ammo being in the 40,600-43,500psi range.

I can get way more directional force by shaping an explosive charge towards something I want to cut a hole in or blow in a direction, than a non-shaped charge of more weight.

We used to do it with water impulse charges using IV bags, as well as the actual shape charges that come in crates pre-made for cutting through reinforced concrete, bunkers, walls, permafrost, ice, etc.

But yes, in cartridge design with the Ackley Improved approach using straightened case walls, bolt thrust is reduced at the same pressures as the parent cartridge. The downside to straight walls is reliability in feeding, but AI's were mainly done in bolt actions.

That said, I haven't heard of many feeding issues for hi-power shooters who shoot 6mm AR Turbo. There may be, but in the guys I know who shoot it, they haven't communicated that.

[A5BLASTER]

Chamber shape has a huge influence on bolt thrust. The more taper there is to the walls, the more thrust there is.

A tapered chamber wall acts like a conical shape charge pushing back towards the bolt face more.

When you straighten the case taper more, you have less bolt thrust, all other things equal.

If you push a 7.62x39 to the same pressure as a 6.5 Grendel, shooting the same bullet weight, even though they have the same head diameter, the 7.62x39 will have more bolt thrust.

There are several reasons why the Russians limit the 7.62x39 working pressure to 45,000psi or less, with factory ammo being in the 40,600-43,500psi range.

I can get way more directional force by shaping an explosive charge towards something I want to cut a hole in or blow in a direction, than a non-shaped charge of more weight.

We used to do it with water impulse charges using IV bags, as well as the actual shape charges that come in crates pre-made for cutting through reinforced concrete, bunkers, walls, permafrost, ice, etc.

But yes, in cartridge design with the Ackley Improved approach using straightened case walls, bolt thrust is reduced at the same pressures as the parent cartridge. The downside to straight walls is reliability in feeding, but AI's were mainly done in bolt actions.

That said, I haven't heard of many feeding issues for hi-power shooters who shoot 6mm AR Turbo. There may be, but in the guys I know who shoot it, they haven't communicated that.

Do you know if any of the guys that have used the 6mm turbo 40 has had luck running it from high cap mags?

I'm sure most target type guys are useing the 10 round mags.

Wonder how well that case type would run from a 26 round cpd mag?

[grayfox]

Now that you put it that way, it makes sense, the conical shape reflects (some of) the pressure wave back towards the bolt face, would this be what happens? that would make a difference.

[Mad Charlie]

Bolt thrust is the main reason that I like to keep my chambers clean and dry, in any of my firearms.

[kmon]

Paul did a good job explaining it and there is I think a little more to it than charge.

A little from the link I posted above on Ackley and his cartridges. This talks about the case taper and gives an example I would not do but then again Ackley was one that tested actions for strength and blew quite a few up intentionally.

zack1.JPGzack2.JPGzack3.JPG

Further in that section it goes into other aspects of the Ackley designs, shoulder angle, neck ength andcase caacity