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Old 11-09-2010, 12:12 AM   #21
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BKT, I am just curious as to what kind of lower and upper you are going to be dealing with, or really just how the rifle sits right now. Are you going to have any kind of weight restriction or is this gonna end up a rifle that goes straight from the back of the truck to the bench/snipers mat?

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Old 11-09-2010, 12:26 AM   #22
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No weight restrictions -- not humping this puppy around. Bench rest is the idea.

Right now, it's a generic carbine with a pencil barrel, no muzzle break, fixed handle...not my cup o' tea, but certainly functional. Fake collapsible stock, which I'd swap out with a heavier more appropriate stock, of course.

I gotta say...I love the boobs. Just...thanks.

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Old 11-09-2010, 12:33 AM   #23
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Ah, the hooters, they are there for everyone's enjoyment. I am going to start posting a different rack each week to keep things fresh, pretty PG13 though, so as to be politically correct.

Just for your info to take into consideration for your build, most of the calibers out there are offered with 24" bull barrels and will have your complete rifle weight in the 13 to 16 pound arena depending on stock, and length/size of optics.

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Old 11-09-2010, 12:36 AM   #24
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Yeah, that's fine. I'm ok with a monster.

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Old 11-09-2010, 07:05 AM   #25
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Default The math really is not that simple if you consider all of this

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Originally Posted by Dillinger View Post
To be honest, I don't know. As I said, this is the opinion of one lowly little shop rat. I don't even have my apprentice card signed off in all categories yet... LOL

In your example above, the 24" .416 barrel has a 1:11 twist, so the bullet will make a little over 2 complete revolutions in the barrel before it launches.

These barrels are Right Hand, or clockwise, twist to the rifling. That means that pressure exerted on the barrel first is going to be fighting not to spin with the direction of the round.

Extrapolate that out and you get the following:

Barrel wants to spin, can't. Barrel is free floated to the receiver, so any pressure to any handguard is going to be negated until.... The Barrel tries to spin in the receiver & can't.

At the point the torque is on the action, it can direct energy along the free float tube to the bipod, but it will sooner transfer torque to the lower receiver from the upper receiver where the energy is stored.

I am no math whiz or a scientist. There is a gal here, Sebbie, who is though and I will PM her to weigh in on this issue.

But in my mind, that pressure is going to reach those two pins in the aluminum faster than it is going to reach the bipod, and with more force as it will be closer to the source of the initial explosion.....
I am a little sleepy tonight but here is what I am reading. You want to know the torque applied to the platform. The math can be done easily by Calculus but here is the shortcut thinking.

For every action there is an equal and opposite reaction. You all know that one from your discussion. I see you are all thinking quite well in what happens mechanically but you are thinking in 2D. The bottom line is the mass ratios, mechanical advantage, and the amount of time torque is applied.

Remember the bullet has to accelerate to full velocity its speed at any given moment in the bore determines its torque. Its mass is quite small compared to just the barrel let alone to the rest of the gun. Its also at a mechanical disadvantage to the barrel being smaller in diameter and its torque reaction energy is only at it greatest right at the end where it has has the highest velocity.

The torque energy is not a constant. That is one variable. The next one is the mechanical advantage to rotational velocity with mass in the calculation. And another different points from the center as example starting from the center bore to the outside of the barrel you have different rotational velocities and mass being applied with the bullet being on the short end of the stick for mechanical and mass advantage.

I can set up the formula for anyone who wants to work the problem after measuring and weighing the mass involved to find the kinetic energy point you want during the differentiation of the formula for the time point you want.

So basically by the time the bullet torque is totally applied and running through the platform the gun mass and with the mechanical advantage of the items force is acting on torque is pretty well dissipated. Its just not applied long enough to make a big difference.

That is not to say guns don't torque much. Handguns show a much closer relationship due to smaller mass differentials. The bigger the bore the bigger the torque is also not quite true. That rotational acceleration rate and time applied has a lot to do with it. Play with a drawing compass and you will get the mechanical advantage idea. Or just weigh the gun convert it weight to grains and take the ratio with the weight of the bullet just for a ratio idea. Its too bad I can't run a CAD program for you on a post.

Hope this little bit helped with the insight. There is so much more.

Also let me know if you really want the rotational energy formula I will have to image it here since I can't get special symbols to work on this. I did try.

Sebbie
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Old 11-09-2010, 08:20 AM   #26
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^^So you have not seen the sites with all of the math done? Or the <alt> numerical keys that have all of the symbols you need to show the math? Ballistic formulas are out there for all to see and use. Most of the sites don't even require a useless math degree to follow. Just sayin...

CAD programs are easily transferred to a post or e-mail. Just sayin again...

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Old 11-09-2010, 02:50 PM   #27
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I know this doesn't apply to what you are looking for -- but if I had the funds, this is one I wouldn't mind having in my stable for those "reach out and touch" scenarios...

Note the stainless receiver?

DavidTubb.com.html

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Old 11-09-2010, 03:34 PM   #28
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Originally Posted by Squirrel_Slayer View Post
You can build a .300 WSSM AR15 and the ballistics are better than that of a .30-06, I think.
The only problem with the SSM rounds is that they generate a lot of wear and tear on the barrel, and unless you are reloading, they are expensive to feed.

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I don't doubt for a minute that the AR platform for a BMG is under-engineered. It's arguably not the perfect tool for the job. Since I won't be flinging more than 100 (if that) rounds downrange, though, is it unsuitable?

But I'm intrigued by Squirrel's suggestions, so the BMG might be moot anyway.
In that round count the honest answer is I am sure it will be fine and even at triple that, it would probably never be an issue.

Thanks to Sebbie for stopping by and adding some insight

As I have tried to express, the view of the platform being "too much" for the lower is mine and mine alone. I didn't consult a team, I don't have a degree in advanced engineering and I am no gunsmith. In just looking at the dynamics of what that round is capable of, and then looking at what platforms it is usually built upon, a thin walled aluminum lower receiver with 2 pins holding it together usually among them.

As I have said, your mileage will vary I am sure. There wouldn't be that many of them for sale if people were getting hurt by them.

This does beg more research though, and it would appear this morning is going to be a slow one.... Hmmmmmmmm
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Old 11-09-2010, 04:38 PM   #29
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J.D.-why is it that some calibers tend to be real barrel burners compared to others? Is it in relation to chamber pressures? I always thought that a .243WSSM AR would be cool to build too, but I have heard that you can only expect about 2000 rounds out of a barrel chambered for .243. It baffles me.

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Old 11-09-2010, 04:54 PM   #30
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Double S - You basically answered your own question. When you take a small size diameter round, like .243 or even a small .30 cal, and you push it at 2800fps or equivalent pressure, you are generating a tremendous amount of heat and pressure on the lands and grooves of the barrel.

All that zero to full bore energy causes excessive wear, so you see breakdown in the fine tooling of the lands and grooves and eventually accuracy suffers.

As an example:

You can load a .243 WSSM with a 55grain bullet and achieve over 4000fps in muzzle velocity. And that is a factory spec number that could be pushed.

Take a .22lr round with a 40grain standard load in comparison and you are looking at about 1200fps.

It's just a ton of energy, being spun pretty fast, with very little surface area to dissipate heat and thus you get more wear per round fired.

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