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Old 11-08-2010, 08:53 PM   #11
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ZEL Tactilite .416 Barret AR upper, goes for around 2,200.00. There you go. Nearly all of the force from the recoil goes into the buttstock and then your shoulder, I wouldn't think that the upper will chew through lowers. All the lower does on one of these big bore conversions is house the hammer and trigger mechanism along with the pistol grip. You nearly strip all of the regular guts out. No need for any magazine release garbage, can't use a magazine. And all of the buffer tube stuff comes out because the rifle is now a bolt action. Run a buffer tube and fill it with some lead to deaden some of the recoil and I think the lower should live just fine. In theory...

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Old 11-08-2010, 08:57 PM   #12
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ZEL Tactilite .416 Barret AR upper, goes for around 2,200.00. There you go. Nearly all of the force from the recoil goes into the buttstock and then your shoulder, I wouldn't think that the upper will chew through lowers. All the lower does on one of these big bore conversions is house the hammer and trigger mechanism along with the pistol grip. You nearly strip all of the regular guts out. No need for any magazine release garbage, can't use a magazine. And all of the buffer tube stuff comes out because the rifle is now a bolt action. Run a buffer tube and fill it with some lead to deaden some of the recoil and I think the lower should live just fine. In theory...
Pricey but very cool. Thanks!
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Old 11-08-2010, 08:59 PM   #13
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bkt - I don't have any evidence to support my position to be flat out honest.

We ordered one of the bolt action style uppers into the shop about 2 years ago. We fitted it to a stock A2 lower, with the fixed plastic stock and we sat it on the counter.

At the same time we had a 33" long 1.5" diameter bull barrel benchrest .50BMG that was in for some fine tuning before B.R. Season sitting on a wood stock with a 4" wide tracking base that probably weighed in the 50# category.

I looked at those two guns and just sat there for a minute thinking it through in my mind's eye.

When that big bastard round gets touched off on the benchrest gun, the whole thing is going to slide backwards on the tracking sled to help ease the recoil. The action was this monster hex design and it was mated up to the wood stock with some serious action screws, along with the back of the receiver being deep inside the "meat" of that wood stock.

And then we had a 30-something inch, monster steel upper with this big friggin tank style break on it mounted to an aluminum AR lower by two pins and that big assed action was sitting behind some plastic in the form of a stock.

Now, what a lot folks don't know, or don't think about, is that there are several factors that happen when a round goes off.

First is the initial back pressure from the explosion of the gunpowder. Everyone expects that.

But almost immediately after that, there is a SERIOUS amount of torque that is applied in the form of that bullet being propelled at force into a set of lands and grooves that cause it to start rifling.

That is the same type of torque as when you are drilling something and as you get to the end of the cut and the drill bit binds up and the drill wants to jump out of your hand and take your arm with it.

That is not lateral, front to back, pressure or force. That is a twisting, turning, wanting to spin that upper receiver like a top, sort of pressure.

And that pressure is being countered by a couple of pins that go through a piece of aluminum.

Now, I am sure people a lot smarter than me probably graduated from MIT and looked at all these forces and compared specs and weights and measures and potential until the cows came home.

But I personally just don't think that is a recipe that I would want to trust.

Your Mileage WILL vary I am sure. This is just one lowly shop rats opinion.

JD

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Old 11-08-2010, 09:16 PM   #14
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J.D.-I didn't even think of the torque the whole rifle undergoes when the bullet hits the rifling. What is your opinion on how this will effect the lower assembly if the bipod is mounted to the hand guard, and the operator fires the rifle without a death grip on the pistol grip. Wouldn't most of the torsional force be transferred to the hand guard?

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Old 11-08-2010, 09:29 PM   #15
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J.D.-I didn't even think of the torque the whole rifle undergoes when the bullet hits the rifling. What is your opinion on how this will effect the lower assembly if the bipod is mounted to the hand guard, and the operator fires the rifle without a death grip on the pistol grip. Wouldn't most of the torsional force be transferred to the hand guard?
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.....
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Old 11-08-2010, 09:50 PM   #16
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That sounds to be very educated extrapalations to me. Used my word of the day right there. It's because of all of the reasons you have listed here, I think I would stay away from AR big bore conversions. I think if you want a .50BMG, you should go out and buy one. Having that empty mag well is dorky in my opinion. There are alot of other options out there for the AR15 platform that have quite a bit of punch and can cover some serious yardage.

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Old 11-08-2010, 10:17 PM   #17
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It's not high on my list of things to build, but down the list a ways there is this idea I have for a serious AR-10 that will chamber and shoot something like the .270 or .30-06 out of a magazine and be semi auto in nature.

I know that COBB has their MCR, but it's more of an AR shape than a real AR platform.

It's an idea I have rolling around that might have to have some R & D done on it in the next couple of years.

A .30-06, magazine fed system that could achieve sub moa accuracy and still be broken down to fit in a backpack?? Now THAT would be a Battle Rifle.

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Old 11-08-2010, 10:19 PM   #18
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You can build a .300 WSSM AR15 and the ballistics are better than that of a .30-06, I think. I was looking into building one of those but then the little woman talked me into building her an AR, so my project got put on the back burner for a while.

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Old 11-08-2010, 10:24 PM   #19
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i know this was posted in a different thread but i think it applies here as well. remember this is a semi and the action absorbs a great deal of the recoil forces. just from watching this and knowing how much a semi auto can damp a rounds recoil forces i wouldnt want to slap a 50 or any other bolt fed large bore round on a ar lower.

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Old 11-08-2010, 11:00 PM   #20
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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.

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