different ways of attaching a barrel+barrel extension to an aluminum receiver

Hello,

I'm a European engineering student (and hunter/gun owner) spending some of my free time daydreaming about designing the most awesome rifle. I've got an idea of what I want the rifle to do but I'm kind of exploring the technical possibilities to determine if it's realistic to realize my concept rifle. So I'm not doing finite element analysis or anything like that but I'm doing rough estimations and ballpark figures to see where I am relative to reality.

Anyway...

My rifle will probably use an aluminum receiver with the locking lugs cut in a steel barrel extension. I'm trying to figure out how to mate these two together. There are a bunch of factors to think about:


i) Able to keep the barrel pointing in the right direction
ii) able to prevent the barrel from being pushed rearwards toward the shooter
iii) able to prevent the barrel from being pulled forwards toward the target
iv) removable with normal tools

Now there is a whole bunch of rifles, most notably the Sauer bolt action rifles, and also the Bergara BX11 and a bunch of similar rifles, who all use the same solution. In the guns the barrel+extension slides into a smooth cylindrical socket in the front end of the receiver. The socket is cut on the underside of the receiver and 1-3 screws are tightened by the used to tighten the fit. See links. This is a great and simple solution, but I'm not so sure it would work reliably on a semi-automatic rifle. You see, there is very little in the solution described that keeps the rifle in the receiver while the gas operating system is bracing itself against the barrel to slam the bolt carrier group rearwards in the direction of the shooter. Unless the barrel is somehow secured, I'm worried that it might be driven out of the receiver.

http://3k27nj16tyvt3jv4dx43narh.wpe...t/uploads/sites/17/2015/03/004_IMG_006101.jpg

https://www.youtube.com/watch?v=rzFC8UYxtDs

Another solution is the one employed in the Robinson XCR series of rifles. This is very impressive, because it relies on a single screw on the underside of the rifle, just in front of the magazine well. I think it works like this (plz correct me if I'm wrong): The barrel extension is inserted straight from the front of the rifle into a slot inside the receiver. The screw is then screwed in from below, mating with a "female" cut in the barrel extension, thus applying pressure to secure the barrel, and preventing it from moving forwards or rearwards or rotating around its own axis. My gut feeling tells me the downside of this system is that it doesn't really hold the barrel all that tightly. I've also read about some XCR's having accuracy problems due to failure to meet tolerances in the interface area between the barrel extension and the receiver. I can imagine that the dimensions have to be just right to make it work.

https://www.youtube.com/watch?v=7p0f6mhpgEU

pics from XCR Forum of b ext:
https://app.onlinephotofiler.com/images/A_2/9/8/6/106892/f590fd3c896848c2a98929a5e0e1f2dc.jpg
https://app.onlinephotofiler.com/images/A_2/9/8/6/106892/6f4b6f508f35403ba9fb992f0df8e7aa.jpg

I kind of don't get how the XCR barrel retention mechanism works though... if anyone has first hand knowledge to share I'd appreciate it.

There is also the possibility of having a barrel nut around the barrel, but that isn't really feasible for my most awesome rifle because the receiver design is already quite fat and I don't want to make it any thicker than necessary by adding a barrel nut.

What other ways are out there? I believe the Browning BAR barrel is very tightly fitted into a heated aluminum receiver which then shrinks around the barrel.

You could do some type of cross pin/bolt that interfaces a groove cut into the extension. Look at AI rifles with quick change barrels. I believe that's how they are setup. I could be wrong though.

There are several ways to hold a barrel with extension into the receiver. you can use a setup like a AR-15 where the extension is a slide fit into the receiver bore and then a external threaded ring holds the assembly in place. This 50 BMG rifle has a bored hole with a seat and the bottom of the receiver is split. The head spaced barrel/extension is slipped into the receiver and aligned with the bolt lugs. On the side of the receiver as you can see are 4 10-32 that clamp the barrel into the bore and there is no load on the receiver seat where the extension sits against. With the Lothar Walthar barrel on the rifle in the photo allows it to shoot small groups at 1,000 yards (under 1 moa) The other photo is not a LW barrel I just wanted to show you the extension and how it slips in. 6 more of these are currently being made 5 50's and 1 416 Barrett.

paracongo308 said:

On the side of the receiver as you can see are 4 10-32 that clamp the barrel into the bore and there is no load on the receiver seat where the extension sits against.View attachment 162416View attachment 162417

Click to expand...

So the screws clamping the receiver together provides enough friction to transfer the recoil from a .50 BMG to the receiver and ultimately to the shooter? If that's true then my worried assumption that the clamping attachment of this style wasn't tough enough for a semi auto is proven false.

The thing about the external threaded ring solution is that I would get an extra layer of metal which I imagine would make the total width of the receiver larger. I have no workshop, but in my sketches, the place where I have a lot of space is right below the chamber, hidden by the handguard. The clamping style barrel attachment would be perfect, but I assumed that when it's used on bolt action rifle, recoil is transferred straight to the receiver seat, as you call it. That would mean that the clamp perhaps would not be strong enough to hold the barrel in place when the operating system is pulling it in the other direction, where there is no seat to hold it still.

jonesturf said:

You could do some type of cross pin/bolt that interfaces a groove cut into the extension. Look at AI rifles with quick change barrels. I believe that's how they are setup. I could be wrong though.

Click to expand...

From looking at this video it seems as if they use a threaded barrel extension that is easily screwed into a steel receiver, and then further tension is added with a screw that I presume clamps the barrel, just like in the example above. Link below to Accuracy International's youtube video showing barrel change.
https://www.youtube.com/watch?v=MXhOkcWdsZo

The Blaser rifles are very popular here in Europe. They have a very compact action which allows the barrel to be so far to the rear that the scope mount attaches directly to the barrel. This means that there is no loss of zero when changing barrels, as the scope and scope mount stays on the barrel. The barrels have a cut in them, in which a recoil lug attached to the receiver fits. The barrels are installed from the top and secured by two screws, which are accessed through the bottom of the stock. The system is very slick and simple! It's not optimal for my design, because it would force the user to remove the scope.

http://www.realguns.com/images/barrelinstldets.jpg

I guess, however, that you could have a system where that recoil lug is attached to the barrel extension and installed like a bayonet into the receiver bore, and then secured with a screw or two. What do you think of that?

Nice grab on the AI video. I was actually thinking of the DTA SRS rifles. I believe they use something like I was explaining for quick swap. If I remember right there were 4 screws on the side you turned. I think one only turned 90 degrees and the other 3 were clamps.

The AI rifles work well. I shoot with some guys on the AI team and they have nothing but good things to say about them. Makes multiple calibers or barrel changes mid season a snap. You can do load development on a new barrel then take it off and put it on halfway through the season and be ready to go.

Transferring the recoil to the receiver should be the least of your worries as it can happen at the rear of the extension or the front of the receiver if there is a shoulder on the extension that bears there. The only forward forces the barrel/extension would see are from chambering a cartridge or maybe some inertial effects. Chamber pressure from firing would be contained by the bolt/extension until it drops to zero.

I am guessing you are planning a direct impingement gas system. If so, do the math on the reaction force. The gas tube is such small diameter that it can't have a very large force. I am guessing that the hole in an AR gas tube is about .125" diameter, which has an area of .012 in[sup]2[/sup]. Supposing a port pressure of 10,000 PSI (just a number here), you would see a 122 lb reaction.

I'm really appreciating this discussion, thank you all for your replies.

I'm looking at using a piston/op rod setup actually.

Doing the math on the reaction force seems tricky to me, because I'm not sure of the mechanics. Is the bolt/carrier accelerated by chamber pressure after unlocking the bolt?

In this report that I googled the internal piston force in an m16 bolt carrier (ie the force separating the carrier from the bolt and slamming the carrier rearwards) is calculated to 777 lbs of force for a gas port about 10 " from the chamber. I'm not so sure I trust that guys calculations, because he has some odd conclusions, but I also think that 122 lbs of force seems kind of low. http://www.ewp.rpi.edu/hartford/~ernesto/SPR/LeBlanc-FinalReport.pdf

What concerns me is that if there is just a slight gap between the operating rod and the bolt carrier, then the op rod will slam into the carrier (when they impact the op rod will have a velocity from the piston and the carrier will be static) and transfer a lot of impulse to the carrier. My gut feeling tells me this is the same as having a small hammer to tap a pin out of a hole. The op rod will tap the barrel out of the receiver bore, if the clamping of the barrel is insufficient.

The clamping method of attaching a barrel is very simple and effective enough for a bolt action rifle, but unless I've confused myself somewhere, it's not really enough for a semi auto that has a piston bracing itself on the barrel to push the carrier to the rear. So far I haven't found a single example of a semi auto (or machinegun) using the clamping method to attach the barrel, but even in this thread we have a bunch of bolt action examples.

So I made a silly sketch in paint. Please tell me what you think.

The idea is that the barrel is inserted straight from the front. In the sketch there is no serious bore or "pipe" for the barrel to be inserted in, but there is no reason why this could not be the case. Attached to the barrel extension is a block with a threaded hole (labeled "glorious hole" in the sketch) drilled from the shooters end of the barrel. When the barrel is mated with the receiver, a bolt (labeled "glorious bolt" in the sketch) is threaded through a hole in the upper receiver. When this is tightened, the barrel is pulled toward the receiver. The interface area between the rear of the barrel assembly and the receiver is large, and the fit is tight as assured by the bolt. When the op rod is slamming the bolt carrier group rearwards, this force is transferred to the receiver via the threads in the glorious hole/glorious bolt to the interface area between the head of the glorious bolt to the aluminum upper receiver.

This attachment alone would probably be insufficient from an accuracy point of view, but I imagine you could combine it with some style of clamping.

Edit: As long as the upper receiver is well extended forward to provide support around the barrel extension and block I can't see why this wouldn't work. As I said, it might be a good idea to add a second screw, for example forwards of the barrel block, to clamp the receiver together around the barrel, but that might be superfluous.

Clamping appears to be adequate for an AR type rifle.

http://www.lewismachine.net/media/MRPBarrelInstructionManual.pdf

That's a great find!

I'll see your informative manual and raise it with one very enchanting and informative video.

https://www.youtube.com/watch?v=tKjO4QuiU38

It does seem as if LMT used the clamp to retain their barrel, but note that there is a cut in the bottom of the barrel extension. There is also an audible click when the barrel is inserted. I imagine that the click and the cut are related. I also imagine that the cut is there to prevent the barrel from walking out during firing. What I can't imagine is what fills that cut in the bottom of the barrel extension. It can't be one of the bolts, can it? The bolts are at the same level but there is only one cut.

I found a video with the .308 LMT that didn't enchant me quite as much as the other one, but each to his own.

https://www.youtube.com/watch?v=OtCbotdz5IA

The screws are placed further apart and the cut in the barrel extension seems to line up with the front screw.

Is the bolt/carrier accelerated by chamber pressure after unlocking the bolt?

The bullet should have cleared the muzzle before the bolt unlocks so there should be negligible pressure then.

From the LMT manual linked above:

"If a carrier group exists, remove it from the upper receiver. Using a
T-30 torx wrench, remove the front bolt (1) from the receiver.
Proceed to the rear bolt (2) and loosen using three complete turns.
***Note, the screw does not need to be removed. Grasp the
muzzle end of the barrel and pull the barrel from the receiver. "

The front bolt must be removed, but the rear bolt only has to be loosened. This supports the assumption that the front bolt fits into the groove on the bottom of the barrel. I guess this is what jonesturf described above.