Posted: 4/6/2010 7:15:46 AM EST
[Last Edit: 7/12/2010 10:35:38 PM EST by LLCoolBeans]
Part 1 - Open the port:
Here's the barrel. AR Stoner 16" SS .50 Beowulf Barrel, with matching bolt (I'm assuming they headspaced using the provided bolt.). Another member mentioned on another thread that he called AR Stoner and found out that their barrels are made either by ER Shaw or Wilson. My guess is that this is a Wilson barrel.
I also ordered an oversized dust cover to use in place of the standard cover. Here is a photo comparing the two. The oversized dustcover is 0.040" taller.
I chose a Vltor upper for this build as the beefy nature seemed to lend itself to this application. Also, they just look cool and Vltor is a local business.
I needed a way to hold the upper at the proper angle on the mill table in order to machine the ejection port properly. I decided to make a mandrel/fixture that would wrench onto the receiver as if it were a barrel, using a standard steel barrel nut.
I ordered some of these really handy blank arbors a couple of years back and they are worth their weight in gold. These have a hardened taper section and a blank portion ahead of the taper which is left soft and machinable. These particular blanks have an MT2 taper, the same taper that fits inside my rotary table.
Machining the mandrel. Copied the dimensions of a real barrel exactly, only the mandrel needed to be slightly shorter to prevent the endmill from touching it while machining the port. Say 0.025" shorter, just enough for clearance.
I then drilled a hole all the way through the mandrel and added a 1/8" spring pin, to take the place of the indexing pin on the barrel. This probably was unnecessary, but I figured it would rule out any possibility of the receiver rotating on the mandrel.
Now that I had my mandrel, I needed some way to support the opposite end of the receiver while doing the machining. I created a nylon plug that fits inside the internal bore of the receiver on one end and fits the dead center of my mill table tail stock on the other.
All meta machine work is now complete. Time to wrench the mandrel to the receiver and set up the mill.
I used dial indicators to ensure the receiver was properly oriented in all three axies.
Here is the ejection port, before any modification...
First I trimmed back the landing for the dust cover by 0.040" as the new dustcover is exactly that much longer than a standard dustcover. I used a 1/2" endmill for this operation as it matches the 1/4" radius on the dust cover.
Next step was to open the port towards the top of the receiver. I used a 3/8" endmill to create a 3/16" radius, in order to square up the corners of the port a bit. This is to to help stop the cartridges from binding on the original large 1/4" radius. I opened the port to the north by 0.035" leaving 0.005 in case I made a mistake, I'd have 5 thou to fudge with.
Next, open the bottom of the port by 0.105", same 3/8 endmill.
Port is now fully opened.
Now, if we want the dust cover to function properly, we need to re-cut the detent cams (I don't know the official jargon, but you understand what i mean) The outer one is easy, a ball endmill makes short work of this task.
Ok, this is the tricky part. How the heck do you cut the inner cam/pocket? Obviously, from the original shape of this cam surface, there is a specialized tool for this operation. I do not have such a tool. I decided to fudge it by eye with the same ball endmill, since all we really need is a place for the detent to rest when the dustcover is in the closed position, and provide a way to compress the detent spring so the dustcover can open when pushed by the bolt carrier. I changed the angle of attack and did this by eye.
That's the end of the mill work on the Upper. De-burr and test oversized dust cover. It works! That's a relief!
Link to video of new dustcover operation
Part 2 - Upper Assembly:
Well, I've got all the parts I need now, excluding the muzzle break and some ammo (which appears to be in extremely short supply right now).
Here's the Gas Block, a LaRue .875" ID 0.300" spacing, low profile gas block.
And the handguard, a LaRue 11" (This thing as a little heavier than I had hoped. Not that a little extra weight is a horrible thing on a heavily recoiling gun. Still, I'd like to keep this rifle reasonably lightweight. This thing is very SOLID however, so that balances out the weight disadvantage. Also, the built in QD sockets are a huge advantage.)
Insert barrel, slather threads with moly goo.
Torque the barrel nut.
Assemble gas tube to gas block and check for proper fit.
I had a heck of a time driving the gas tube roll pin into the gas block. I used the spring pin provided by LaRue in the gas block packaging. Eventually, I got it in there, but it was a PITA with the bench block I have. Also, just FYI, as far as I know, all Beowulf uppers use a Mid-Length gas system. Therefore, what you see here is a Mid-Length gas tube.
Install handguard. This is a little tricky and it took me a moment to figure it out. You have to put the handgaurd onto the barrel, but don't thread it on. Then, insert the gas block with gas tube attached. Now that the gas block is in place, scoot the handguard back and forth until one of the little holes on the bottom of the handguard lines up with one of the set screw holes in the gas block. Insert set screw, repeat for other hole. Then and only then tighten and secure the handguard. Once again, I can't stress enough how solidly this thing mounts. It's a rock.
M4 feedramps should in no way interfere with the feeding of Beowulf carts.
Here's where the Mid-Length gas block ends up under the 11" handguard. Looks like I could have gotten away with the 10", but this way the gas block is completely shrouded from bashing, and I do get a bit more sight radius.
Well, here she is. Technically, she is ready to shoot. I could slap some sights on her and take her out today, assuming I had even one round of ammunition.