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gunco irregular
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3,461 Posts
Discussion Starter · #1 ·
I like how for a semiauto you can cheaply and easily make a new barrel for an AR15/308AR and if you don't like it, you're only out the barrel blank if you are unable to repurpose it. I like to use unturned blanks if available as they're usually much cheaper and for the most part you're going to be turning the entire surface of the blank so why pay for something you're just going to turn off anyway? After determining what you're going to chamber the barrel in and select a barrel blank that will work and turn it to whatever is going to be the largest diameter of the barrel.
Cut your thread tenon.

AR15 .619" long .8125" x 16TPI. .540" diameter feed ramp at 120 degree included angle.

308AR 1.323" long 1"-16tpi with a .25" long front thread relief at .920" diameter, .625" diameter feed ramp 120 degree included angle.

Chamber the blank and as you sneak up on the correct depth, use a feeler gauge to measure the gap between the barrel stub shoulder and barrel extension with a GO gauge and bolt inserted in the extension and snugged up the barrel extension by hand until there is a little resistance to turn the bolt. The feeler gauges will tell you how much deeper to cut the chamber, but keep in mind the headspace will close up around .003"-.005" when you torque on the barrel extension. If you're trying for minimal headspace then set the extension off of the go gauge, then once the extension is torqued on go back and take the last couple thousands off that were lost when you tightened the extension. The quick and dirty version for barrels that have the standard .004"-.005" difference between GO and NO-GO length is to rough chamber as above using the NO-GO gauge and stop when the bolt will just close on the NO-GO. When the extension is tightened down the headspace will close by .003"-.005".

If you end up with too much headspace you can easily remove a little material from the shoulder and stub end to get it to headspace within reason.

Before you torque down the extension, polish the feed ramp and break the sharp edge at the transition between feed ramp and chamber wall. Also if you reload, look at your barrel extension and lightly remove the sharp corners from the front side of the extension on either side of the extractor slot. Those two sharp corners will scratch and gouge your cases upon extraction.

Figure out what your gas system length is going to be and keep in mind not all calibers will work with all lengths. If using a non standard caliber it can be helpful to look at a ballistics program like QuickLoad to see where on the barrel the pressure will be 12Kpsi and choose the gas system (possibly custom) that will be the closest to that location.

I like to drill the gas ports with the barrels set up in a dividing head if they're going to be using a setscrew type gas block. The gas port is .295"-.300" in front of the step for the gas block journal. Once the gas port is drilled if the barrel is going to be used with a low profile gas block, rotate the dividing head 180 degrees and put a dimple for the rear setscrew to lock into. Makes lining up the gas ports super easy.

Finish the muzzle as you see fit, I prefer threaded with a 11 degree crown and lightly polish with a red 3m pad in forward and reverse to knock any burrs off of the edges of the rifling. Apply finish of your choice, install and enjoy.

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gunco irregular
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3,461 Posts
Discussion Starter · #3 ·
For common calibers generally I'll just buy one if it's not much cheaper, but I do like making them. The one pictured is a wildcat in 44BGM or 44x1.8" which came from a long running thread here on Gunco. Don't think I did a build thread on that rifle, but it is shown in my 44BGM 1891 Mauser thread along with the same round shoehorned into an ar15. 1891 Mauser in .444-1Biggun I kept the chamber pressure lower on the 1891 Mauser since it is over 120 years old and didn't have as good of gas handling as later Mausers. For bullets up to 270gr out of the AR10/15 with 16" barrels it will beat the velocity I get with my 444 Marlin lever action with a 22" barrel. Heavier or long for weight bullets start taking up more case capacity so the longer 444 Marlin surpasses it there. So far I've only recovered 1 bullet fired from one of the 44x1.8's and it stopped just under the skin of the left shoulder after about 36" of penetration on an angling away shot on a buck.
 

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aka: SDK1968
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7,780 Posts
Dan(moleman) is way way above most people in this hobby with his ability level & skillset.

he makes some fantastic off the path builds. makes me jealous of his skills with every fancy build he does.
 
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gunco irregular
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3,461 Posts
Discussion Starter · #6 ·
Thanks Tommo,

I mentioned above about dehorning the barrel extension because it can cause gouges and scratches on your brass which can effect case life. Depending on the case there can be 1 or 2 dings above the shoulder (if there is one) and then usually 1-2 scratches/gouges on the shoulder and again on the neck area. The gouges on the neck area of the cases if bad enough are the ones that usually will result in a case split.
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The once fired LC cases on the left were fired in my sons BCA 7.5" SBR. It's mainly leaving just one ding, but they're deep enough that I'd expect to only get 2-3 firings out of them before the necks split like the 243 cases fired from an ar10 on the right. What is causing it is when extracted the ejector is trying to push the case off of the bolt face and once clear of the chamber the case is sliding over the barrel extension until it clears and gets pushed out of the action. The ejector pushes the case which pivots on the extractor which causes the case to slide over the slot where the extractor goes into the barrel extension. If the front side corners are nice and sharp they will cut/scratch/ding/gouge the cases. The worst example would be a 9mm luger DI which if you don't time the action or add a rail or rollpin to keep the case from tipping into the barrel extension once it clears the chamber.
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Hard to tell from the pic but the one on the left has been beat between the barrel extension lugs, and the nickel plating has been scratched off where it rubbed against the barrel extension lugs.

If you dehorn the front corners on the two lugs where the extractor will ride the cases may still get dented, but usually will not be scratched or gouged. Basically you want to remove just the tip of the corners and lightly polish them. I use either a cratex bit or a stone bit followed up by a felt bob and red rouge. You're not removing much material.
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The extension on the left has been dehorned and installed on a 16" mid-length 5.56 barrel. It still dents the necks, but no cutting of the cases or brass glitter inside the barrel extension when you're done shooting. The difference in the barrel extension color on the finished barrels is from alloy differences between two extensions manufacturers.
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