Chambering And Profiling A M4 Barrel

[dave2176]

I was building a AR15 5.56 and wanted to make my own barrel. I purchased a barrel blank from Green Mountain in 4140 steel, 1:8 twist, 25" long.

I purchased a 5.56 live pilot finisher reamer from PTG.

I needed to have a range rod to setup the barrel. Using 3/16" O1 steel I cut an area for the pilot bushing and threaded it 4-40.


Using the range rod I adjusted the barrel in the 4 jaw and spider until it was perfectly centered. It took quite a while. With the bushing under the aluminum straps in the jaws I adjusted the 4 jaw until the DTI was steady. Moving out to the end of the barrel I adjusted the spider until the DTI was steady. The process was repeated until the DTI no longer moved as the barrel was rotated with the range rod bushing in either position. Note that the range rod only needs to be held in the tailstock drill chuck so only the bushing rotates with the barrel.


Once the barrel was centered I cut the breach end to just clean it up. The barrel was 1" already so I didn't need/want to remove much. D=.980"


The area for the barrel extension was next. .620" long x .850" is the spec. I went a few thousand smaller so the thread profile wasn't so sharp. I'm turning with a left hand tool mounted upside down. The lathe is running reverse so I cut from the shoulder out to the end. Nice and easy, no more than .005" at a time.

 

[dave2176]

After the thread was completed I drilled the chamber with an 'M' size drill to remove some bulk so the reamer doesn't have to work so hard. You can see I marked the bit with a sharpie so I don't go to deep.


The chamber was cut next. I used generous amounts of Viper's Venom oil, cutting .050" at a time and then clearing the chips from the reamer and barrel. Here's few pics.



The barrel was then cut to length on the bandsaw and turned around in the 4 jaw. It was adjusted using the range rod and DTI to make sure the barrel bore was centered. It was cut with a 60* angle for the tailstock center to hold.


The barrel was moved out to the live center and 4 jaw. It was adjusted for center using the DTI.


Turned down to .850". Coated with dykem the barrel was marked for the different diameters and parted near the finished size. The first taper off the breach was cut with a HSS bit that was sharpened at an angle and stepped down for the taper.


The sections were turned to diameter, filed and polished with emery cloth.


The muzzle was threaded using the 3 wire method since it couldn't be fitted to the muzzle break with affecting the mounting. After it was threaded the barrel was again mounted in the 4 jaw to have the waste cut off and an 11* crown cut on the muzzle.

 

[Milehimachine]

Not an expert but your work looks pretty good.

 

[ebgb68]

I know nothing about turning a barrel but I always see the person using a drive dog plate ? I'm looking to do just what you did here thats why I'm asking . As Milehimachine said your work looks good.

 

[dave2176]

Not an expert but your work looks pretty good.

Thanks. The rifle shoots quite well. Need to take a photo of it and post it along with a target.

I know nothing about turning a barrel but I always see the person using a drive dog plate ? I'm looking to do just what you did here thats why I'm asking . As Milehimachine said your work looks good.

A lathe dog would be used if turning between centers. I would be concerned with the center damaging the critical edge of the chamber. Also a reason I left some waste on the muzzle to part off and crown. Guess I could use a crown saver on it instead.

All in all a very enjoyable project. Looking forward to the next one.
Dave

 

[ebgb68]

Thanks for the answer that makes sense. The other barrels were for bolt guns 5hat may be the difference.

Sent from my SAMSUNG-SM-G900A using Tapatalk

 

[drhall762]

Very good job.

 

[aametalmaster]

Great write up and pics. Thanks...Bob