Build your own AR Barrel from a blank | 358 Legend barrel Build

Building your own AR barrel a complete guide.


I ordered these parts to complete this 350 Legend (Modified 358) AR15 Barrel:

Barrel manufacturers recommend cutting off 1/2 to 1 inch from each end of the barrel. This is because the blanks are turned in centers with a lock dog and center tail stock. The centering tools wobble and wallow out the openings.


I chose to stick with a simple heavy barrel profile that uses a .936 Gas Block. Consulting with other barrel manufacturers who are making 350 Legend barrels, I acquired the gas port hole spec to be a carbine length #41 or #42 drill bit for ~.090'' gas hole.


Barrel Spec Sheet

A crude sketch lol



Contouring the barrel

Sticking with this simple design, I turned down the barrel starting at 1 inch from the chamber end. I then indicated the bore to 3 ten thousandth run out to get ready to cut the barrel extension tenon.

Cutting the tenon for the barrel extension

Since the AR locks up between the barrel extension and bolt face, it's crucial to setup the barrel extension tenon exactly plus ~5 ten thousandths for crush factor. This will setup for a better fit and headspace thus increasing accuracy. For this its recommended to use the bolt head that will be used for this barrel, matching its clearance. To figure out the barrel tenon length a depth gauge measurement is measured from the barrel extension face to the top portion of the bolt head. Most prints call for this measurement to be .625". Verify with the bolt head you plan to use and add 5 ten thousandths to 1 thousandths for clearance. Also notate the measurement to the bolt face to barrel extension. This number will be used to determine chamber depth when it comes down to chambering.

The diameter of the barrel extension tenon is stated .812" some prints will state .813". Threading for the tenon is 16 threads per inch. Once threaded, check fitment with the extension for a tight snug fit


Chambering the barrel

With the depth measurement taken from the bolt face earlier subtracted by the depth measurement to the top of the bolt head. That measurement should be the over hang from the back of the cartridge. To double check, a measurement taken from the bolt to bolt face

For this cartridge, the measurement is .125" Cartridges that utilize a 223 case should have the same amount for proper case support. SAAMI spec will state a datum line for measurement on bottle neck cartridges, that number can be used to to determine reaming depth minus the case head overhang. As example on this cartridge, the Go gauge measures 1.710" to match max SAAMI. With a .125" over hang (1.710 - 0.125) puts this barrel reaming depth goal at 1.585". ALWAYS GIVE YOUR SELF CUSHING. It's recommend to ream to a measurement that stops approximately 5 thousandths from final measurement. Using a dial indicator on the tail stock along with a make shift jig will help determine your chambering depth


Once the final chambering depth is near, I found putting the lathe in neutral gear and hand spinning the jaw while feeding the tail stock and watching the indicator has helped sneak up exactly to the measurement goal. This will be a tedious step here as you'll be taking frequent measurements with the depth gauge to meet your case head overhang number


Torquing the Barrel Extension, Check bolt head fitment

Before final torquing the barrel extension first check proper fitment with the Go-Gauge and the bolt. The bolt should pass the barrel extension lugs and not have more than 1 thousandth play. Barrel extension should be torqued to 30-150 lbs. I settled with 90 ft lbs. Use Red Loctite once it's determined the proper fitment of the go-gauge passes the barrel extension lugs. As a quick tip, a piece of scotch tape on the back of the go-gauge will add about 2 thousandths. Cut off the excess. This will act as a No-Go gauge. As a triple check before final torquing of the barrel extension, the bolt head should not pass the extension lugs with the scotch tape. Drive in the barrel extension pin as a final install

Cutting the Gas Block shoulder

All measurements from here as based on the face of the barrel extension. For this build, a carbine length gas system is used. That measurement is 6.746''. There is slight play with this number as the gas tube will have a little room between the BCG gas key. If your lathe is equipped with a DRO, zero out from the extension face and score a mark at the 6.746". If you don't have a DRO, a caliper measurement will suffice. I turned down the barrel to fit a .936" gas block from this shoulder to the muzzle. To allow relief so the gas block to slide down the barrel, I then stepped down to .930" two inches from the gas block shoulder. The barrel was then thread for a 5/8x24 muzzle and the crown was cut at a 90* face. Chamfer the edges of the gas block ledge as well as the shoulder. I then touch the front of the bore opening with a rubber emory bit just to break the edges of the lands and groove to finish off the crown.


Drilling the gas hole

Print's call for the position to drill the gas hole is .300" from the gas block shoulder. However, the gas hole on the gas block is slightly smaller so if you but up the gas block to the shoulder the alignment will be off. If you plan on using a traditional handguard stick with the .300" measurement, else take a depth measurement to the center of the gas hole on the gas block. For this build, that measurement ended up being .273"


I used a mill equipped with a DRO to drill the gas port. Setting up the barrel leveled and clocked directly at 12 O clock is simple. Using an quality upper receiver on the barrel extension, I used the picatinney rail to level the barrel


A edge finder was used to find and set zero for the x and y axis then jogged over to .273" to drill the gas port with a #41 drill bit for a .090" gas port hole


As a final step. I recommend marking the barrel the caliber and barrel twist. You never know where this barrel will end up. Well folk's that wraps up the barrel build. If you have any question's please comment below. I hope this helps!


-Eagle Eye Shooting



Some pictures where taken from the article written from https://rifleshooter.com/2014/05/building-machining-an-ar-15m16-m4-barrel-from-a-blank/


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