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Another update - barrel profiling and installing the barrel extension.

I didn't take many pictures of barrel profiling, was too busy dodging chips. (just turning the blank down from 1.25" to 1" makes a lot of chips, then profiling started from there.) I kept the profile simple - 1" at the shank, .875" to the gas block, .750" at the gas block and .746" to the muzzle. I may add some flats or fluting later, but not at this point.
Note the different chips piled up - piles of golden bronze C and 6 shapes from roughing with heavy cuts, and then fine curly silver hair from the finishing cuts.
[Linked Image from i.imgur.com]

Piles of this stuff everywhere, including my pockets, hair, and they stick in your shoes. I had to clean the floor after every pass down the barrel. The gold/bronze color is from heat; if we were pushing a little harder the chips would be blue and purple.
[Linked Image from i.imgur.com]

A bit of polishing with emery cloth and then Simichrome on a paper towel cleaned up the barrel to a nice shine. (I don't use shop rags on the lathe, but paper towel can just rip if it gets caught instead of winding up your fingers/hand/etc.)
BTW that ugly chunk of cardboard back there was to keep chips and cutting oil off of the wall, but it fell down during my project and I didn't bother to fix it.
[Linked Image from i.imgur.com]

No pics of contouring or polishing the front end of the barrel, so on to crowning the muzzle. I cut an 11° target crown on this one and finished the OD with a small chamfer.
[Linked Image from i.imgur.com]

A little polishing, and the crown is done (sorry for the fuzzy pic):
[Linked Image from i.imgur.com]

ARIC

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On to installing the barrel extension permanently. I made this barrel vise years ago, and it's pretty out of the way attached to a workbench in a back room so I've never bothered to move it. The two 5/8" fine thread grade 8 bolts give LOTS of clamping pressure, and with a properly fit set of steel blocks it'll handle 175+ ft-lb easily without slipping or marring the barrel. I make those steel blocks for each barrel size by boring a hole through a solid block then sawing it in half. With a smooth finish in the blocks, it doesn't even scratch the polished surface of this barrel.

I use red 271 Loctite on the barrel extension threads, and torque to 100 ft-lb. Torque is applied via 1/2" drive torque wrench and that Brownells barrel extension tool pictured at the beginning of this thread (in the receiver lapping post).
[Linked Image from i.imgur.com]

Installing the extension pin is straightforward. I just cut a piece of 1/8" O-1 drill rod (annealed, not hardened) to .185" with a chamfer on both ends, and tap it in place. The barrel extension hole is sized to be the right press fit for 1/8" rod.

Comment on this part - some companies install hardened pins here. I don't like that because I've had to fix a couple that cracked and broke (from abuse, but still). An annealed pin is much easier to remove if it's damaged, and O-1 is plenty strong even in its softest state. This pin is only there to locate the barrel in the upper so the feed ramps are lined up, it should never be stressed by barrel or muzzle device installation (one reason I really do not like receiver blocks, use an extension tool / reaction rod instead).
[Linked Image from i.imgur.com]

This barrel is now ready for drilling the gas port hole, that'll come in the next update. The top barrel is my extra 243 LBC blank, still unprofiled at 1" diameter.
[Linked Image from i.imgur.com]

Last edited by Yondering; 12/05/19.
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Gas port drilling

When I drill a gas port, I like to drill a matching set screw dimple to help with gas block alignment. To do this, I like to use a collet block, which is simply a precision ground block that holds a 5C collet in the center to allow repeatable positioning of a part in different orientations. I used a square collet block for this job, with a 7/8" 5C collet (that's why I turned the barrel to .875", rather than .850" which I don't have a collet for). I also use a stop on the vise jaw, so I can bump the collet against that to repeat the same position when the collet block is removed and replaced in the vise.

Aligning the barrel in the collet is important too; I mostly do this by eye and align it so the barrel extension pin is straight up. That can be checked in the mill once it's all in place.

[Linked Image from i.imgur.com]

Using an edge finder, I can locate both sides of the barrel to find the center. This one is an audible edge finder so it clicks when it jumps out of position as it finds the edge, that's a convenient feature.
[Linked Image from i.imgur.com]

With the center of the barrel located, I locate the barrel shoulder at the gas block journal, and move forward .300" to the gas port location.
[Linked Image from i.imgur.com]
[Linked Image from i.imgur.com]

Drilling the gas port - from research online it sounds like the appropriate gas port size for this barrel length and cartridge is usually between .088" and .093". I drilled first to .076" and then stepped up to .082"; that reduces the burr on the inside of the hole. I'll test fire it at .082" and step up the size to .088" or larger if needed.
[Linked Image from i.imgur.com]

With the gas port drilled, now I flipped the collet block upside down to drill the set screw dimple. The mill table position is unchanged, so this dimple is directly in line with the gas port, as desired.
[Linked Image from i.imgur.com]

With that part finished, I removed the barrel from the collet block and cleaned the bore in preparation for test firing.

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Fun side note - in cleaning this 20 caliber bore, I discovered that it's a nice tight fit for Q-tips (I use a lot of those for cleaning shop projects). With the shop air nozzle stuck in the chamber end, that barrel will shoot a Q-tip through a cardboard box at close range. I may have wasted some time firing Q-tips around the shop. whistle

Thought I'd show what I use for barrel installations, this is the fastest, easiest, and most secure method I know of, and doesn't stress the upper receiver at all. It's great for removing/installing muzzle devices too.

1/2" drive breaker bar in the vise. I made this one from a broken 1/2" extension, but a regular breaker bar or even a ratchet works too.
[Linked Image from i.imgur.com]

Barrel extension tool sits on the breaker bar
[Linked Image from i.imgur.com]

Upper receiver and barrel sit on the extension tool. This allows easy visibility of the gas tube hole for barrel nut alignment, and all of the barrel nut torque is reacted straight into the barrel extension rather than twisting the upper receiver.
[Linked Image from i.imgur.com]

Installing the gas block - note the dimple under the rear set screw hole; that hole is directly in line with the gas hole in the gas block, so lining up the set screw with the dimple automatically aligns the block with the barrel's gas port.
[Linked Image from i.imgur.com]

Double checking gas tube length - looks just right. (compare it visually with the half-circle cutout in the upper for the bolt cam, should be a little past center)
I also spend a little time tweaking the gas tube if necessary so it lines up with the gas key on the carrier without bumping or flexing.
[Linked Image from i.imgur.com]

Upper is assembled and ready to test fire. I won't bother with the handguard until the gas port is the correct size.
[Linked Image from i.imgur.com]

Oops, wrong chambering for this upper. This'll never work.
[Linked Image from i.imgur.com]

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Originally Posted by Mackay_Sagebrush
Outstanding!

Love the pics!


Thanks for the comments guys!

IC B2

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Like your work.

What brand of lathe do you have?

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Originally Posted by plainsman456
Like your work.

What brand of lathe do you have?


It's a Jet 13x36, belt drive model from 1991. I wish it was the gear drive model just for faster speed changes, but otherwise really like it; it's a good lathe for gunsmithing work. The 1.5" spindle bore means plenty of room for barrels and actions, and when building my Form 1 suppressors I often use 1.5" stock so that's an important feature.

I bought a matching Jet mill from the same guy, that is a clone of the 9x42 Bridgeport.

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Final assembly and test firing happened this weekend.

For final permanent assembly, I decided to use a BCM upper since this'll be more of a precision rifle. (This was originally a left-over parts build, but has morphed into two complete uppers with the extra parts I picked up along the way.) I like the BCM uppers because they are a tighter fit to the barrel extension, and are usually a couple thousandths press fit rather than the typical loose slip fit of most uppers. Heating the upper with a heat gun expands it enough to install the barrel:
[Linked Image from i.imgur.com]

I stamped the cartridge and twist rate on the barrel as well. This isn't a fancy way to mark the barrel, but is quick and expedient, and I'm more interested in function than looks on this one anyhow. I do mark a line on the barrel to index the stamps against, and always test each stamp on a piece of scrap first (note the aluminum block near the top of the pic). Stamping a letter upside down is embarassing, testing on scrap helps avoid that. Stamped letters always look a little bit drunk, but IMO this one looks worse in the pic than it does in person.
[Linked Image from i.imgur.com]

Inital test firing went pretty well, other than the chrono not picking up any readings. Cycling was just a little sluggish though, with the bolt not locking back on most loads, so I stepped up the gas port size to .089" (from .082").
[Linked Image from i.imgur.com]

Indexing the gas port hole is pretty simple - I use the previous .082" drill bit in the chuck backwards, and use the drill bit shank to locate the hole. When the drill bit can enter the hole without any deflection, it's centered. This is accurate to within a thousandth or so if a guy is careful.
[Linked Image from i.imgur.com]

Final assembly is complete. Blue #243 Loctite was used for all scope, handguard, and gas block screws, as well as between the gas block and barrel for sealing. Green #620 was used between the handguard and barrel nut per Midwest's instructions. The rifle is topped with a 3-9x HD SS on a 20 moa riser. Still couldn't get reliable velocity readings, but the rifle cycles well with both 32 and 40 gr V-max over TAC, 2460, 8208, and Benchmark.
[Linked Image from i.imgur.com]

Just for fun I fired a 32gr V-max @ ~3900 fps (if the 2 chrono readings are correct) into this 5/16" steel plate. Shown for comparison is an impact crater from a 75gr 5.56. It's not a completely fair comparison, 24" vs 16" barrel, but the 75gr 5.56 won't punch through from a 24" barrel either. Max loads with that 32 gr V-max should add another ~200 fps or so.
[Linked Image from i.imgur.com]

I'll follow this up with some velocity and accuracy results when I can, but for now this build is done. Thanks for reading.

Last edited by Yondering; 12/09/19.
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Looking good.

Hope it shoots tight groups.

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Originally Posted by plainsman456
Looking good.

Hope it shoots tight groups.


Haha me too! It'll be embarassing if it doesn't, after all this work! smile

Thanks.

IC B3

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Thanks for putting this up, competence can make for a compelling read. And thank you for taking the time to make those well composed and informative photographs. Good gun writing lives here.

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Great job, great pics & great narrative.

Thanks for taking the time to post it all up.............that in itself takes a while.

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Wow, that looks fantastic. thanks for sharing all those details.

...where knowledge and skill converge. Now that is a 'build'.

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Thanks for the comments guys. This is the stuff I enjoy doing, much more than debating brands and headstamps.

MontanaMan - glad to do it. I've thought of documenting one of these in the past, but happened to have time to post while making this barrel so it all worked out. About half my rifles have my own barrels like this, but usually I'm too busy to take pics and write it all down. Hopefully this might help someone else who's making their own barrel for the first time; I didn't include every detail but tried to capture the basics anyway.

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I haven't had a chance to test any loads for accuracy yet, but did burn some powder finding max loads for a few different bullets, as well as poking holes in steel.

Out of a bunch of different powders I tested, I'm liking XTerminator the best so far based on consistency, how it runs the rifle, and velocity. No surprise given the excellent results I've had from it in 5.56 and lighter bullets. Max loads for good brass life at this point appear to be ~3,850 fps with the 40gr V-Max and 39gr TNT, and ~4,180-4,200 fps with the 32gr V-Max.

20 Tac with those three bullets shown here alongside a 243 LBC with 95gr VLD
[Linked Image from i.imgur.com]

I plinked some steel to see what happens with the 32gr V-max load. Looks like I'll probably need to restrict this one to at least 200+ yards on my AR500 steel targets.
[Linked Image from i.imgur.com]

Last edited by Yondering; 12/17/19.
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Great post. Thanks for sharing.

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Thanks for this posting.
You did an awesome build, and showed so much to learn from.
Hats off to you sir.

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Yondering, any more on this one?


'Four legs good, two legs baaaad."
----------------------------------------------
"Jimmy, some of it's magic,
Some of it's tragic,
But I had a good life all the way."
(Jimmy Buffett)

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Originally Posted by Son_of_the_Gael
Yondering, any more on this one?


Actually was just loading some more ammo for it last night. I should take some pics of accuracy targets but haven't done that yet (I really prefer shooting longer distance at steel and other targets once I figure out a load); however, I found it shoots the best (and IMO very well!) with the 40gr V-max over 31gr of Lever for 3917 fps avg at the muzzle. I ran a seating depth test with varied powder charges at each depth, and found that 2.255" OAL shot really well at 100 even with a full 1.3gr variation in powder charge, so I'm loading in the middle of that range and calling it good. I did confirm through a lot of testing that Lever is THE powder for the 40gr V-Max in this rifle; nothing else ran as fast or shot as well.

The 32gr V-Max shot reasonably well with X-Terminator, but struggled to pass 4100 fps without really pushing the brass, and at that speed it gives up a lot to the 40gr load so I didn't bother with it further.

The 24gr NTX is an interesting one I've been experimenting with. There is factory data for it in the 20 Tac, like the Sierra manual, but if you really look at the numbers and powder choices is severely underloaded and hampered by powders that are too slow and fill the case before reaching full potential. Since I've got a pretty full selection of Ramshot and Accurate powders I did the wildcatter thing and started messing with those, stepping faster in burn rate until things fell into place with AA2200. The max load with that one put that 24gr NTX a bit over 4,800 fps (no joke) but I found a decent accuracy load in the low 4,700 fps range. That one doesn't shoot as well as the 40gr V-Max, and loses steam fast, but it's a neat little bullet to fire at warp speed. It also has almost no perceptible recoil other than the bolt carrier movement; it feels like less than my 22 Hornet.
An interesting side note though is that even though it's doing almost 1,000 fps faster than the 40gr loads, it doesn't penetrate steel plate any better. I think that powdered copper core is just so frangible that it comes apart too fast. I did find one example when I had my target mounted to a cardboard box that it split the side of the box and fully fragmented within ~3 inches of entry on just the cardboard.

Anyway, it's a really fun round and a blast to shoot in the AR. It's very low recoil compared to the 5.56 which is already pretty mild, and is an absolute laser beam within ~300 yards. Also, as usual I'm really happy with this Black Hole barrel. I've got another build to post up when I get time; this one is a 19" octagon barrel in 243 LBC, set up as a mid-light weight walking rifle in the AR15 platform. That one includes a thumb surgery story too since I did a very dumb thing and found out what the bone looks like under my thumbnail.

Last edited by Yondering; 07/02/20.
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Thanks, the cartridge intrigues me.

Sorry about the thumb.


'Four legs good, two legs baaaad."
----------------------------------------------
"Jimmy, some of it's magic,
Some of it's tragic,
But I had a good life all the way."
(Jimmy Buffett)

SotG
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