Custom AR-15 Barrel Contour

smores · Apr 25, 2010

Ok, I'm going to be threading, contouring, chambering etc. a .300/221 barrel to my AR-15 barrel extension this week and I have a few questions about the contour of the barrel.

Below is the contour diagram I found at Shilen's website:

My main concern are dimensions D, E, F and H.

What is the correct measurement (D) for a pistol-length gas system?

Secondly, what barrel diameter is correct (E, F) for .300/221 or can I follow the standard contour? Once I determine the proper diameter for H, the rest of the barrel will follow that diameter until the muzzle where a .308 Phantom flash hider will be installed.

For (H) under the gas block, the diagram shows the barrel diameter should be .760", yet the gas block I have from JP Enterprises is supposedly .750". I guess since it's a clamp-on type, a difference in .010" won't be that big of a deal. I guess I can measure the ID of the gas block when I get home and go from there. Any advice?

As for the gas port, I decided I'm going to just drill it then ream it to the proper diameter after speaking with someone very knowledgeable about accurate AR barrels.

canuck · Apr 25, 2010

check out ar15barrels.com

CKOD · Apr 26, 2010

For an AR-15 barrel I wouldnt want to have to give all that information to my barrel maker (I.E. they should know most of what you need, and should be able to give you a dimensioned drawing for the profile they will use for you to Okay for them to use. )

As you know, you want a pistol position gas block for subsonic .300-221, and you want a .750 journal on the barrel. and yes, .010 can be the difference between fitting and not. a set screw one can have the journal too big, and even a clamp on one can only be pried open so far. If you have a gas block in mind, send the block in, and have it fitted to the barrel, and even pinned in place if they will do it for you.

The gas port on my .300-221 is .075, and Ive hag good luck getting it working with the loads that Ive made, and its also pretty small, leaving you plenty of room to go bigger if you determine that you need to. However, on something where *every* round you shoot though it is hand loaded, if youre having problems cycling, change your powder before you change your gun.

Also, make sure you get the end threaded concentric to the bore, and have a nice shoulder and crown for mounting a suppressor, or mount. I'd suggest 1/2-28 (standard AR-15) because then you can use 9mm suppressors for subsonic only, or get a 5/8-24 adaptor for .30 caliber rifle cans for both sub/super use.

If you think you dont want a suppressor for it I can arrange some range time and change your mind

Iver head good stuff about white oak armament for AR-15's

Numidian · Apr 26, 2010

CKOD said:
For an AR-15 barrel I wouldnt want to have to give all that information to my barrel maker (I.E. they should know most of what you need, and should be able to give you a dimensioned drawing for the profile they will use for you to Okay for them to use. )

As you know, you want a pistol position gas block for subsonic .300-221, and you want a .750 journal on the barrel. and yes, .010 can be the difference between fitting and not. a set screw one can have the journal too big, and even a clamp on one can only be pried open so far. If you have a gas block in mind, send the block in, and have it fitted to the barrel, and even pinned in place if they will do it for you.

The gas port on my .300-221 is .075, and Ive hag good luck getting it working with the loads that Ive made, and its also pretty small, leaving you plenty of room to go bigger if you determine that you need to. However, on something where *every* round you shoot though it is hand loaded, if youre having problems cycling, change your powder before you change your gun.

Also, make sure you get the end threaded concentric to the bore, and have a nice shoulder and crown for mounting a suppressor, or mount. I'd suggest 1/2-28 (standard AR-15) because then you can use 9mm suppressors for subsonic only, or get a 5/8-24 adaptor for .30 caliber rifle cans for both sub/super use.

If you think you dont want a suppressor for it I can arrange some range time and change your mind

Iver head good stuff about white oak armament for AR-15's

Smores is in gunsmith school, he's making the barrel from a blank himself

Fustercluck · Apr 26, 2010

CKOD said:
However, on something where *every* round you shoot though it is hand loaded, if youre having problems cycling, change your powder before you change your gun.

QFT

You will definitely want a faster-burning powder for your weapon design. If you try Varget or a similarly slow-burning powder, you will blow little green chicklets out the end of your barrel. SBR AR's love fast powder. H-322 would be a good place to start. Easy to meter, kinda clean, and good accuracy potential.

smores · Apr 26, 2010

Thanks for the help guys. Got the info I needed. Numidian is correct, I'm turning this one down myself. I've got a Shilen 1:8 .308 blank sitting here that needs a lathin'.

As for cycling issues, I went with the JP Enterprises adjustable gas block, which will help me dial in the right amount of pressure along with tailoring loads. I went with an M16 BCG from BCM and an H buffer so this thing should run nice, slow and smooth.

And CKOD, I will definitely take you up on the offer. I've never fired a suppressed weapon... I'll PM you closer to the next time I'm heading back to PDRM.

CKOD · Apr 26, 2010

'Doh

Well I dont know what D would be, but pistol position gas port is 4.7" and I'm pretty sure thats from the reference point used above front of bolt face/rear of chamber I believe. If you have the gas block you can obviously measure off of that, its always convenient when you just slide the block up against the shoulder.

You can follow the standard contour for the profile of the barrel nut/chamber area since its based off a 5.56 case after all. The gas block is pretty far back, so there wont be that much of a weight penalty just doing the short taper from the OD for the chamber to the smaller OD for the barrel journal. My .300 whisper barrel is pretty close to "Medium" here http://www.ar15barrels.com/prod/med.shtml in fact that would probably work fine with a little shortening of the first 2.5" section, just add a shoulder for the gas block to seat on for easy installation too.

As far as the muzzle end
http://www.silencertalk.com/threading/THREAD GUIDE_072709.pdf Page 6 for a standard AR-15 1/2-28 silencer quality thread spec.

As for twist rate, 1:8 or 1:7 I hear 1:8 can be a bit touchy with 240 SMKs at subsonic speeds, but havent tried it out myself with my noveske barrel. And powders, I'd suggest VV N110 for upto 175 grain bullets, and something slower like IMR 4227 for heavier bullets. Ive used the 4227 for up to 200 grain bullets ATM, and will try it with 220's when I get a chance.

I'm currently running noveske 16" barrel, pistol length gas system, .075 gas port, standard AR-15 BCG and CAR buffer. because thats what I had, I havent gotten a H or H2 buffer to play with.

smores · Apr 26, 2010

I'm going with the "Heavy" profile. With the H buffer and M16 BCG, my rifle rear of the delta ring weighs almost as much as my complete 16" A2 upper AR carbine! With all that weight, felt recoil will be real low.

Not to mention I'll have less machining operations to do with the Heavy profile, and won't have to cut a taper. A guy at school has .300/221 reamers and gauges luckily, but he leaves by Friday. I'm gonna have to crank it out relatively quickly.

As for threads, I got a 5/8 - 24 flash hider (.308 Phantom 5C2). Thanks for the link to the thread specs. They'll be helpful on many future projects...

CKOD · Apr 27, 2010

Oh and damned noveske said they are working on a switchblock tuned for the .300 fireball barrels they just did a special run on a few months ago.

As much as I like the one I have now (slide to one side to shut off, slide to the other for subsonic rounds to cycle) being able to run semi-auto with supersonic or subsonic ammo would be awesome, but I need something that I can get milled out and put over it, and doesnt thread on like the YHM forend.

Hopefully they sell pin-on version that will match up with my exisiting pin holes :party29:

smores · Apr 27, 2010

CKOD said:
Oh and damned noveske said they are working on a switchblock tuned for the .300 fireball barrels they just did a special run on a few months ago.

As much as I like the one I have now (slide to one side to shut off, slide to the other for subsonic rounds to cycle) being able to run semi-auto with supersonic or subsonic ammo would be awesome, but I need something that I can get milled out and put over it, and doesnt thread on like the YHM forend.

Hopefully they sell pin-on version that will match up with my exisiting pin holes

I did read about the switchblock but figured the JP set-screw adjustable block allows for finer tuning. JP has a good reputation as well. I'll obviously report once I get it all set up.

Had to work on a different project today, hopefully I'll get this barrel chambered and headspaced tomorrow, and test fired. I want to test fire it before I go through the trouble of contouring it. I've got a 1.250" blank, and I've decided to go with a 1.000 contour from the barrel extension to the gas block, .750" for the section under the gas block, and .730 forward to the threads at the muzzle.

CKOD · Apr 28, 2010

smores said:
I did read about the switchblock but figured the JP set-screw adjustable block allows for finer tuning. JP has a good reputation as well. I'll obviously report once I get it all set up.

Had to work on a different project today, hopefully I'll get this barrel chambered and headspaced tomorrow, and test fired. I want to test fire it before I go through the trouble of contouring it. I've got a 1.250" blank, and I've decided to go with a 1.000 contour from the barrel extension to the gas block, .750" for the section under the gas block, and .730 forward to the threads at the muzzle.

If youre doing just subsonic, then the JP should be fine, the advantage for the switchblock would be super, sub, and off with one twist.

Oh and proof loads ( :party29:

) or just normal test loads? I have a copy of quickload an I can roughly estimate some chamber pressures for you if you know which bullet, OAL, case length and powder.

Just be concious of your throat length and your case length. The "Actual" trim to length for .300-221 is 1.355, not 1.400 that some people use because thats the trim length for 221 fireball(per the now defunct quarterbore forums). Either or is fine though, as long as your cases are cut to work with your throat. But you dont want a 1.4" case thats nice and stretched from a few loadings going into a rifle with a throat at 1.360.

Also, make an test chamber (forget the term now) with a stub of left over barrel for checking your loads, it'll be handy for playing with seating depths/rifling engagement to not have to put a round in the gun. But I bet you were going to do one of those already.

smores · Apr 28, 2010

CKOD said:
If youre doing just subsonic, then the JP should be fine, the advantage for the switchblock would be super, sub, and off with one twist.

Oh and proof loads ( ) or just normal test loads? I have a copy of quickload an I can roughly estimate some chamber pressures for you if you know which bullet, OAL, case length and powder.

Just be concious of your throat length and your case length. The "Actual" trim to length for .300-221 is 1.355, not 1.400 that some people use because thats the trim length for 221 fireball(per the now defunct quarterbore forums). Either or is fine though, as long as your cases are cut to work with your throat. But you dont want a 1.4" case thats nice and stretched from a few loadings going into a rifle with a throat at 1.360.

Also, make an test chamber (forget the term now) with a stub of left over barrel for checking your loads, it'll be handy for playing with seating depths/rifling engagement to not have to put a round in the gun. But I bet you were going to do one of those already.

For test loads I like to use factory ammo. I ponied up for a box of Cor-Bon 150gr soft point rounds. The loading die set I wanted from Redding was on backorder so I haven't gotten that yet.

Good idea on the test chamber, I've got a section of crappy ER Shaw barrel I'd rather use for that. I'm gonna save the unused portion of this Shilen for an SBR which may be built in the future. :innocent0

The morning was tied up with working on this lame project that's going nowhere, afternoon I got the barrel ends faced off properly (90* to the bore) and the rear section of the barrel turned down almost to 1.000. I want to get that section done, and chamber/headspace before the guy with the .300/221 reamers and gauges leaves on Friday. Then I plan to get my Howa chambered and bedded before I finish this barrel. Pic below:

CKOD · Apr 28, 2010

how long is that blank? only need 16.1 for the title one upper, and 7-10" makes a fine SBR upper length. Same performance as a 16" barrel since your limited on your velocity, and stiffer barrel compared to the long one, so accuracy has the potential to be better.

clandestine · Apr 28, 2010

Be sure to single point thread the muzzle on centers if you are going to suppress it.

smores · Apr 28, 2010

CKOD said:
how long is that blank? only need 16.1 for the title one upper, and 7-10" makes a fine SBR upper length. Same performance as a 16" barrel since your limited on your velocity, and stiffer barrel compared to the long one, so accuracy has the potential to be better.

The blanks come pretty long, this one was 28" before I cut the button off the end (stamped with Shilen markings in case I ever sell it, I have proof it's really a Shilen).

Final length will be 16.25" from end of shank to muzzle.

smores · Apr 28, 2010

clandestine said:
Be sure to single point thread the muzzle on centers if you are going to suppress it.

By that do you mean keep the muzzle in a live center, and cut the threads using a tool bit on the lathe (after indicating to ensure the bit is square to the bore)?

I hear some people use dies to cut threads, and that's not how we're taught.

clandestine · Apr 28, 2010

smores said:
By that do you mean keep the muzzle in a live center, and cut the threads using a tool bit on the lathe (after indicating to ensure the bit is square to the bore)?

I hear some people use dies to cut threads, and that's not how we're taught.

Correct. You got it, its a pain in the rear but your doing it right.

Stay away from the die methods, it will cut threads off center and off axis.

smores · Apr 28, 2010

clandestine said:
Correct. You got it, its a pain in the rear but your doing it right.

Stay away from the die methods, it will cut threads off center and off axis.

Cool. I've made tons of tools, practice barrel shanks and did my practice chambers all using that method. I'll be sure my bit is a perfect 60* as well. :thumbsup:

I love learning this stuff. Just hope I can find a job where I'll be able to use these skills! I'd love to specialize in pistols but I don't want to confine myself to a specific niche just yet.

smores · Apr 29, 2010

Chamber end of barrel down to 1.000", shank prepped:

Shank done (threads look dirty, just a bit of cutting oil etc. before cleaning up):

Barrel extension torqued on, go gauge in chamber:

Bolt closed on go gauge. Not a chance with no-go:

Should have the barrel chopped (it's 28" right now lol) and the rest of it turned down within the next few days when I get more lathe time.

midcountyg · May 2, 2010

smores said:
By that do you mean keep the muzzle in a live center, and cut the threads using a tool bit on the lathe (after indicating to ensure the bit is square to the bore)?

I hear some people use dies to cut threads, and that's not how we're taught.

Single point cutting in a properly set up lathe is the only way to know your threads will be concentric to the bore. I understand using special dies/mandrels when truing receivers, but I still feel single point threading is superior there to. People use dies because it's easier, and they call it good enough.

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