Designated cast AR15

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shastaboat posted this 23 July 2018

When I last attempted to get my AR,16", 1 in 9" twist AR to shoot and cycle with 55 gr LEE over 10 gr of 2400 I could not get it to cycle even with a cut down recoil spring and a lightened buffer.  I'm planning to build a designated AR15 for this cast load only.  I want to use a 20" 1 IN 12" twist; preferably non-chrome lined barrel.  I've found Green Mountain Barrels sells a 1 in 12" chrome line barrel but I prefer a regular steel barrel and not even stainless steel.  Does anyone know of a source of original 20", 1 in 12" twist new M-16 barrels.

Also I would like info on what length gas system would work better on this project; if a reduced poundage recoil spring and buffer are available and what size gas port is needed.  I understand if I use an adjustable gas port that I may be able to also shoot standard jacketed 55 gr rounds but that is a secondary need and would not be pursued if not readily possible using 10 gr of 2400 and 55 gr cast bullets.  Thanks in advance for any suggestions.  

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Geargnasher posted this 24 July 2018

If you want it to cycle popgun loads with fast powder you need a very short gas system, i.e. port close to chamber to tap pressure earlier in the cycle before it drops off and to increase dwell time as much as possible. SP1 barrels are rifle barrels, having rifle-length gas systems. The retro-AR craze soaked up most if that kind of parts, so you may have to do or have done some modifications. If you want a 20" rifle with delta ring and standard handguard, you can have one made with a pistol-length gas system and low-profile gas block concealed under the hand guard and still install a "dummy" front sight gas block with the front handguard escutcheon tack welded or retained with a screw through the gas tube hole. Doing that will require custom gas block journals unless you can find a premade barrel like that, similar to the dissipator concept but in rifle length. Such a system will be insanely over-gassed for full power 5.56 loads and you will need to find an EFFECTIVE adjustable gas block (such as the ones thar vent excess gas rather than restrict it) and make a way to access it without removing the handguards.

Or you could just build a standard rifle and use enough IMR 3031 to make it run with your cast bullets.

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shastaboat posted this 24 July 2018

OK, so the trick would be to secure a barrel with a short gas tube set up (pistol-length) with a low profile adjustable gas block.  I have absolutely no use for a military style dummy front sight and when scoped the only thing they are good for is hanging a rifle on a nail.  Along with the low profile gas block I would be using a free floating smooth forearm.  I will have to contact Green Mountain or another company to see if they would make me up a special 20" barrel with a pistol length gas port and preferably no chrome lining.  I want to continue using my load of 10 gr of 2400.

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Geargnasher posted this 24 July 2018

If you don't care about it looking a certain way, then yes, a short-for-barrel gas system (pistol-length on a 20" rifle barrel) is an easy solution, just install the barrel with a FF handguard of your choice and be done. You will still likely have to enlarge the gas port to work with your loads because 5.56 pistol gas ports designed for full power ammo are tiny.  

I recommend a venting-type adjustable gas block with multiple/variable settings, not a "Off/On/Suppressed" type or one that uses a pointed screw like a carburetor jet across the port...those leak so much gas around the threads that even fully "off" they will push enough gas through to cycle....and probably break stuff with full power ammo and the port size/location it will take to run your load.  Several companies make gas blocks which are adjustable from the front, very convenient.

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shastaboat posted this 24 July 2018

Now  I only need to find a source of a barrel as I specified with a 7.5" gas port.  I can open the port easy enough and install the adjustable gas port.  No one catalogs such an barrel and I've sent out inquiries to several of the AR15 barrel manufacturers.  Do you have any suggestion regarding what company may be able to facilitate my needs?  I have no intent to shoot full power loads in this rifle build but a properly designed gas port would be in order.   

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Geargnasher posted this 25 July 2018

I don't think 7.5" is going to do it.  See the other thread, I'll see what I can come up with using modeling first.

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Geargnasher posted this 26 July 2018

Looks like 20K peak pressure....at 2" of bullet travel.  That means that you'll have about the same peak pressure at standard pistol port placement that a 20" barrel will have at standard rifle gas port location.  So standard pistol-length gas system, start with a .08-something" port diameter and plan to enlarge it to .093 to .096" before it will lock the bolt back on the last round every time, even when dirty.

 

Carbine gas system is a no-go, port pressure will be around 12K psi and I don't think you can realistically make a large enough port hole in a .22 barrel to make it work.  My Blackouts with carbine gas systems and 16.25" barrels require a minimum port presssure of 5.5K psi to function with subsonic ammunition....using a .120" port hole.

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shastaboat posted this 26 July 2018

So do you think a 7.5" will work or not?  Is that the standard pistol gas spacing?

 

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Geargnasher posted this 26 July 2018

So some research on standard port placements and review what I wrote above.  Understand that "bullet travel" is just that, from where the base of the bullet is located in the barrel before firing.  Standard port distance is measured from the breech face, same as barrel length, so you will need to factor in the difference for proper port placement.

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shastaboat posted this 26 July 2018

Other than building a custom barrel with a odd for barrel length port distance, I would want to use common manufactured parts including gas tube and adjustable gas block.  The port size can easily be altered and opened up as necessary

.

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Ken Campbell Iowa posted this 26 July 2018

just a thought about " main ... buffer .... recoil ... " springs.

( from recent mental games about using lighter springs in my Astra 400 in order to shoot functioning light plinking loads  ) ...

the lightest spring you can use is one that from full open still reliably strips a new cartrtridge from the magazine, then pushes it into chamber and locks the " bolt " ....   thus that is the limit of spring rate, not the maximum rate that will allow the bolt to fully retract.

since test springs are not always available, i was going to add several shorter springs of the required rate from a ( Lee? ) commercial spring supplier .  note that adding springs reduces the combined rate. ( due to the longer torsion bar ) .

thanks guys for the interesting thread .....  i like stories about making things work that didn't  ... kinda like marco polo crossing the alps ... ( g ) .

BTW i have heard pretty good things about green mountain barrels ....  over-achievers they seem to be ...

ken

 

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shastaboat posted this 26 July 2018

Hi Ken,

Green Mountain barrels are excellent barrels and I have used several in conventional rifle builds.  However they won't change or alter their standard designs for less than a 100 piece order.  They do make a 20", 1 in 12" twist AR barrel but will not change the gas port location.  Therefore I will have to find a decent machinist that is capable of building an AR15 barrel to my specifications from a rifled blank.  I am not capable.

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shastaboat posted this 26 July 2018

I just ordered a custom AR15 barrel from McGowen barrels to my specs with the pistol length gas port.  Under $300.00.  10 week delivery time.  I may have to drill out the gas port hole as discussed to get it to cycle.  Also ordered a low profile adjustable gas port and pistol length gas tube from Brownells.  I'm going to rebarrel one of my AR's that has a lighter bolt group.  (non m-16)  It also sports a 2 lb Jard trigger and floating forearm.  I may install a longer free-floating forearm in the future.

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Geargnasher posted this 27 July 2018

Perfect. Standard pistol length, about 4" iirc from the breech face, will tap into that 20kpsi peak and should work very well for you. The only issue I could forsee is case rim damage from unlocking so soon, while the chamber pressure us still high, but at 20k the obturation of the case in the chamber won't take much force to break.

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shastaboat posted this 27 July 2018

I'm not sure what size gas port hole will be drilled but the adjustable gas block should give me the ability to adjust the weapon to full cycle at the least needed gas.  And if the gas is insufficient I can always open the port size.  We'll see.  I received an email from McGowen regarding the unusual specs of my barrel order.  I advised them what I am attempting to do.  I'll see if I get any feedback.  It should work.

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shastaboat posted this 14 September 2018

After waiting about 7 weeks my custom McGowan AR15 barrel was delivered last Wednesday.  I had to wait until Friday to receive my slick sided upper to install the 20", 1 in 12" twist .223 Wilde chambered, pistol ported barrel.  A quick install with an adjustable gas block and rifle length floating forearm and test fire was giving me 50% cycling.  With a weighted carbine buffer and full power spring and the gas port shut down all the way, full power jacketed rounds cycled as normal.  I was headed out on Saturday for a few days of Nevada Desert camping so I took my tools and a few parts with me.  When I got camp set up I installed a buffer with no weights and a recoil spring that I had cut down 2".  YAHOO!  the AR functioned 100% and was printing less than 1" groups at 100 yards.  I decided not to clean the barrel until accuracy subsided.  After about 100 rounds and a 15 mph crosswind I think it's time to clean out any fowling.  I could drill out the gas port but since I was successful in the cycling and accuracy goals, I'm going to leave it alone.  With the adjustable gas block and a full power spring and fully weighted carbine buffer I can make it a jacketed firing AR.

The barrel cost me $290.00 which was a bit high but that's the price of experimentation.  This AR is going to be my spring ground squirrel primary weapon and it will be nice to run with a 30 round mag instead of continual loading of a 4 round bolt mag.

I just wish that a barrel manufacturer would see fit to make a 100+ order to these specs so others can do the same build with a bit less cost.  I figure at 69 years old, I can't take my money to the grave so I might as well spend it now.

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delmarskid posted this 14 September 2018

Cool beans! I'm glad you could pull it off.

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Scearcy posted this 14 September 2018

It would be outstanding if you would do a short How To article for The Fouling Shot. I am one of the unwashed masses who doesn't own an AR but would gladly buy one if I could make it run on cast. The challenge is that most of us don't speak the lingo but there is significant interest in doing what you did. Good show!

Jim

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shastaboat posted this 14 September 2018

Well Jim, other than a basic AR15 build the changes are quite easy.  I've pretty well covered it here.

Custom 1 in 12" twist barrel with pistol porting; adjustable gas block, lightened buffer; 2" cut off a standard recoil spring.

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Ken Campbell Iowa posted this 14 September 2018

hi  shastabout ...   congrats on new toy .... the price on that special barrel was pretty decent ... have to remember those guys .... ....   i am not clear on a couple things ...

are you going to shoot cast in your rig ?? ....  

with your mj loads, are you loading them down 

... or does the adjustable gas valve cover that situation .

thanks and keep us informed of your further adventures.

ken

to make you feel better, i once ( once ! ) had a special barrel made by wilson ...$700 ....... it was unuseable ... big at both ends   ( except maybe a foot in the middle )....   makes for great stories though ...

 

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shastaboat posted this 15 September 2018

To answer the above questions.  I will be shooting cast bullets in this AR pretty much exclusively.  If I choose to shoot jacketed bullets at full power I will have to change out the light weight buffer to a normal carbine buffer and I will have to install a normal recoil spring.  The adjustable gas port will be used in both cast and jacketed.  Basically the Upper cost me $400.00 to build and the other modified buffer and cut down recoil spring I already had.

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