Follow Along for my First AR Build

[stagpanther]

The 6mm ARC is among the cartridges on Hodgdon's load data web page. I use their results to "cross-correlate" approximate QL predictions when I compare velocities from my hand-loads. With about 30 grs of CFE 223 I'm getting 2700 fps with a 95 gr ballistic tip, probably almost the same as an accubond would I suspect.

 

[Metal god]

I don’t know what the best gas system on the ARC is based on the barrel length but 18” with carbine length gas system sounds way over gassed . That’s 11” of dwell time which sounds like a lot .

EDIT , did a quick search 18" barrel seems to like rifle length .

My next step or maybe one of the things I’d try is a mechanical zero on the gas block . Meaning set it fully closed the open it fully keeping track of how many turns it takes . Then split the difference and set the gas block there . Setting it there allows to return to a know location to adjust one way or the other from there .

I just feel this is a gas/pressure issue . I know the first time I ever tried an adjustable gas block it did not cycle fully closed or fully open . Granted it was 223 Wylde and never locked up but as Stag points out adjustable blocks rarely are best at the extremes of there adjustments.

EDIT , I asked this before but don't think it was answered . You originally had a carbine buffer tube , spring and buffer . When you switched to the PRS did you also change to a rifle buffer spring and buffer or did you use the spring and buffer from the carbine kit ? FWIW carbine length gas system does not mean use carbine buffer and spring . That is based on the length of your buffer tube .

 

[Brian Pfleuger]

EDIT , I asked this before but don't think it was answered . You originally had a carbine buffer tube , spring and buffer . When you switched to the PRS did you also change to a rifle buffer spring and buffer or did you use the spring and buffer from the carbine kit ? FWIW carbine length gas system does not mean use carbine buffer and spring . That is based on the length of your buffer tube.

Sorry, I lied... the gas system is rifle length.

The buffer is still carbine length, I didn't have to replace it for the PRS stock.
Also has a Armsaspec Stealth captured recoil spring, of normal weight (3.8oz I think?).

I don't have to count the turns on the gas block... they tell you, it's 4 1/2 turns to Max Pressure, So 2 1/4 would be "1/2" open.

The 6mm ARC is among the cartridges on Hodgdon's load data web page. I use their results to "cross-correlate" approximate QL predictions when I compare velocities from my hand-loads. With about 30 grs of CFE 223 I'm getting 2700 fps with a 95 gr ballistic tip, probably almost the same as an accubond would I suspect.

Oh yeah, I always forget you have to change it to "rifle" on their site.
Note that the bullet they use is solid copper Barnes LRX... not really a fair comparison.
For what it's worth, checking QL with that bullet and OAL, QLs predictions aren't even close to what they show... it thinks the pressures and velocities would be much higher.

 

[Metal god]

I'm going to do this one as a separate post because I'm stunned right now . I just got off the phone with Hornady and they said they have no data on how to configure the barrel for there cartridge .

I asked , but it's your brand new to market cartridge how do you have no data an gas system lengths , port sizes or anything ? how did you conclude the cartridge would even work in a AR system ? He said we don't manufacture firearms so we don't have that info and that I need to talk with the manufacture as to how and why they build the barrels the way they do .

That blows me away that there is nothing Hornady can help with when it comes to building a rifle for there cartridge , STUNNED is all I can say .

 

[stagpanther]

STUNNED is all I can say .

Why? As long as their cartridges are within SAAMI Specs that they submitted why would they comment on what other manufacturer's issues are?

 

[Metal god]

I didn't say there were issues only "I" planned on building a 18" 6mm ARC and what length gas system do they recommend . There answer was we have no opinion or data on that . How does a company make a brand new cartridge designed to work in a specific type of firearm and have zero data on how to optimize it in any way in that platform ?

How did they test it ?

Did they ever even fire the new cartridge from a AR platform ?

did it cycle

If so what barrel length

what gas system

They must have data on what worked or do you think they were just like Brian here and in the dark as to why some times it works and some times it doesn't ?hmm Lets put a pistol length gas system on a 24" barrel and We'll just call the one that cycles GTG . Who cares why or if no other length will work out because of the powder we had to use to get it to have the right pressure at the gas port at pistol length ????

Them saying they have no data on all that is bull period . My only guess is by refusing to give any guidance leads me to conclude all standard mil-spec dimensions are what should work . Carbine length on 14.5" , mid on 16" , rifle on 20" etc . gas ports are likely the same way .

The stunned is that there was no guidance at all , ZERO on a cartridge they developed . It would be different if I called Blazzer and asked what gas system would work best for there 6mm ARC ammo . I can see them saying we don't have data on that we just manufacture the ammo to SAAMI/Hornady specs . But for the creator of the cartridge to have no guidance set me back a little .

What if McGowen or other call up Hornady and say we are making barrels so you can sell your 6mmARC ammo what gas system and port sized worked for you so we can be sure your ammo will work in are barrels . Hornady's reply is um IDK we just got lucky I guess , good luck .

 

[HiBC]

I can acquire a lot of experience building 5.56 and 7.62 NATO AR pattern rifles and come up frustrated due to jumping into a non-standard cartridge.

I can load to expected reasonable peak pressure levels,select powders per burn rate,etc...all based on a lot of semi-informed assumptions.

I do not have your answer. I can tell you of a project. An AR 15 chambered in a 6.5 version of the Remington Bench Rest cartridge. Its not my rifle,so I don't have all the details.(Port size,lengths,etc)

I can tell you that after about a year of frustrating dead end range trips, somehow it was under gassed in effect and the cure,which is working fine,was lightweight BCG and buffer components along with a reduced weight spring.

If I remember right,(but maybe I don't)he's getting 2700 or 2800 with a 120 gr bullet,so its not about low peak pressure. Pressure curve down the barrel is more complex than I myself can fully grasp.

I'm not saying those are the solution for you. I'm just suggesting keep your options open.

I would guess somewhere there is a group of enthusiasts for your 6mm ARC cartridge. Its likely they have done a lot of R+D and discovered what works.

You may not need to be the guy who figures it out to radius the corners on the wheel.

 

[Drm50]

I just bought my first AR. Isn’t here till Saturday. The only thing I’m going to change is the stock. I want conventional type. I plan to sight it in, clean it and hang it up in closet over top the door. Out of sight. I’m going to purchase a ammo/ magazine bulk deal for it. Just got it for social engineering if it’s necessary. Never had use for them before.

 

[stagpanther]

When you switched to the PRS did you also change to a rifle buffer spring and buffer or did you use the spring and buffer from the carbine kit ? FWIW carbine length gas system does not mean use carbine buffer and spring . That is based on the length of your buffer tube .

I just caught this detail rereading the thread--your PRS may not operate properly with just a standard carbine tube/buffer and spring assembly. Mine won't, but it's on a 308. Rifle gas should be OK on a 18" barrel, but I could see the rifle gas system making the system a bit "touchier" functionally when lower pressures are generated and there is heavier resistance from the buffer assembly and cartridge fit to chamber.

 

[Brian Pfleuger]

your PRS may not operate properly with just a standard carbine tube/buffer and spring assembly.

I don't understand? It comes with an adapter for a carbine tube.

COMPATIBILITY

Mounts to rifle-length receiver extension tube without A2 spacer but will also accommodate Mil-Spec sized M4 Carbine and many A5-length tubes with standard Castle Nuts and End Plates

https://magpul.com/prs-gen3-precision-adjustable-stock.html?mp_global_color=118

 

[stagpanther]

Here's my 6mm ARC build--complete opposite of your's, made from the cheapest leftover parts that I had lying around--except for the 6.5Arms monster 20" barrel (which Mel88 on this forum uses to great effect) which didn't cost very much and the Odin hand guard blem which I found on sale for a hundred bucks, I didn't spend anything on making it. Not nearly as nice as your's is potentially, but sometimes simplicity is a good thing. If I lived closer to you I'd offer to swap your barrel out and see how it worked on my rig which runs reliably with my handloads.

 

[Metal god]

Mounts to rifle-length receiver extension tube without A2 spacer but will also accommodate Mil-Spec sized M4 Carbine and many A5-length tubes with standard Castle Nuts and End Plates

Interesting , forgive my ignorance I had no idea that was a thing . I just assumed you switched to a A2 buffer tube as all my A2 style fixed stocks use . I've mixed and matched uppers and lowers ( carbine and A2/rifle) and they all work with one another so I guess you should be GTG . I will say virtually all my lowers that run a A2 style buffer tube , rifle buffer and spring run smoother regardless of upper on them . This isn't to say you can't get the carbines to run smooth . It just seems the A2's seem to get there quicker with minimal tweaking .

 

[Brian Pfleuger]

Well gents, for whatever the reason, I guess we can quit wonder why the gun don't work because it magically works just fine now.

I loaded up 9 rounds at 25.5gr 2495 under 95gr Nosler BT and figured if they shot reliably I would try the Hornday. I set the target up at 35 yards and got this result, 2 shot groups with scope adjustments between:

The rounds all extracted fine but a couple jammed. I also noticed that when the bolt locked back on the final round the bolt head was forward, so it would not catch a round from a mag. I assume this was a symptom of low power rounds also.

So, I moved the target to 100 yards and shot my last 3 rounds... marked "HL" below. Assuming the full-power 100gr Hornady would be higher at this range, I adjusted the scope 2" down and fired 2 rounds (Group 1), adjusted the scope 2" down and 1/2" right and fired 3 shot group 2:

So apparently not only does the gun now shoot... it shoots pretty darn well. This was done prone, with the handguard resting on the front of a Boyt rest that is designed to support front and back but the AR doesn't fit because of the grip, so I used it "backwards". So, here she is in her natural element:

 

[stagpanther]

Well gents, for whatever the reason, I guess we can quit wonder why the gun don't work because it magically works just fine now.

You gave it flowers and took it out on a nice date? You player, you!

 

[HiBC]

Don't you love it when a plan comes together?

 

[Metal god]

Great , looking good . So you did nothing different since last time out and now it works ? If so boy would I hate that , I’d me me confused then ever .

 

[Brian Pfleuger]

Great , looking good . So you did nothing different since last time out and now it works? If so boy would I hate that , I’d me me confused then ever.

That is correct... and I kind of hate it too, but not as much as I hated it when it didn't work.

Who knows? After the last jam with the Hornady ammo, I took the BCG out, basically looked at it said to myself, "WTH? :shrug:", took it apart, cleaned and oiled it (again) put it back together and the gun has run ever since. The BCG was and has been always properly cleaned/lubed every time and I didn't do anything different... so...

But sometimes you just have to shut the hell up and shoot, I guess.

 

[Scorch]

Sweet. And it shoots good! Gotta love it! Congratulations!

 

[Shane Tuttle]

Normally speaking, even well discussed Threads don't last past 3 pages...moreless 174 comments....before one is closed.

This one bucks the trend. And, by looking at the end product, it was well worth most every post.

Congratulations, Brian. That AR of yours turned out to be SICK!

 

[Metal god]

Yeah and Stag and I are only like 170 of those