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Magnum AR Project

M

Masczek

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Full Member
Minuteman
May 3, 2010
39
54
Cody, WY
I thought some of you would find the project I've been working on interesting. This is my AR10 style rifle chambered in 6.5 PRC and 300 WSM:

OZBQ5230.JPG
IMG_4290.JPG

The lower receiver is my design, it accepts AICS short action mags. The mags allow for slightly longer OAL for the 300 WSM and factory length 6.5 PRC cartridges, plus they're a lot more stout than AR mags IMO. I'm still experimenting with the gas system and buffer and spring rates, both 300 WSM and 6.5 PRC guns are slightly over gassed. Other than that, the guns feed very well, no complaints on the magazine performance. I'll post more info as I get out and test more.
 
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3D Printed lower?
Or just a modified standard AR10/LR308 lower?

This is gonna be a great thread...

The hardest part about getting a magnum AR to feed so far is that the current PMAGs are set up for a .308 sized cartridge/brass...make a few mods, and it feeds like a dream.

So having the ability to use magnum AICS mags is awesome!
 
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Earnhardt said:
3D Printed lower?
Or just a modified standard AR10/LR308 lower?

This is gonna be a great thread...

The hardest part about getting a magnum AR to feed so far is that the current PMAGs are set up for a .308 sized cartridge/brass...make a few mods, and it feeds like a dream.

So having the ability to use magnum AICS mags is awesome!
Click to expand...
It's a CNC machined lower that I designed. I have a small machine shop along with a type 7 FFL, so why not. Yeah, the AICS mags need no modification at all, they work great!
 
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Update:

I've hit a snag after testing a new 300 WSM barrel this weekend. I believe the gun is still over gassed and the gas port in the barrel is too large, but I'd like some input on this theory.

I have a SLR adjustable gas block installed with a +2 rifle length gas tube, I'll adjust it until I can get the rifle to eject (there is no bolt stop, I'm still developing this). The bolt will come out of battery slightly with the empty case still in the chamber. I'll open the gas block one more click and the brass might eject at the 1:30-2:00 position. If the empty case doesn't eject it's stove piped. The brass has a very slightly raised rim where the extractor claw grabs the case rim (dwell time too short). I have a carbine Griffin Armament +15% extra power spring along with a 5.5oz buffer.

My theory is the gas port in the barrel is probably too large, it measures at .115 in diameter. With the port that size, I think I cross a very fine line when I open the gas block one click. Maybe a gas block that has a finer adjustment might help? Possibly a heavier buffer may help too, but I want to try and restrict the gas more. I think if I have a barrel with a gas port around .075 it may solve the problem.

Thoughts?
 
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Any AGB that just has a threaded adjustment screw with no click detents will be adjustable as fine as you can get. Seekins, Wojtek etc.
Honestly +2" isn't excessive on a long barreled 6.5CM, I could see a PRC needing longer.
 
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I'd be running a very heavy buffer. What length receiver extension are you using? A gas port of 0.115" is about as big of a gas port there realistically is. The 3rd step in the horizontal bore of a mil-spec gas key is 0.116"(+0.005").
 
Masczek said:
It's a CNC machined lower that I designed. I have a small machine shop along with a type 7 FFL, so why not. Yeah, the AICS mags need no modification at all, they work great!
Click to expand...
This would be awesome for use with a Uintah Precision bolt-action upper... The AICS mags are much better than AR10 mags, with much longer COAL, especially if you use the MDT mags without the binder plate.
 
Masczek said:
Update:

I've hit a snag after testing a new 300 WSM barrel this weekend. I believe the gun is still over gassed and the gas port in the barrel is too large, but I'd like some input on this theory.

I have a SLR adjustable gas block installed with a +2 rifle length gas tube, I'll adjust it until I can get the rifle to eject (there is no bolt stop, I'm still developing this). The bolt will come out of battery slightly with the empty case still in the chamber. I'll open the gas block one more click and the brass might eject at the 1:30-2:00 position. If the empty case doesn't eject it's stove piped. The brass has a very slightly raised rim where the extractor claw grabs the case rim (dwell time too short). I have a carbine Griffin Armament +15% extra power spring along with a 5.5oz buffer.

My theory is the gas port in the barrel is probably too large, it measures at .115 in diameter. With the port that size, I think I cross a very fine line when I open the gas block one click. Maybe a gas block that has a finer adjustment might help? Possibly a heavier buffer may help too, but I want to try and restrict the gas more. I think if I have a barrel with a gas port around .075 it may solve the problem.

Thoughts?
Click to expand...
It sounds like too much gas. What length is the barrel? I would have prob gone smaller on the port. It’s a lot easier to open a port than close it! A block with a threaded adjustment as suggested above is prob the next thing to try. With higher pressure stuff there can be a really fine line between not enough gas and way too much gas. A smaller port and maybe a heavier buffer would prob be the best way to widen that window on the next barrel.
 
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Masczek said:
Update:

I've hit a snag after testing a new 300 WSM barrel this weekend. I believe the gun is still over gassed and the gas port in the barrel is too large, but I'd like some input on this theory.

I have a SLR adjustable gas block installed with a +2 rifle length gas tube, I'll adjust it until I can get the rifle to eject (there is no bolt stop, I'm still developing this). The bolt will come out of battery slightly with the empty case still in the chamber. I'll open the gas block one more click and the brass might eject at the 1:30-2:00 position. If the empty case doesn't eject it's stove piped. The brass has a very slightly raised rim where the extractor claw grabs the case rim (dwell time too short). I have a carbine Griffin Armament +15% extra power spring along with a 5.5oz buffer.

My theory is the gas port in the barrel is probably too large, it measures at .115 in diameter. With the port that size, I think I cross a very fine line when I open the gas block one click. Maybe a gas block that has a finer adjustment might help? Possibly a heavier buffer may help too, but I want to try and restrict the gas more. I think if I have a barrel with a gas port around .075 it may solve the problem.

Thoughts?
Click to expand...
I encountered this problem tuning gas on magnum AR's. The adjustment on the SLR gas block is too coarse. I found success using an adjustable gas key on the BCG allows for a finer adjustment. I also found there is a tendency for gas blocks to come loose from the barrel when they only have 2 set screws or clamp screws on a magnum. A JP clamping gas block works better. Also set screws will loosen and back out as you are firing (on adjusted gas blocks which use one set screw to adjust gas and a 2nd set screw to tighten against gas screw) will cause a more overgas situation that could damage your rifle.
 
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Thanks for the replies. Here's a little bit more info on the current setup:

The rifle has a milspec carbine buffer tube with a 22" barrel. The reloads are 200gr Hornady ELD-X with 60.5gr of H4350. I know it's at the upper end of things, but I want to make sure the system will work with higher pressure loads. I've got an Superlative Arms gas block I'm also going to try.

When I ordered the barrel from X-Caliber, I specified I was using an adjustable gas block. They built me the barrel with the .115 gas port. Before I order another barrel I'm going to call them and see if I can have them drill the gas port around .062. Like you guys said, I can always drill it bigger if I need to.
 
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Masczek said:
I thought some of you would find the project I've been working on interesting. This is my AR10 style rifle chambered in 6.5 PRC and 300 WSM:

View attachment 8127645View attachment 8127646
The lower receiver is my design, it accepts AICS short action mags. The mags allow for slightly longer OAL for the 300 WSM and factory length 6.5 PRC cartridges, plus they're a lot more stout than AR mags IMO. I'm still experimenting with the gas system and buffer and spring rates, both 300 WSM and 6.5 PRC guns are slightly over gassed. Other than that, the guns feed very well, no complaints on the magazine performance. I'll post more info as I get out and test more.
Click to expand...
Great job, buddy. AR-10 in 6.5prc and 300wsm is definitely worth the project. Since you don't have any feeding issue, you should enjoy shooting the rifle.
 
Haven't had a lot of time to do some extensive testing, but here's a video I got of it last week.


This is the 6.5 PRC upper, it works a bit better, but is still over gassed. I tried both the Griffin Armament +15% extra power spring, and standard AR 308 carbine spring, both seem to operate the same when firing. I've got the Superlative Arms bleed off adjustable gas block installed on it now, so I'll see if this helps the over gassed issue by bleeding the excise gas out. I'm also going to try this on my 300 WSM barrel as well. I haven't ordered another test barrel yet, but I can get one with a .064 sized gas port. I'll start testing with that and drill the hole bigger if I need too.
 
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Masczek said:
Haven't had a lot of time to do some extensive testing, but here's a video I got of it last week.


This is the 6.5 PRC upper, it works a bit better, but is still over gassed. I tried both the Griffin Armament +15% extra power spring, and standard AR 308 carbine spring, both seem to operate the same when firing. I've got the Superlative Arms bleed off adjustable gas block installed on it now, so I'll see if this helps the over gassed issue by bleeding the excise gas out. I'm also going to try this on my 300 WSM barrel as well. I haven't ordered another test barrel yet, but I can get one with a .064 sized gas port. I'll start testing with that and drill the hole bigger if I need too.
Click to expand...

Very awesome!
I'll just throw out another gas tuning option if your gas journal is 0.75". It might not be as user friendly as an AGB, but is bullet-proof and cheap. YHM came out with a small passage, low profile, set-screw gas block a couple years ago. The gas passage (bore that connects the gas port to the gas tube) of many lo-pro 10-32 set-screw blocks is 0.159". The YHM small passage block is 0.059", IIRC. It works as-is for many applications, but can be easily drilled out larger if needed. You obviously have to remove the gas block to drill it out, but for under $20 and no moving parts, it might be something to consider.
 
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Masczek said:
Haven't had a lot of time to do some extensive testing, but here's a video I got of it last week.


This is the 6.5 PRC upper, it works a bit better, but is still over gassed. I tried both the Griffin Armament +15% extra power spring, and standard AR 308 carbine spring, both seem to operate the same when firing. I've got the Superlative Arms bleed off adjustable gas block installed on it now, so I'll see if this helps the over gassed issue by bleeding the excise gas out. I'm also going to try this on my 300 WSM barrel as well. I haven't ordered another test barrel yet, but I can get one with a .064 sized gas port. I'll start testing with that and drill the hole bigger if I need too.
Click to expand...

This is right up my alley man. I’m a gas gun guy at heart. Let me know when I can buy one!
 
msgriff said:
Very awesome!
I'll just throw out another gas tuning option if your gas journal is 0.75". It might not be as user friendly as an AGB, but is bullet-proof and cheap. YHM came out with a small passage, low profile, set-screw gas block a couple years ago. The gas passage (bore that connects the gas port to the gas tube) of many lo-pro 10-32 set-screw blocks is 0.159". The YHM small passage block is 0.059", IIRC. It works as-is for many applications, but can be easily drilled out larger if needed. You obviously have to remove the gas block to drill it out, but for under $20 and no moving parts, it might be something to consider.
Click to expand...
Thanks for the info, I may have to experiment with this in the future.

Precision Underground said:
This is right up my alley man. I’m a gas gun guy at heart. Let me know when I can buy one!
Click to expand...
PM sent
 
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Very interesting.

I also would guess the gas port is a little to big.
How much ? Hard for me to guess.... the closest thing I have to a Magnum SA AR is "kinda" my 300WM Browning BARMk II Safari. So not really.
 
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I tried the gun out this afternoon, here's what I found:

With 2 rounds in the magazine, the bolt seems to short stroke. Sometimes it will eject the spent case at the 4-5 o'clock position, sometimes it will barely unlock from battery. It never picks up the next round, and I don't have a bolt lock installed yet, so it don't know if it will lock back on empty (I'm guessing it probably won't). I see these symptoms through all the adjustments I made with the gas block, even with it all the way open, it acts like it's not getting enough gas.

I believe I'm on the right track. My theory is that the gas port is just too small, even with the gas block open all the way, the port is just not letting enough gas through. I'm debating on opening up the gas port around .073" but don't want to go too big too quick. What do you all think?
 
Masczek said:
I tried the gun out this afternoon, here's what I found:

With 2 rounds in the magazine, the bolt seems to short stroke. Sometimes it will eject the spent case at the 4-5 o'clock position, sometimes it will barely unlock from battery. It never picks up the next round, and I don't have a bolt lock installed yet, so it don't know if it will lock back on empty (I'm guessing it probably won't). I see these symptoms through all the adjustments I made with the gas block, even with it all the way open, it acts like it's not getting enough gas.

I believe I'm on the right track. My theory is that the gas port is just too small, even with the gas block open all the way, the port is just not letting enough gas through. I'm debating on opening up the gas port around .073" but don't want to go too big too quick. What do you all think?
Click to expand...
I would expect that the port size might not be correct the first time around. Whether you go up one number size bit at a time or skip a few is a judgement call. I would also de-burr/chamfer the inside of the port each time it was enlarged. Taking a video from the side during testing would help track BCG travel increases.
 
300 WSM, 22" barrel and 200gr Hornady ELDX. I realize they are hot loads, but I'd like to get reliable cycling with as much speed as possible. After that, I'll work on accuracy.
 
Masczek said:
300 WSM, 22" barrel and 200gr Hornady ELDX. I realize they are hot loads, but I'd like to get reliable cycling with as much speed as possible. After that, I'll work on accuracy.
Click to expand...
You mighy ask KAK Industries what size gas port they used on their magnum uppers. If I still had mine I could check but alas I stupidly traded it
 
I've thought about buying a KAK upper and seeing how well it works with my receiver. Yes, 200gr is heavy for the 300 WSM IMO.
 
I finally found that a .077 gas port is the size that works best for the setup that I currently have. Dwell time still seems a bit quick since some of the rims are only very slightly raised, but not all of them. I currently have a 5oz buffer, I'm thinking a slightly heavier buffer will give me the perfect recipe.

That being said, has anyone had their AR have cycling issues when having a heavy optic installed on it? I'm also getting ready to build a version 3 receiver, more on that to come as well.
 
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Masczek said:
I finally found that a .077 gas port is the size that works best for the setup that I currently have. Dwell time still seems a bit quick since some of the rims are only very slightly raised, but not all of them. I currently have a 5oz buffer, I'm thinking a slightly heavier buffer will give me the perfect recipe.

That being said, has anyone had their AR have cycling issues when having a heavy optic installed on it? I'm also getting ready to build a version 3 receiver, more on that to come as well.
Click to expand...
Sounds like you could use a little more delay before unlock.

Make sure your gas system is clearance right. Bolt tail - bore, ring run in carrier. Use good 3 pc rings. If there is a lot of leakage it'll be less responsive to fine adjustments similar to what you described above. That said my guess would be .085 - .096 max for the port using adj block.

+3" might help with your current load, but it may become real finicky if you switch projectile weight lt - hvy.

Rifle Speed gas block is worth a look. The one I have seems very well designed & has a large/fine adjustment range. Not prone to seizing up either based on design.

I'd suggest switching the tube to a A5 & trying a 8.5oz XH Carbine buffer from here. https://heavybuffers.com/ar10carbine.html
That setup also uses a std length rifle spring. I've got this buffer with a JP spring in a 308 & it runs like a champ. Your not stuck with the weight at 8.5.
I played around swapping aluminum spacers for the tungsten down to around 4.5oz.

As for the heavy optic............No. I don't even understand the rational behind that. If it was true than same logic would apply to a heavy bi-pod, rail weights, free or relaxed shoulder pocket tension.
 
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357Max said:
Sounds like you could use a little more delay before unlock.

Make sure your gas system is clearance right. Bolt tail - bore, ring run in carrier. Use good 3 pc rings. If there is a lot of leakage it'll be less responsive to fine adjustments similar to what you described above. That said my guess would be .085 - .096 max for the port using adj block.

+3" might help with your current load, but it may become real finicky if you switch projectile weight lt - hvy.

Rifle Speed gas block is worth a look. The one I have seems very well designed & has a large/fine adjustment range. Not prone to seizing up either based on design.

I'd suggest switching the tube to a A5 & trying a 8.5oz XH Carbine buffer from here. https://heavybuffers.com/ar10carbine.html
That setup also uses a std length rifle spring. I've got this buffer with a JP spring in a 308 & it runs like a champ. Your not stuck with the weight at 8.5.
I played around swapping aluminum spacers for the tungsten down to around 4.5oz.

As for the heavy optic............No. I don't even understand the rational behind that. If it was true than same logic would apply to a heavy bi-pod, rail weights, free or relaxed shoulder pocket tension.
Click to expand...
Never heard of Rifle Speed gas blocks, I'll give them a look. I'll be trying a heavier buffer as well.

The only reason I ask about heavy optics is that I used to work for a company that sold big bore AR's. One of the customers could not get their gun to cycle and sent it back in to have us look at it. After taking the scope off, the gun functioned as it should, put the scope back on, no dice. That all being said, those guns were already extremely finicky and were operating on the line of failure anyway.
 
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Masczek said:
I finally found that a .077 gas port is the size that works best for the setup that I currently have. Dwell time still seems a bit quick since some of the rims are only very slightly raised, but not all of them. I currently have a 5oz buffer, I'm thinking a slightly heavier buffer will give me the perfect recipe.

That being said, has anyone had their AR have cycling issues when having a heavy optic installed on it? I'm also getting ready to build a version 3 receiver, more on that to come as well.
Click to expand...
Yes, on a Xanthos ultra light build, a heavier scope would affect how the gun cycled.
Also the brake would affect the gun’s “tune”…

Once I figured the scope and brake I would permanently use, i set the AGB, and buffer up for that.

Now I just use a Tubbs Flat Wire spring, and a Stoner 9 oz rifle buffer in my 6.5 CM, and the 12-15oz XXH Heavy buffer in the WSMs…that’s the easy button for me now

By the way Masczek…this is a great thread man!
 
I'll be using a simple 3 ribbed brake. I'm still debating on the scope, but I'm leaning toward a Leupold Mark 5 or US Optics FDN17.

I'll post more once I get out shooting more, I can't wait to get some loads developed!
 
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Earnhardt said:
Yes, on a Xanthos ultra light build, a heavier scope would affect how the gun cycled...
Click to expand...


How?

It’s a non-reciprocating part, and it’s not attached in any way to any part of the system that does move…or that moves the gas that causes the other parts to move.

Maybe I have the dumb today, but this statement makes no sense whatsoever to me.

—-

…obligatory pic of my Xanthos-based lightweight build:

i-VW3DkvS.jpg
 
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91Eunozs said:
How?

It’s a non-reciprocating part, and it’s not attached in any way to any part of the system that does move…or that moves the gas that causes the other parts to move.

Maybe I have the dumb today, but this statement makes no sense whatsoever to me.

—-

…obligatory pic of my Xanthos-based lightweight build:

i-VW3DkvS.jpg
Click to expand...
Is your load “average” or “hot”?

All my loads are hot…

Set your load up, set your AGB, along with buffer, remove the light scope (in my case, a Leupold VX3 at 12oz…the install a 30-40 oz scope…try it

Do the same thing with your brake…set your gun up with no brake…then install a very effective slab brake…try it

The lighter the BCG and buffer, and the hotter the load, the more my cycling was effected by the changing of scopes and brakes
 
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I believe you. And I can somewhat understand the brake having an impact, at least a little bit; similar to adding a can with the commensurate increase in back pressure, only the opposite.

However, the scope mounted should have zero impact on how the reciprocating mass cycles. The only way I could see it having any impact is if the upper is out of spec, or maybe the mount was installed without torquing the bolts in the correct order, and/or massively over-tightened to the point where you get enough deflection in the upper receiver for the bolt carrier to drag along one side or another, or even misalign it enough for the bolt carrier to strike an edge.

I’d think you’d have more serious cycling issues under those circumstances though.

If this does impact it in some way, someone please enlighten me…I’m an engineer by training (Aeronautical though, so not much help here, LoL), but don’t claim to be any kind of expert in AR gas systems, and certainly not a gunsmith by any stretch of the imagination.

Not trying to be a jerk here, truly want to understand.
 
Momentum! and relative motion of masses

Light gun follows the reciprocating mass. Imagine mounting the carrier onto a mass of equal weight. Now drive the system backwards under recoil while trying to accelerate the carrier away from the mass. Gun tries to keep up

Heavy gun. Now mount the carrier onto as mass that weights 500 times the carrier weight. Recoil barely moves the system so the carrier can be accelerated away at the same rate but separates without the gun following it.

Example - 24" Creedmoor with plain barrel cycles cleanly with a 1.3mm diameter gas port. Fluting the barrel caused the gun not to cycle until the gas port was enlarged to 1.5mm diameter.
 
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Be interested to see the results, a good basic experiment. See if you can measure how much weight you applied, the base weapon weight and the BCG weights.

Observed that the AR10 (Armalite pattern) has one of the heavier carriers of the 308 class guns and this promotes slower initial acceleration for the input force so the guns are more suited to using suppressors without dropping primers

Extremely intrigued in the work of the OP into large caliber AR type mechanism. Posts are informative and much appreciated.
 
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I've heard of but never personally seen the optic causing malfunctions phenomenon. In the several minutes I've spent thinking about it I've always theorized its gotta be a little like limp wristing a pistol or holding an inertia shotgun so tight you stall it. Maybe you can limp wrist a pistol and it works. Stick a heavy comp and a red dot on the slide, change the reciprocating weight and all of the sudden it doesn't. That's the non scientific analogy that pairs well with my even less scientific theory.
 
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Sendaloaf said:
This build is super cool!

Do you mind my asking what your parts list was for this project?
Click to expand...
Most of the parts were things I had laying around. The barrels I got from X-Caliber, I had a Magpul PRS lite stock and Magpul standard grip, Geissele Dual Stage trigger, Spuerlative Arms adjustable gas block. I have two bolts, one from KAK Industries and one that is modified (not sure what brand).
 
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What kind of weight are we talking about for the naked rifle?
 
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