7mm SAUM AR

[Schanen51]

Posted on another forum too. Maybe some of you will enjoy this!

You may be asking, "why?!" I asked myself that question as well, honestly. I had a hard time answering it too. I guess it just fits in the 'unique' moderately useful rifle category. Some of you picky folks might call me out for calling this an AR10 and not an LR308, but for the sake of easy description, we're going to use AR10.


I have officially assembled a 7mm SAUM AR10. This whole thing started when I found an Aero M5 lower on sale for $98... I had X-Caliber cut a 24" barrel, using a KAK magnum bolt that I sent them for proper headspacing. While the barrel was in queue, I started gathering parts. I used a KAK magnum bolt carrier group, as stated earlier, and a KAK magnum stripped upper. They essentially just mill a larger ejection port. It's a little bit cheesy though, because they mill it AFTER anodizing. I really don't care, as it'll probably get rattle-canned somewhere down the line anyway. I put an adjustable gas block on it, as well as a jp heavy scs. It should be (relatively) tunable to ensure proper function. I threw on a luth-ar fixed stock because it was one of the few I could find with adjustable comb.


Why 7mm SAUM you ask? The SAUM line has piqued my interest for awhile, actually. And full disclosure, had I been able to find 300 SAUM brass, I'd probably have gone with that instead. I like the idea of having a semi-auto magnum... maybe it just sounds cooler than it really is, but oh well. It's definitely capable of taking any game in my local area, and if I want to shoot it long range, I can do that too. I think the 7mm is incredibly adaptable to whatever you want It's use to be. I have larue SR-25 mags that have a COAL limit of 2.870, and book COAL on 7mm SAUM is 2.825, so I have a little bit of room to play with for seating depth. Let me tell you, those little bastards ain't cheap! Consensus on accurate barrel life seems to be about 2000rds. I picked up 300 pcs of ADG 7mm SAUM brass, that will (hopefully) survive the life of the barrel. I figured 6 loadings should be ok, and I only need about 60% of them to last a 7th. Time will tell, I guess. 

Dimensions are just shy of 46" and 12.5lb without suppressor, with suppressor, low-mid 50"s and about 14lbs. Way larger than I originally anticipated, but I wanted a 24" barrel to take advantage of the cartridge.

Let me know what you think. I only took one picture so far, so that's the one I put up with it, but if you want anything specific, don't be afraid to ask. And if you want a full build list, I can do that too. Might even be able to throw a price list together, if desired. 

I'm not sure how to post a picture directly onto the thread, so I've just attached it, and I had to resize it super small to fit the max file size... Hopefully you can see it ok.

[stagpanther]

Very cool! Brave project. Are you going with a single stack magazine and some kind of special straight-ahead feedramp like Tromix has for the 458 socom?

[Schanen51]

Thank you!! It should be fun to play around with. The SR-25 mags are traditionally double stack with 308, but realistically with the diameter of the 7mm SAUM case, it becomes a single stack. They're 20 rd mags, so I'm not 100% sure what the actual case capacity will be with the 7mm SAUM yet. The follower may or may not need some adjustment, but its a plastic follower, so I'm going to have to be really sure about where material needs removing before I start grinding at the follower. I don't [I]anticipate[I] feeding problems from the mag, but maybe I'm just being hopeful haha!

The good thing is, I'm not the first to take on this project. In fact, maybe 10 or more years ago, DPMS actually made a 7mm SAUM upper. I think lack of sales, and lack of available factory ammo killed the product. Someone might know better than me, but I'm fairly certain they used steel 308 mags. I'll update when I can finally get out and shoot.

[tangolima]

I'm tempted. I have a regular AR-10 in .243 Winchester. Do I need to upgrade the bolt? Any other barrel makers offer this caliber? Thanks.

-TL

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[Schanen51]

Yeah, you will need a magnum bolt. The SAUM brass is quite a bit larger than .308 brass. I used KAK industries, and that seems to be the most recommended route for magnum chambered ARs. You'll probably need to open up the ejection port on your upper if you're trying to convert it, but KAK sells one that's already milled. The 2 manufacturers that I found for barrels were X-Caliber and Tactical Ordnance. I have read not so great reviews about tactical ordnance barrels, so I went with X-Caliber. It's a 24" 1:8 twist stainless barrel. FWIW, they do a sale occasionally around big holidays, so if you're not in a hurry, it might pay to wait.

[georgehwbush]

Schanen51; yeah, i like. let us know how it goes when you start tuning thing.


and i totally agree that the 7mm is a very good cal. the only reason i went 243 for my ar-10 was because i had all the hardware for reloading 243 and didn't have it for 7-08 else i would have probably went 7mm

[tangolima]

Originally Posted by Schanen51

Yeah, you will need a magnum bolt. The SAUM brass is quite a bit larger than .308 brass. I used KAK industries, and that seems to be the most recommended route for magnum chambered ARs. You'll probably need to open up the ejection port on your upper if you're trying to convert it, but KAK sells one that's already milled. The 2 manufacturers that I found for barrels were X-Caliber and Tactical Ordnance. I have read not so great reviews about tactical ordnance barrels, so I went with X-Caliber. It's a 24" 1:8 twist stainless barrel. FWIW, they do a sale occasionally around big holidays, so if you're not in a hurry, it might pay to wait.

Thanks. The bolt cost $150. I have a side charging upper, and the ejection port is already enlarged, so it could be ok already. The barrel probably cost about $500? Reloading supplies etc. I reckon $750 ish I would be in business. Still quite a sum, but I will see.

6mm - 7mm is pretty cool. It is the range of ballistician's delight. Bullets are just a bit expensive though.

-TL

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[Schanen51]

and i totally agree that the 7mm is a very good cal. the only reason i went 243 for my ar-10 was because i had all the hardware for reloading 243 and didn't have it for 7-08 else i would have probably went 7mm

It's funny that you mention 7mm-08, because I was SO close to doing that instead of the saum. I had actually spec'd a barrel and was in the checkout process for it, and changed my mind last minute. I'll update this thread when I can find a few free moments to test some some stuff out.

Thanks. The bolt cost $150. I have a side charging upper, and the ejection port is already enlarged, so it could be ok already. The barrel probably cost about $500? Reloading supplies etc. I reckon $750 ish I would be in business. Still quite a sum, but I will see.

6mm - 7mm is pretty cool. It is the range of ballistician's delight. Bullets are just a bit expensive though.

-TL

Yeah, I got the barrel on a sale actually, so I think I paid less than $400 for it. Not to say that's cheap by any means, but I thought it was a fairly decent deal. If I could go back and reorder the barrel, I might call X-Caliber and see if they could drill a rifle +2 gas system instead of just rifle. In another thread, someone mentioned dwell time possibly being an issue. I was more worried about it cycling, and didn't really consider the possibility of dwell time being a problem. Food for thought I guess!

[tangolima]

24" barrel could benefit from longer gas tube, but a rifle length + adjustable gas block should suffice too.

-TL

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[tangolima]

OP. What barrel profile did you choose? Regular HBAR ok? X-caliber is running a sales. I'm thinking about it.

I'm getting the bolt from kak. Did you need to upgrade the firing pin? No change to the bcg?

Thanks.

-TL

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[HiBC]

I'm relatively certain they have discontinued it, but Armalite for a while offered the 300 SAUM in the AR-10. At least the upper.

He had to wait for quite a while,but my brother bought one.

Its a double stack,but it may not be a perfect stack. It works for him.

[Schanen51]

Originally Posted by tangolima

OP. What barrel profile did you choose? Regular HBAR ok? X-caliber is running a sales. I'm thinking about it.

I'm getting the bolt from kak. Did you need to upgrade the firing pin? No change to the bcg?

Thanks.

-TL

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OP. What barrel profile did you choose? Regular HBAR ok? X-caliber is running a sales. I'm thinking about it.

I'm getting the bolt from kak. Did you need to upgrade the firing pin? No change to the bcg?

Thanks.

-TL

TL I'm sorry it took me 3 months to see your question... I dropped the ball on enabling notifications. I used the SPR contour that X-Caliber offers. I did the full KAK bolt carrier group, and that came with the high pressure firing pin. AFAIK, the bolt would be a drop in if you wanted to use another carrier. As for whether the high pressure fp is a must... I have absolutely no idea. I don't want to give you bad info. Let me know if/when you decide to pull the trigger on building one. I've loaded up some ammo, but still can't seem to find time to shoot it. I started with h4350, but I have some h4831 and h1000 in reserve in case the h4350 doesn't do the trick. Loading 175 eld-x bullets for now

[tangolima]

No problem. Thanks for getting back.

I got the bolt, the die set, and the barrel. The barrel profile I chose was ar-10 heavy. Goodness it is heavy. I don't plan to shoot the gun mobile. So why not? The price is the same.

Fund is tight. Instead of getting another set of upper and BCG, I'm going to swap out the barrel of my current ar-10 in .243 win, till I recover from the credit card bill.

I still need brass. I plan to convert from WSM brass, which is more available at more reasonable price. What say you?

-TL

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[50 shooter]

Have you ever looked at the 7mm Valkyrie for the AR-15 platform?

Nice setup, check out their page. https://www.7mmvalkyrie.com/about.html

[tangolima]

First time heard about this round. Thanks. Looks interesting. Everything is expensive though.

I actually have been looking at 6mm Max, which is more affordable. Problem is the shop stopped making the barrels.

-TL

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[tangolima]

Mine finally works. The last issue was unreliable ejection. The extractor on the kak magnum bolt needed some fitting. It didn't have any cam-out with a brass head in the bolt face pocket, so that the extractor unhooked from the extraction groove prematurely.

The x-caliber barrel is top notch. The rifle shoots close to 1moa, occasionally better than, 10-shot from cold bore. It kicks rather noticeably, pushing 140gr bullet close to 3000fps, so I put a brake on even I don't like the noise. Burns a lot of powder and the brass costs small fortune. After some serious tinkering I can now convert from .300 wsm at a fraction of the price.

Now what's the next silly caliber I should try?

-TL

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[Metal god]

Mine finally works.

I can confirm yes it does , and shot a pretty nice group at 300yds

[tangolima]

Thanks Metal. That was the first group at the distance, but wasn't the best it has shot. Will try harder next time. The frustration to put the shot on paper initially distracted me from reading the wind. I need to focus on the mirage more.

-TL

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[tangolima]

Actually the extractor went through 2 rounds of fitting. I wouldn't be surprised as the bolt is advertised for "magnum", and there are variations among different magnums.

When I first started, the bolt couldn't go into battery. The inclined plane on the top of the extractor was too small to be cammed out by the brass head to engage the extractor groove. I filed the inclined plane larger to make it work. I didn't check the cam-out with a brass in the pocket at the time. It required taking out the dual ejectors.

-TL

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[stagpanther]

What is extractor cam-out--I've never heard of that before?

[tangolima]

Originally Posted by stagpanther

What is extractor cam-out--I've never heard of that before?

When the brass head is in position on the bolt face, it must be actively engaged by the extractor. The extractor needs to be cammed out (pushed away) by the brass groove by certain amount. That amount is the extractor cam-out. I want to see 0.005" ish.

In this particular case, the extractor is limited by a pad under the hook. I filed some material away from the pad to allow the extractor hook reach in further to engage the brass extraction groove. That makes a huge difference.

Having said that, this round probably hits the limit of ar platform. The brass has fat body and relatively small bullet. The difference between these 2 diameters dictates the angle of brass in the action before it gets kicked out by the ejector. Bigger this angle, the closer is the ejector to "run out of gas". The angle is pretty big for this round. The ejection pattern is neither "definite" nor "energetic", but acceptable. There are a few minor things I can try to improve, but I don't expect much.

-TL

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[stagpanther]

I see. I have a couple of their dual ejector bolts on ARs and have found when I've had issues they've been attributable to "too much" power in the ejectors or extractor--or possibly the opposite with gummed up ejector springs or broken ejector pin.

[tangolima]

I actually don't like the dual ejectors. It is over powered that it slams the brass body, shoulder and neck into the teeth of lock lugs and drag while pressing on it. One of the things I will try is to tweak the springs, putting weaker and different strengths. Different strengths can steer the trajectory of the ejection.

2 ejector holes on the bolt face reduces support to the brass head too.

-TL

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[stagpanther]

I don't have fun taking those things apart, but I've found bolt issues can often be traced to simply the inner parts being gummed up with residue--which are also getting regularly heat-blasted.

[tangolima]

It is actually not too hard to disassemble and reassemble. Much easier than, say, Rem 700 bolt. Gumming up is possible but I found it not very often. It is moving parts. The motion doesn't really allow easy deposit of foreign matters.

Wear and tear is more likely. I have just replaced a extractor pin in another ar. It had been sheared into 3 sections, causing extraction and ejection irregularities.

-TL

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