7mm SAUM AR

Schanen51

Private
Minuteman
May 16, 2024
4
7
Central Oregon
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.

Dimensions are just shy of 46" and 12.5 lb. My plan is to shoot it suppressed, so the actual dimensions while shooting will be in the lower 50s and around 14lb.

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.

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.

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That's going to have a ton of dwell. Any reason that you decided to go with rifle length gas instead of magnum (+2) length gas? Did you spec your gas port diameter or use their default, and can you post that diameter?
You're right, it will. Main reason was I only found 2 barrel makers that make 7mm SAUM barrels for an AR, and X-Caliber was the better of the 2. X-Caliber didn't have a magnum option when I ordered the barrel. I could have called and maybe had them do magnum instead of rifle, but I didn't. I was more concerned with cycling, than losing velocity, to be honest. The gas port is oversized, by how much I don't know. Didn't measure it before installing the gas block. The gas block is adjustable, so I'll progressively open the gas block until I'm getting enough gas to cycle. I can also adjust the spring tension in the silent capture system. Will it help? Who knows. I plan to find out though.
 
Good to know I’m not the only one trying this one.
I went with an 18” barrel with a rifle + 2” gas length from X-caliber. I’m planning on using 140/150gn bullets primary for hunting.
Schanen51,
How did you open up your ejection port and what magazines are you using.

I’m playing around with 3D printed followers in 20rd steel mags.
 
Good to know I’m not the only one trying this one.
I went with an 18” barrel with a rifle + 2” gas length from X-caliber. I’m planning on using 140/150gn bullets primary for hunting.
Schanen51,
How did you open up your ejection port and what magazines are you using.

I’m playing around with 3D printed followers in 20rd steel mags.
Nice! What kind of velocity are you thinking you'll get out of the 18" barrel? I have a bunch of 162s for my 7mm RM that I shoot, so I'm planning on using those for sure, and I also have some 175s, but the magazine might stop me from using those.

KAK Industries sells a magnum AR10 stripped upper receiver that is machined to have a larger ejection port. They machine it AFTER anodizing though, so it's just exposed aluminum.

For mags I'm using LaRue mags. Larue mags and Knights Armament SR-25 mags have the longest possible OAL at 2.870". Someone also said ASC mags have a longer OAL, but the reviews on ASC mags are mixed. If you come up with a good solution, let me know.
 
Nice! What kind of velocity are you thinking you'll get out of the 18" barrel? I have a bunch of 162s for my 7mm RM that I shoot, so I'm planning on using those for sure, and I also have some 175s, but the magazine might stop me from using those.

KAK Industries sells a magnum AR10 stripped upper receiver that is machined to have a larger ejection port. They machine it AFTER anodizing though, so it's just exposed aluminum.

For mags I'm using LaRue mags. Larue mags and Knights Armament SR-25 mags have the longest possible OAL at 2.870". Someone also said ASC mags have a longer OAL, but the reviews on ASC mags are mixed. If you come up with a good solution, let me know.
I originally was going to build a 300 WSM like KAK. Unfortunately I couldn’t use their upper as I’m building on a MA-10 rifle. After plenty of online research and a email to ADG I’m aiming for 2800-2900 with 150s. That will give me 1800fps to 700yds.

I’m using the ASC mags as I had a few for the 308 upper. I’m trying to understand the fine points of magazine design to solve the mag bulging issue.
 
I have a 7wsm bolt gun and just enough spare parts lying around that I feel like it’s needs a gas operated sibling… please keep us updated on progress. Especially what works and what doesn’t
 
Something interesting I noticed tonight; the X Caliber 7SAUM barrel has a single feed ramp rather than the traditional “M4” style dual ramps. Given this I’m going to proceed with an MJF printed single stack magazine designed to fit in a “SR-25” mag well.
 

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Sorry if I’m high jacking your thread OP.
The rest of my parts will be in by the end of the week. White Oak gas tube, Riflespeed gas block and SLR handguard.
starting loads will be 150 & 175 ELD-X over H4831SC in ADG brass. I’ll update how everything goes this weekend.
A prototype of the single stack mag may be printed this week as well.


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I designed it based off of some research and other magazine dimensions. The first print has some issues. The rear webbing being so thin allows the side walls to bow out.
This is just the first rev.
 

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I’ve got mine together for some initial load testing. I had to steal the handguard off my 308 as I ordered the wrong barrel nut for mine.
Here is the upper parts list.
Mega upper
KAK bolt carrier and magnum bolt
Xcaliber barrel - 18”, 1/8 twist, 14” gas system
Handguard is a SLR as is the new one.
Rifle speed gas block
 

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My first loads were 150 ELD-X over H4831sc in ADG brass. The initial shots went well, I had the rifle speed set at six and single fed rounds over an empty mag. The gas was enough to lock the bolt back and the brass would just clear the port. Grouping was “good” for a load testing. Recoil was more than a 6.5 Grendel less than a 308 both from mid length gas systems.

Overall I’m pleased, the rifle comes in at 12lbs empty, it’s on the heavy side but it’s well balanced. Other than load development the last problem to solve is the mag.
 
Velocity?
Amount of powder you were able to put in the load, and still get the gas system to function properly?
Load psi?
Thanks

Velocity?
Amount of powder you were able to put in the load, and still get the gas system to function properly?
Load psi?
Thanks
I was using Hornadys load data which is considerably lower than Noslers. My max load was 58gn with an overall length of 2.820”. Velocity at 58gns was 2617 measured at the muzzle with a magneto speed. Nosler suggested max load is 62.2gns of the same powder. I’ll be shooting the new test loads this afternoon working up to 62.2 in .2 increments.

All my load work is done with the suppressor on. I’ll probably hold off fine tuning the gas setting until I have a load.
 
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That’s not my target velocity, that’s just what the Hornady load manual produced. I don’t know why the Hornady max load for that powder is 4gns less that Noslers max load. It’s also an 18” barrel.

I should sort it out this afternoon as I’ve got new loads worked up to 62.2gns.
 
“shrugs” I did lots of reading.

Anyway,
The rifle continues to shoot well, I worked up to a max of 62.2gns of H4831 over a 150 ELDX. Velocity was 2832. No pressure signs or swipe marks on the base.
The only issue I ran into was too much gas causing deformation in the rim where the extractor grabbed the rim.

What I thought was the cases barely ejecting at #6 gas setting was the cases bouncing off the top of the ejection port. I turned the gas down to #3 and didn’t see the extractor issue again.

The next load set will be once fired brass at 61.4gns to further tune the gun and gas. I removed some material from the top of the ejection to see if that helps ejection.

Edited a typo on the charge weight above.
Any insight on the 7SAUM loads would be great. I want to get the gas more dialed in before I try other loads or push the current load further.
 
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Now that I’ve got more data I’d like to get input from the site.
I’ve done ladder test with the 150 ELDX over H4831SC & 7828SC and a 10 shot string with 61.5 of H4831SC. The H4831 load had an average velocity of 2809 with an SD of 10, grouping was 1”. With 7828 I was able to get a velocity node at 50fps over H4831 with a max of 2923.

All that said if the 7828 load groups well I’ll be looking at a load with the ballistics shown on the attached screenshot. I’d like to know where y’all would take the load. Try a different powder or try the 162 ELD? I’d like to go ahead and try anything new now so I can dial in everything for deer season.

The 7828 load(external ballistics) is within my original “design spec” for 500yd capable deer rifle. Grouping will need to improve but the velocity is there.
 

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Small update:
The 150 ELDX / 62.3gn of 7828 load did very well today.
Average velocity of 2857 / SD 9.4 and a nice tight group minus a first round flyer. I’ll load up 20 or so rounds and stretch it out to distance.

also tried the 162s /7828, found a velocity node around 2770.

I think I’ve solved the magazine issue as well.
 

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What was the trick to the magazine?
Designing two inserts that convert the mag to single stack.

Spring is a 6.5 Grendel 25rd spring. It holds 8rds in a ASC 20rd 308 mag and locks back on the last round. So far it feeds flawlessly, but more testing is needed to really prove the concept.
 

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Update: currently in the middle of load development, the rifle is functioning very well with no major issues. I’ve got three loads I’m working with.
1) 139 Interbond w/ 64gn @ 3045fps
2) 150 ELDX w/ 62.3gn @ 2875fps
3) 162 ELDM w/ 60.9gn @ 2783fps

All in ADG brass using 7828ssc. I did switch from CCI 250s to 200s in this last round to see what difference that makes. The 250 seemed over kill for 60gns of powder.

What’s the go to brass prep for a gas gun? I’m sure there is a thread on that here.

Tweaks: I need to order a couple of the larger plungers for the rifle speed gas block. Currently it’s on setting 2 with the stock plunger. This gives me plenty of room for more gas but nothing below. A longer plunger should give me a more refined adjustment range.
 
Ok, so I am looking to do the same thing. I was trying to decide between a 7 WSM or 7 SAUM. I build a lot of AR's for PRS and was thinking of playing around with one for ELR...I know...WHY! LOL Just to do it! So what issues did you run into that I should know about? My plan is Areo Enhanced M5 kit, 26" xcaliber barrel, KAK BCG, because I think they are the only one who makes one. +2 gas with adjustable GB, Super heavy buffer and david tubbs flat wire spring. What did you learn and what should I know? thx!
 
It’s honestly pretty straight forward. Some flavor of SAUM would be my suggestion, but ELR may not be the best purpose. I did some initial loads with 175s, they fit and feed but they are waaaay short of the COALs everyone is running for 180 class bullets. 2.825” will be your COAL.
As far as modifications go you’ll need to open up your ejection port. Specifically the top edge. I did mine with the infamous dremmel(KAK magnum uppers already have this done).
I’m not sure what my spring and buffer combo was but it’s was the same setup I used in my 16” 308 upper. The key to the gas is the Riflespeed block. Get it with the longer plungers so you have better range of adjustment. I still need to order one.
 
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Ok, so I am looking to do the same thing. I was trying to decide between a 7 WSM or 7 SAUM. I build a lot of AR's for PRS and was thinking of playing around with one for ELR...I know...WHY! LOL Just to do it! So what issues did you run into that I should know about? My plan is Areo Enhanced M5 kit, 26" xcaliber barrel, KAK BCG, because I think they are the only one who makes one. +2 gas with adjustable GB, Super heavy buffer and david tubbs flat wire spring. What did you learn and what should I know? thx!
Do you reload for your PRS gas guns? What is your brass prep and load development for gas guns?
 
It’s honestly pretty straight forward. Some flavor of SAUM would be my suggestion, but ELR may not be the best purpose. I did some initial loads with 175s, they fit and feed but they are waaaay short of the COALs everyone is running for 180 class bullets. 2.825” will be your COAL.
As far as modifications go you’ll need to open up your ejection port. Specifically the top edge. I did mine with the infamous dremmel(KAK magnum uppers already have this done).
I’m not sure what my spring and buffer combo was but it’s was the same setup I used in my 16” 308 upper. The key to the gas is the Riflespeed block. Get it with the longer plungers so you have better range of adjustment. I still need to order one.
Thanks for the information! I'm going to run a 7 SAUM, much better long range ballistics than the 300 SAUM. I run build and run Gas Guns in PRS and have been pretty successful in the GG class. I regularly take mine (6GT and 6.5 Creedmoor)out over 1,000yds with good success. But I have a friend who shoots ELR and wants me to come shoot. So I figured it would be fun to build an ELR gas gun. Is it the best choice, nope. But it will be a hell of a lot of fun! lol Did you do anything to your magazines to get them to run? I'm planning on using my Larue, it gives me the most clearance.
 
Thanks for the information! I'm going to run a 7 SAUM, much better long range ballistics than the 300 SAUM. I run build and run Gas Guns in PRS and have been pretty successful in the GG class. I regularly take mine (6GT and 6.5 Creedmoor)out over 1,000yds with good success. But I have a friend who shoots ELR and wants me to come shoot. So I figured it would be fun to build an ELR gas gun. Is it the best choice, nope. But it will be a hell of a lot of fun! lol Did you do anything to your magazines to get them to run? I'm planning on using my Larue, it gives me the most clearance.
What are the ranges shot in ELR? With the 162 load I’ve been working on it stays supersonic to 1496yds. That’s at more or less sea level and going 2770fps from an 18” barrel. So your 26” barrel should much faster. I don’t know if X-Caliber would do it but you may want to look into having the chamber cut for the load/seating depth you want to use.

On the mags, scroll up a few post. I’m still testing them.
 
What are the ranges shot in ELR? With the 162 load I’ve been working on it stays supersonic to 1496yds. That’s at more or less sea level and going 2770fps from an 18” barrel. So your 26” barrel should much faster. I don’t know if X-Caliber would do it but you may want to look into having the chamber cut for the load/seating depth you want to use.

On the mags, scroll up a few post. I’m still testing them.
ELR goes out over 2,000yds, so I'm trying to push my speeds to 3,000ish.
 
Load development update
150 ELDMs have proven to be the sweet spot for this barrel. 62.2 grains of 7828 provides good groups and around 10fps SD.
IMG_7291.jpeg

8rds of the above load in fired ADG brass
Average velocity was 2910
IMG_7290.jpeg

5rds of the above load in fresh ADG brass velocity average was 2808

Is a 100fps difference common from formed to fired brass? I’ve never really gotten this granular with load development.
 
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