ar10 caliber choice decision tree for elr

[banditbump]

I am gonna build a elr ar10 platform rifle. Been researching and planning for years. Below I am gonna list a decision tree on determining step by step of my caliber choice by first choosing the case, then the caliber, and finally the catreridge. The goal is to choose the parameters that can shoot sub moa out to the furthest distance. Some written in stone rules:

1. Must function out of a 20 round magazine
2. Amount of recoil is not important
3. Would prefer a standard caliber but a popular custom round like 6.5 saum would be okay.
4. Minimum 6.5 caliber, I don't want a 6mm caliber. If it has longer extreme range because I may use the rifle for long range hunting in the future.
5. Barrel length will be 22 to 30 inches depending on the catridge, caliber and bullet choice.
6. Would like to be able to choose a setup that allows using less expensive lower bc bullets like amax, smk, accubond lr, as well as the pricier, harder to find, and higher bc berger bullets. I know in the end this may force me to make a chamber compromise but I will figure that out once I get there.

Step 1. Case choice: can the 284 win, saum, or wsm cases be used, in a 20 round standard magazine, with obviously a few round lower magazine capacity. If the answer is yes one of these cases will work in a standard magazine, will this extra case capacity translate to increased velocity all else being equal, example 260 vs 6.5x284 out of the same barrel length?

Step 2.

Projectiles in consideration

6.5 the 130 to 140s. I have read the 6.5 creedmoor has problems with 140 grain factory amax ammunition. is this a factory ammo problem or a problem just running heavy 6.5 bullets in the ar 10 platform?

7mm 162 and 180

308 208 amax and 230 berger can these be run hard in an ar reliably with a heavy barrel?

338 since the 230 berger has a g7 bc of nearly the 300 grain 338 I don't see how this route would work because velocity would be much lower than the 308 .


In summation:
1. Can a bigger than 308 case work in a standard 20 round magazine, it's okay if it works with magazine modification, as long as I can modify the magazine or pay someone to do it.

2. If so can using the larger case capacity translate into increased velocity without degrading reliability or exceeding pressure limitations of the cartirdge and ar 10 platform.
3. All else being equal, same case size and barrel length, which caliber is capable of the longest supersonic distance? A. 6.5 caliber130s and 140s, b. 7mm caliber 160s to 180. C. 308 caliber running 208s to 230s.

I greatly appreciate any advice or feedback you can give me.

Best,
Daniel

 

[Black max]

I am gonna build a elr ar10 platform rifle. Been researching and planning for years. Below I am gonna list a decision tree on determining step by step of my caliber choice by first choosing the case, then the caliber, and finally the catreridge. The goal is to choose the parameters that can shoot sub moa out to the furthest distance. Some written in stone rules:

1. Must function out of a 20 round magazine
2. Amount of recoil is not important
3. Would prefer a standard caliber but a popular custom round like 6.5 saum would be okay.
4. Minimum 6.5 caliber, I don't want a 6mm caliber. If it has longer extreme range because I may use the rifle for long range hunting in the future.
5. Barrel length will be 22 to 30 inches depending on the catridge, caliber and bullet choice.
6. Would like to be able to choose a setup that allows using less expensive lower bc bullets like amax, smk, accubond lr, as well as the pricier, harder to find, and higher bc berger bullets. I know in the end this may force me to make a chamber compromise but I will figure that out once I get there.

Step 1. Case choice: can the 284 win, saum, or wsm cases be used, in a 20 round standard magazine, with obviously a few round lower magazine capacity. If the answer is yes one of these cases will work in a standard magazine, will this extra case capacity translate to increased velocity all else being equal, example 260 vs 6.5x284 out of the same barrel length?

Step 2.

Projectiles in consideration

6.5 the 130 to 140s. I have read the 6.5 creedmoor has problems with 140 grain factory amax ammunition. is this a factory ammo problem or a problem just running heavy 6.5 bullets in the ar 10 platform?

7mm 162 and 180

308 208 amax and 230 berger can these be run hard in an ar reliably with a heavy barrel?

338 since the 230 berger has a g7 bc of nearly the 300 grain 338 I don't see how this route would work because velocity would be much lower than the 308 .


In summation:
1. Can a bigger than 308 case work in a standard 20 round magazine, it's okay if it works with magazine modification, as long as I can modify the magazine or pay someone to do it.

2. If so can using the larger case capacity translate into increased velocity without degrading reliability or exceeding pressure limitations of the cartirdge and ar 10 platform.
3. All else being equal, same case size and barrel length, which caliber is capable of the longest supersonic distance? A. 6.5 caliber130s and 140s, b. 7mm caliber 160s to 180. C. 308 caliber running 208s to 230s.

I greatly appreciate any advice or feedback you can give me.

Best,
Daniel

IMO you should look into a different platform for elr shooting. Why limit yourself with an AR platform?

 

[banditbump]

I want to see how far an ar 10 can go. I think in the ar 15 it's pretty well determined with 556 6.5 grendel and 6mm rounds. I have bolt guns and Luke them too, I just would like to see how far the ar-10 and catridge that work in it can be accurate.

 

[LawnMM]

ELR and AR10 don't go in the same sentence. You want an auto loader for ELR you are looking at minimum something like a Nemo OMEN in 300wm and they don't sell receiver sets for a custom build.

High pressure cartridges and auto loaders don't usually play very nice. Know going into it you will likely have issues feeding from a standard mag with magnum and wsm calibers.

You are better off going with a 6.5 caliber and long barrel for muzzle velocity. 123 scenars out of my 22" barrel are supersonic to around 1600.

How far have you shot so far?

 

[VaRandy]

I also don't understand the choice of a AR-10 platform. You say "I want to see how far an ar 10 can go". Ballistics software can tell you that and save you a lot of time and money.
ELR shooting is not just how far you can toss a projectile, but how accurate as well. I suspect your definition of how accurate is an accurate rifle would fail short of a real ELR platform.

If you think about it, your goal is more realistic such as how good of a long range platform can I build for X dollars on a AR-10 platform.

I want to see how far an ar 10 can go. I think in the ar 15 it's pretty well determined with 556 6.5 grendel and 6mm rounds. I have bolt guns and Luke them too, I just would like to see how far the ar-10 and catridge that work in it can be accurate.

 

[banditbump]

I apologize if I am not communicating my thoughts well on what I'm trying to accomplish. I have shot out to 5 and 600 yards, I have access to much farther. Perhaps this topic was better put in the semi auto forum, I want to build a big ar and was trying to figure out which cartirdge I should get a barrel chambered for. I thought that perhaps others had tried this, I believe jp and gap make 6.5 creedmoor ar's that are capable of shooting past 1k, I was just seeing if others had tried short mag or 284 win case sized ar 10s. I'm guessing that just won't work well as is mentioned above due to pressure issues. With that being the case it's 6.5 7-08 or 308 depending on which one can shoot the highest bc bullet at a good velocity without pressure issues. 6.5 creedmoor would be the only chambering with factory elr ammo...

 

[peaceatwar]

I apologize if I am not communicating my thoughts well on what I'm trying to accomplish. I have shot out to 5 and 600 yards, I have access to much farther. Perhaps this topic was better put in the semi auto forum, I want to build a big ar and was trying to figure out which cartirdge I should get a barrel chambered for. I thought that perhaps others had tried this, I believe jp and gap make 6.5 creedmoor ar's that are capable of shooting past 1k, I was just seeing if others had tried short mag or 284 win case sized ar 10s. I'm guessing that just won't work well as is mentioned above due to pressure issues. With that being the case it's 6.5 7-08 or 308 depending on which one can shoot the highest bc bullet at a good velocity without pressure issues. 6.5 creedmoor would be the only chambering with factory elr ammo...

Banditbump, You are communicating your thoughts very well. For the purposes of snipershide and the ELR forum I would recommend you post to "Semi-Auto Rifles" as this sub-forum generally considers ELR or (Extended Long Range) to be 1,000 yards and beyond.

With that said you seem to have answered your own question in that 6.5 Creedmoor is your only option for widely available factory match ammo that isn't .308

 

[Blackstone]

.260 Rem.
.139gr. Scenar
This is an effective caliber/bullet combination that maximizes the ELR potential of the AR10 platform, IMHO.
BH and SW both have satisfactory factory ammo , if that is a factor.

 

[FCS]

I've done 243AI, 260, 300 WSM (rebated rim, WSM bolt wasn't readily available) on AR-10 platform.

6.5 Creed / 260 / 7 Creed (aka 7 SAC) / 7-08 from AR-10 platform is about knee of the curve. Even with these to make them run well you need to run a longer gas system, heavy up buffer. You'll need to make custom gas tube to get port out far enough.
There are ways to get "factory ammo" in any caliber you want.

Knowing what I know now, I'd go SAUM over WSM as case geometry of SAUM is more favorable to AR magazine. I also wouldn't do another short mag in an AR knowing what I know now.

Armalite magazine allows a bit longer OAL than does the Knights/DPMS pattern magazine.

You can have a 284 Win, 6.5-284, 7 SAUM, 300 SAUM or possibly a 6.5 SAUM made up in AR platform but you will not see same performance as you see with a bolt rifle as the 2.850" magazine constraint is going to hamstring you a bit.

6.5 Creed / 7 Creed with 24" bbl, longer gas system is going to be your "practical" AR configuration for LR / ELR. 260 / 7-08 is going to be pushing the limits of what you can get from an AR platform. 6.5 / 7 / 300 SAUM, the 300 will be least troublesome to get running, 6.5 the most troublesome as you'll be running slower burn rate powders and that creates port pressure / gas volume issues.

I'd get an extended gas system 6.5 Creed, see how you like that. If you want more then go to a little more overbore cartridge like a 260, IMHO a 6.5-284 that is magazine feed is not going to gain you any appreciable amount over a 260 due to OAL constraint of magazine. Just more gas volume, port pressure to deal with.

Chamber, you'll pretty much be limited to a more conventional match bullet profile such as found on the SMKs. VLDs can be made to work but....well, I just haven't found a huge advantage to VLDs in a gasser over conventional match bullets like SMK, Amax, Scenar or similar.

GLW your project!

 

[banditbump]

I've done 243AI, 260, 300 WSM (rebated rim, WSM bolt wasn't readily available) on AR-10 platform.

6.5 Creed / 260 / 7 Creed (aka 7 SAC) / 7-08 from AR-10 platform is about knee of the curve. Even with these to make them run well you need to run a longer gas system, heavy up buffer. You'll need to make custom gas tube to get port out far enough.
There are ways to get "factory ammo" in any caliber you want.

Knowing what I know now, I'd go SAUM over WSM as case geometry of SAUM is more favorable to AR magazine. I also wouldn't do another short mag in an AR knowing what I know now.

Armalite magazine allows a bit longer OAL than does the Knights/DPMS pattern magazine.

You can have a 284 Win, 6.5-284, 7 SAUM, 300 SAUM or possibly a 6.5 SAUM made up in AR platform but you will not see same performance as you see with a bolt rifle as the 2.850" magazine constraint is going to hamstring you a bit.

6.5 Creed / 7 Creed with 24" bbl, longer gas system is going to be your "practical" AR configuration for LR / ELR. 260 / 7-08 is going to be pushing the limits of what you can get from an AR platform. 6.5 / 7 / 300 SAUM, the 300 will be least troublesome to get running, 6.5 the most troublesome as you'll be running slower burn rate powders and that creates port pressure / gas volume issues.

I'd get an extended gas system 6.5 Creed, see how you like that. If you want more then go to a little more overbore cartridge like a 260, IMHO a 6.5-284 that is magazine feed is not going to gain you any appreciable amount over a 260 due to OAL constraint of magazine. Just more gas volume, port pressure to deal with.

Chamber, you'll pretty much be limited to a more conventional match bullet profile such as found on the SMKs. VLDs can be made to work but....well, I just haven't found a huge advantage to VLDs in a gasser over conventional match bullets like SMK, Amax, Scenar or similar.

GLW your project!

Thanks alot for the info FCS! A couple follow up questions:

1. Will the short mags and 284 win based cases fit in a standard 20 round mag?
2. Did you have feeding / function issues with the 243ai as it has less taper than a standard 243 win?

I think I will start on a 6.5 either creed or 260, but if 284 cases fit in a standard 20 round mag I may try the straight 284 or 284 improved. That way you can utilize a standard ar 10 bolt and get more case capacity. But if those fatter cases don't work in a standard sized magazine I am less interested...

Thanks all for input!

 

[LawnMM]

6.5 Creedmoor and be done with it.

 

[FCS]

Thanks alot for the info FCS! A couple follow up questions:

1. Will the short mags and 284 win based cases fit in a standard 20 round mag?
2. Did you have feeding / function issues with the 243ai as it has less taper than a standard 243 win?

I think I will start on a 6.5 either creed or 260, but if 284 cases fit in a standard 20 round mag I may try the straight 284 or 284 improved. That way you can utilize a standard ar 10 bolt and get more case capacity. But if those fatter cases don't work in a standard sized magazine I am less interested...

Thanks all for input!

1) WSM not so great but works from shorter magazines, 20 rd mags the body taper on WSM causes issues. Body of magazine will swell much beyond 5 rds in a 20 rd magazine. I have some ideas on mods to make it work better but it'll end up as a 7 rd mag. 284 is better in fit department but bullet will be buried in case with the bigger, higher BC bullets.

2) 243AI only feeds reasonably well from some Gen 1.5 magazines (these are different feed lip and follower then Gen 1 or Gen 2, hence me calling them Gen 1.5) and even at that it was a bit of a PITA to get the upper to feed reliably. I don't know as it is body taper so much as the sharp shoulder and slightly longer body from shoulder being blown forward as the 243 doesn't have all that much body taper to begin with. I'd expect the 6.5-284 / 284 to be even worse as body is longer, there isn't all that much taper on body and has a 35 degree shoulder. However, I've not tried a 284 in an AR-10 magazine beyond putting an empty 284 case into magazine and looking at some dimensions.

Shoulder angle seems to have a larger impact on feeding in the large frame ARs, I could go into geometry and theory of why but I don't have that much time right now. Longer the body, sharper the shoulder the more gyrations you need to do to make it feed well in my experience with building large frame ARs in calibers besides 308 / 308 parent.
Problem you'll have with the extra case capacity is you'll most probably be running a slower burn rate powder. Slower burn rate powder with bullet deep in case will give you lower peak pressure but you'll still have more port pressure and gas volume to deal with using 284 or short mag cartridge than a 308 parent / Creedmoor / X47 Lapua based round.

Based on my "odd" ball caliber, large frame AR building experiences, I've pretty well decided that 308 parent case is as large as I'd go. I'm also sticking to the 20 degree shoulder if 308 parent. Shorter body Creed seems to work well with the 30 degree shoulder when feeding from AR magazine. There is a pretty solid reason for the 6 Creed / 6.5 Creed in a gas gun. Look at actual performance from a 243 AR vs a 6 Creed AR or a 260 AR vs. 6.5 Creed. Bigger case isn't always going to boost your performance from an AR. You can push port out, size port properly, heavy up buffer, tinker with the feed ramp angle and putz with magazines until you get everything running smooth. Then you chronograph to find all you are really doing is burn a whole lot more powder for a negligible gain. I wanted to tinker with the large frame AR, see what I could really ring out of it much like you are describing and had delusions of grandeur about what a particular cartridge would net me in performance based on that round in a bolt rifle. Didn't work out quite like as I'd planned. I'm not saying you should abandon your project, just go into it with your eyes open. I pretty much enjoyed my projects, exploration of what a large frame AR can do in cartridges with more case capacity then 308 / 308 parent. Looking back.....I'd take the money, the time and an AR in 6 / 6.5 / 7 Creed or 6 / 6.5 / 7 X 47 Lapua properly configured (port location, port size) as knee of curve solution and enjoy a well behaved AR with pretty darn nice ballistics.