Comparing The AR-15 5.56X45 With The Sig Spear 6.8X51

BY Herschel Smith
1 month, 4 weeks ago

Tim makes the comparison and contrast.  I really like Tim and have exchanged email with him, with Tim being helpful with advice in a purchase I wanted to make.

But I think they are wrong in their conclusions on almost every account.  First, don’t shoot practice ammunition when you can shoot the real thing.  No, .38 Spl versus .357 magnum isn’t a good comparison.  If you intend to carry .357 magnum, you’d better be shooting .357 magnum at the range.

Next, I think the high pressures this cartridge generates is going to be problematic, including scorching the chamber and throat.  Also, until you see this gun running its high pressures in hot environments for years, you have yet to see what sorts of problems will develop.  Next, I think the weight is going to be problematic, both for the gun and the ammunition.  Next, I think no one is ever going to shoot this in full auto – it will be completely uncontrollable.  I could go on, but I think you get my main points.

What to do then?  The Stoner platform is great, and so is the 5.56X45 for distances short of about 400 yards.  For those who need to shoot further than that, they should have considered the 6mm ARC.  No, they shouldn’t have considered it, they should have purchased upper receivers chambered for this cartridge as fast as they could.

The upper receiver will fit an AR-15 lower.  The cartridge fits within the AR platform.  Weight stays the same.  Operation stays the same.  A new upper receiver and new magazines is all that is required.  For that, you get a 6mm bullet that weighs almost twice what the 5.56X45 does (103 grains, 105 grains, 108 grains) with the velocity of the 5.56X45, and a long bullet that has the ballistic design for effectiveness at 1000 yards. You do all of that with only 1 – 2 pounds more recoil.

Leave it to the DoD to make stupid decisions and waste money.


  1. On June 15, 2022 at 11:40 pm, Georgiaboy61 said:

    @ Herschel

    Agree with you on virtually all of your points. The Sig Spear 6.8×51 is a “solution” in search of a problem. Leave it to the guys inside the five-sided puzzle palace to choose the most-expensive and least-proven system out there, in other words a debacle waiting to happen.

    As noted, the 6mm ARC is a fine choice for a caliber upgrade for the AR platform. Various makers (DPMS once did, for example) have offered .24-caliber (6mm) cartridges in Eugene Stoner’s rifle, and by all accounts, it performs very well. In that caliber, projectiles in the 100-110 grain range exhibit the exterior ballistics performance needed for 1000 yard capability, and also have superior punch closer in than the 5.56×45 now in service.

    I’d prefer something in 6.5mm (.264-cal.) myself, something like 6.5 Grendel, but we are in agreement, I think, that something in the .24-.26 range would be ideal.

    It is mysterious to me why the army ordnance people specified 6.8mm (.277-cal.) in the RFP, when projectiles in the 6-6.5mm range have better performance in the weight range being considered. 6-6.5mm projectiles have significantly better BC and SD numbers in the 100-140-grain range than 6.8 does, which means they are more-efficient and need less propellant to do the job.

    The Spear project is a mistake waiting to happen, at least as a general issue weapon. Hybrid case technology is not ready for prime time. Sig errs in believing that their weapon and ammo are soldier-proof. Believe me, and ask your son, Herschel, Marines and soldiers can wreck a steel ball, let alone a new and complex system like the Spear.

    Why use a thirteen-inch barrel, when shortening the barrel imposes so many detriments on the rest of the design? A shorter barrel means less dwell time, a faster cycle time, and increased heat, dirt/fouling, and wear on all of the working parts of the rifle. That is only increased by the high operating pressure of the design, which – if memory serves – is 80,000 psi. A figure which is 15-20% higher than most service rifle cartridges. 80K is more than the proof pressure for the 7.62×51 NATO cartridge the 6.8×51 is intended to replace.

    A shorter tube means greater muzzle blast, hence a louder weapon, plus increased muzzle flash (which is a factor in night fighting or low-light conditions), and a louder report compared to a longer barrel. Sig addressed these concerns by fitting a suppressor, but this approach is a band-aid on the real problem, which is inadequate barrel length.

    And oh, by the way: That 13-inch tube you have put on the rifle to save length, when you add a permanently-affixed can, the barrel plus suppressor end up being pretty much as long as a standard service rifle anyway! Duh!

    This weapon may – emphasis “may” – end up being a good special ops/special missions weapon, since JSOC is abundantly endowed with funding and resources not possessed by the line military. But as a general-issue rifle, I’m sorry – but I just don’t see it.

    As you noted, the weight alone of the rifle and its ammo is problematic. How are all of those “small stature soldiers,” what the army calls female personnel, going to like a new weapon which is significantly heavier, which has heavier recoil, and whose ammo weighs significantly more than the present standard issue cartridge?

    Then there is doctrine. Does the adoption of the Sig Spear 6.8×51 signal that the army is revising its doctrine on small arms use? The question seems valid, since the 6.8×51 is another class of weapon entirely from the proven AR15 family of weapons – M4, M-16, etc. – which are assault rifles using intermediate ammunition, whereas the Sig Spear is more of a battle rifle, using a heavier and more-powerful cartridge which is really more akin to a full-sized traditional center-fire cartridge like the 7.62x51mm/.308 Win. than an intermediate.

    I won’t go off on a rant about the new whiz-bang “intelligent” Vortex sight which is part of the Spear, except to say that I’ll believe the claims about it when I see them, and I doubt that sucker will last long under field conditions before breaking, going on the fritz, or otherwise becoming a high-tech paperweight.

    Typical Pentagon (non)solution to a simple problem, namely needing more and better marksmanship training for their recruits. There’s nothing seriously wrong with the existing optical or iron sights now in use that more/better training wouldn’t fix, but then, such common-sense solutions don’t get senior officers promoted, or line the pockets of defense contractors.

  2. On June 16, 2022 at 2:44 am, Chris said:

    There is NO WAY the Soys and Women are going to be doing 20 mile rucks with this New Weapon and Loadout.
    NOT.GONNA.HAPPEN..Period Full Stop.

    Forget the possible “Gun” issues themself.

    (I Believe) it’s either going to be SOF Specific OR it will be go the way of the M14.

    (I also believe) it has been …chosen because of Good Ol’ Arms style Pay’ola.

    That is all.

  3. On June 16, 2022 at 5:55 am, Wes said:

    The approach utilizing this new shiny object still has me wondering where the data is supporting the need. AARs that say “Gee, if only I’d had a cartridge with better legs I could’ve engaged that target at 800m ’cause thing are so relaxed in a firefight.” Where are they?

    RE GeorgiaBoy’s very valid raising of the doctrine issue: Even a 10.3″ or 14.5″ bbl shooting Mk.262 can be adequate with proper training GIVEN what seems the current (and likely) future doctrine. That is, “Oh, you have a beltfed? I’ll see that & raise you some mortar rounds on top of you. Mortars? Gimme the radio. I’ll see that & raise you an AH-64 Apache that is going to make you cry.” (Not made up; told to me.)

  4. On June 16, 2022 at 8:32 am, J said:

    Come on guys…you can’t possibly argue with Hornady’s comments, “it’s sexy and looks wicked!” These are the same mental giants that cratered to their slave masters and brought about layoffs because their worker bees would not submit to the clot shot.

    Now that I cleared all of this up, Herschel post the next Hornyday propaganda fluff piece from ewe-toob.

  5. On June 16, 2022 at 8:36 am, Mountain Rat said:

    I think the 6.5 Grendel would have made a good choice as well. I love mine.

    I think the driving force behind this choice was money. Not nearly as much money to be made with a simple upper swap as a complete new system that will need all new magazines, cleaning and maint. gear, etc.

    I think the weight is going to be a real issue, and the crazy high chamber pressure is almost certainly going to lead to high cost of maint.

    In the end the taxpayer is getting fleeced once again.

  6. On June 16, 2022 at 9:29 am, Drake said:

    The Army wrote up the requirements for a new rifle in incredibly stupid ways – short barrel but long-range penetration, and no bull-pups. Probably didn’t bother with specs on recoil or weight.

    So they get this crazy thing that sacrifices everything – weight, barrel-life, cost of ammo, recoil… to make a short SPR / DMR rifle.

    I’m imaging handing an HK91 or M14 to a 100lb female recruit and laughing as the recoil knocks her down. Forget marching with that beast and 250 rounds of ammo.

  7. On June 16, 2022 at 9:45 am, IA Brooks said:

    I think the ‘practice’ ammo will become the ‘standard’ ammo, due to concerns about barrel life and ease of rifle qual (can’t have too many women failing, y’know…) and then the whole thing will be dropped, after wasting billions of dollars. Good news, though: that’s only ten million or so in pre-Biden bux.

  8. On June 16, 2022 at 11:35 am, blake said:

    Most firearms have a mean time to failure and the failures become fairly predictable after a few million rounds have been expected.

    I believe, with the pressures created by this round, failures will not be easily predicted, nor will those failures be confined to one or two known areas.

    Nothing like breaking what isn’t broken.

  9. On June 16, 2022 at 11:44 am, JoeFour said:

    Agree 100% with your observations Cap’n Smith and also with each of the above reader comments!

    Its almost like the Army/DOD carefully read and studied every point Eugene Stoner ever made so they could be sure to do the exact opposite…

    I smell corruption in the procurement process.

  10. On June 16, 2022 at 1:11 pm, Red Man said:

    It’s all about the $$$$. Not providing the best option. A couple improvements were made to the original platform IMHO. Ambi controls, the short stroke piston keeps the bolt clean, removes the buffer tube for a folding stock and has been proven in many weapons. Easily removable barrel (though why remove a barrel not being a machine gun), but two charging handles is baffling. As a former 11B1P, the weight of the weapon is a trade off. The trade will be ammo! Gaining fire superiority is key, not running low. I guess ‘Light Infantry” ain’t light anymore. I’ll keep my ACOG for now, haven’t changed the batteries in it yet, going on 8 years!! Haha! I’d like to hear Stu Schellers opinion on the weapon. With all that, Georgia Boy said it all.

  11. On June 16, 2022 at 8:53 pm, George 1 said:

    Just watched a video by S-2 Underground. The author makes the case that the cartridge was selected, as stated by the DOD, for it’s effectiveness in penetrating body armor. But he infers it is not to counter body armor used by Russia or China.

    He points out that right behind the U.S. government in use of body armor are civilians in the U.S. In the entire world U.S. civilians are the number two owners of body armor. So, it might be that the U.S. Military is preparing for something they are not divulging.

  12. On June 17, 2022 at 12:48 am, Georgiaboy61 said:

    @ George 1

    Re: “He points out that right behind the U.S. government in use of body armor are civilians in the U.S. In the entire world U.S. civilians are the number two owners of body armor. So, it might be that the U.S. Military is preparing for something they are not divulging.”

    If is going into action against the civilian population, then we the people have a whole heck of a lot more problems than what the army chooses for its newest rifle.

    That scenario scares the pants off of me, I don’t mind admitting, for the following reasons… (historians worry too much anyway)

    Since the armed forces would likely fracture along ideological, political and cultural lines, we’d be looking at two or more factions of the government and its armed forces, contesting for control. A putsch or civil war, in other words.

    If the U.S. descends into a civil war-type scenario, it will almost certain invite foreign intervention inside of our current borders, or perhaps in Canada, where Premier Trudeau has gotten mighty friendly with President Xi Xinping of China. There have not been foreign troops on mainland American soil in large numbers for more than two centuries, dating back to the War of 1812.

    China, Russia, Iran, go down the list…. and don’t forget UN troops, too, are a possibility. And that doesn’t even get into all of the street gangs, thugs, criminals and narco-terroristas (like MS-13) who will have weapons and the urge to use them.

  13. On June 17, 2022 at 8:20 am, Ned said:

    The 6mm ARX is like a 6X45 or 6mmTCU that gets 87 grain velocities in 100+ grain pills without jacking the chamber pressure up to 80,000 psi.

    Yeah, I’m also a 6.5 Grendel fanboi, and so may not set up a 6mm ARC.

    But of course this obvious solution is ignored by TPTB. Let’s instead choose a heavier rifle platform for our female and gay SOF operators and soldiers to hump.

    Because grunts everywhere always say “I sure wish my ruck was heavier, including the one I’m carrying for this female…”

  14. On June 17, 2022 at 8:49 am, George 1 said:


    Bottom line is we are in a heap of trouble!

  15. On June 18, 2022 at 1:15 am, Georgiaboy61 said:

    @ George I

    Re: “Bottom line is we are in a heap of trouble!”

    Yeah, you’re right about that! But hang tough, no way out but through…..

RSS feed for comments on this post. TrackBack URL

Leave a comment

You are currently reading "Comparing The AR-15 5.56X45 With The Sig Spear 6.8X51", entry #30677 on The Captain's Journal.

This article is filed under the category(s) Firearms,Guns and was published June 15th, 2022 by Herschel Smith.

If you're interested in what else the The Captain's Journal has to say, you might try thumbing through the archives and visiting the main index, or; perhaps you would like to learn more about TCJ.

26th MEU (10)
Abu Muqawama (12)
ACOG (2)
ACOGs (1)
Afghan National Army (36)
Afghan National Police (17)
Afghanistan (704)
Afghanistan SOFA (4)
Agriculture in COIN (3)
AGW (1)
Air Force (38)
Air Power (9)
al Qaeda (83)
Ali al-Sistani (1)
America (22)
Ammunition (197)
Animals (169)
Ansar al Sunna (15)
Anthropology (3)
Antonin Scalia (1)
AR-15s (331)
Arghandab River Valley (1)
Arlington Cemetery (2)
Army (81)
Assassinations (2)
Assault Weapon Ban (27)
Australian Army (7)
Azerbaijan (4)
Backpacking (2)
Badr Organization (8)
Baitullah Mehsud (21)
Basra (17)
BATFE (139)
Battle of Bari Alai (2)
Battle of Wanat (18)
Battle Space Weight (3)
Bin Laden (7)
Blogroll (3)
Blogs (24)
Body Armor (23)
Books (3)
Border War (16)
Brady Campaign (1)
Britain (38)
British Army (35)
Camping (4)
Canada (14)
Castle Doctrine (1)
Caucasus (6)
Center For a New American Security (8)
Charity (3)
China (16)
Christmas (13)
CIA (29)
Civilian National Security Force (3)
Col. Gian Gentile (9)
Combat Outposts (3)
Combat Video (2)
Concerned Citizens (6)
Constabulary Actions (3)
Coolness Factor (3)
COP Keating (4)
Corruption in COIN (4)
Council on Foreign Relations (1)
Counterinsurgency (218)
DADT (2)
David Rohde (1)
Defense Contractors (2)
Department of Defense (198)
Department of Homeland Security (26)
Disaster Preparedness (4)
Distributed Operations (5)
Dogs (12)
Donald Trump (27)
Drone Campaign (3)
EFV (3)
Egypt (12)
El Salvador (1)
Embassy Security (1)
Enemy Spotters (1)
Expeditionary Warfare (17)
F-22 (2)
F-35 (1)
Fallujah (17)
Far East (3)
Fathers and Sons (2)
Favorite (1)
Fazlullah (3)
FBI (33)
Featured (187)
Federal Firearms Laws (18)
Financing the Taliban (2)
Firearms (1,555)
Football (1)
Force Projection (35)
Force Protection (4)
Force Transformation (1)
Foreign Policy (27)
Fukushima Reactor Accident (6)
Ganjgal (1)
Garmsir (1)
general (15)
General Amos (1)
General James Mattis (1)
General McChrystal (44)
General McKiernan (6)
General Rodriguez (3)
General Suleimani (9)
Georgia (19)
Google (1)
Gulbuddin Hekmatyar (1)
Gun Control (1,450)
Guns (2,063)
Guns In National Parks (3)
Haditha Roundup (10)
Haiti (2)
Haqqani Network (9)
Hate Mail (8)
Hekmatyar (1)
Heroism (4)
Hezbollah (12)
High Capacity Magazines (16)
High Value Targets (9)
Homecoming (1)
Homeland Security (2)
Horses (1)
Humor (60)
ICOS (1)
IEDs (7)
Immigration (96)
India (10)
Infantry (4)
Information Warfare (2)
Infrastructure (3)
Intelligence (23)
Intelligence Bulletin (6)
Iran (171)
Iraq (379)
Iraq SOFA (23)
Islamic Facism (64)
Islamists (95)
Israel (19)
Jaish al Mahdi (21)
Jalalabad (1)
Japan (3)
Jihadists (80)
John Nagl (5)
Joint Intelligence Centers (1)
JRTN (1)
Kabul (1)
Kajaki Dam (1)
Kamdesh (9)
Kandahar (12)
Karachi (7)
Kashmir (2)
Khost Province (1)
Khyber (11)
Knife Blogging (5)
Korea (4)
Korengal Valley (3)
Kunar Province (20)
Kurdistan (3)
Language in COIN (5)
Language in Statecraft (1)
Language Interpreters (2)
Lashkar-e-Taiba (2)
Law Enforcement (4)
Lawfare (7)
Leadership (6)
Lebanon (6)
Leon Panetta (2)
Let Them Fight (2)
Libya (14)
Lines of Effort (3)
Littoral Combat (8)
Logistics (50)
Long Guns (1)
Lt. Col. Allen West (2)
Marine Corps (273)
Marines in Bakwa (1)
Marines in Helmand (67)
Marjah (4)
Media (62)
Medical (142)
Memorial Day (6)
Mexican Cartels (39)
Mexico (58)
Michael Yon (6)
Micromanaging the Military (7)
Middle East (1)
Military Blogging (26)
Military Contractors (4)
Military Equipment (24)
Militia (6)
Mitt Romney (3)
Monetary Policy (1)
Moqtada al Sadr (2)
Mosul (4)
Mountains (25)
MRAPs (1)
Mullah Baradar (1)
Mullah Fazlullah (1)
Mullah Omar (3)
Musa Qala (4)
Music (24)
Muslim Brotherhood (6)
Nation Building (2)
National Internet IDs (1)
National Rifle Association (86)
NATO (15)
Navy (24)
Navy Corpsman (1)
NCOs (3)
News (1)
NGOs (2)
Nicholas Schmidle (2)
Now Zad (19)
NSA (3)
NSA James L. Jones (6)
Nuclear (61)
Nuristan (8)
Obama Administration (221)
Offshore Balancing (1)
Operation Alljah (7)
Operation Khanjar (14)
Ossetia (7)
Pakistan (165)
Paktya Province (1)
Palestine (5)
Patriotism (7)
Patrolling (1)
Pech River Valley (11)
Personal (70)
Petraeus (14)
Pictures (1)
Piracy (13)
Pistol (2)
Pizzagate (21)
Police (581)
Police in COIN (3)
Policy (15)
Politics (957)
Poppy (2)
PPEs (1)
Prisons in Counterinsurgency (12)
Project Gunrunner (20)
PRTs (1)
Qatar (1)
Quadrennial Defense Review (2)
Quds Force (13)
Quetta Shura (1)
RAND (3)
Recommended Reading (14)
Refueling Tanker (1)
Religion (351)
Religion and Insurgency (19)
Reuters (1)
Rick Perry (4)
Rifles (1)
Roads (4)
Rolling Stone (1)
Ron Paul (1)
ROTC (1)
Rules of Engagement (75)
Rumsfeld (1)
Russia (35)
Sabbatical (1)
Sangin (1)
Saqlawiyah (1)
Satellite Patrols (2)
Saudi Arabia (4)
Scenes from Iraq (1)
Second Amendment (497)
Second Amendment Quick Hits (2)
Secretary Gates (9)
Sharia Law (3)
Shura Ittehad-ul-Mujahiden (1)
SIIC (2)
Sirajuddin Haqqani (1)
Small Wars (72)
Snipers (9)
Sniveling Lackeys (2)
Soft Power (4)
Somalia (8)
Sons of Afghanistan (1)
Sons of Iraq (2)
Special Forces (28)
Squad Rushes (1)
State Department (23)
Statistics (1)
Sunni Insurgency (10)
Support to Infantry Ratio (1)
Supreme Court (34)
Survival (101)
SWAT Raids (57)
Syria (38)
Tactical Drills (2)
Tactical Gear (7)
Taliban (168)
Taliban Massing of Forces (4)
Tarmiyah (1)
TBI (1)
Technology (18)
Tehrik-i-Taliban (78)
Terrain in Combat (1)
Terrorism (95)
Thanksgiving (12)
The Anbar Narrative (23)
The Art of War (5)
The Fallen (1)
The Long War (20)
The Surge (3)
The Wounded (13)
Thomas Barnett (1)
Transnational Insurgencies (5)
Tribes (5)
TSA (24)
TSA Ineptitude (13)
TTPs (4)
U.S. Border Patrol (5)
U.S. Border Security (16)
U.S. Sovereignty (20)
UAVs (2)
UBL (4)
Ukraine (9)
Uncategorized (65)
Universal Background Check (3)
Unrestricted Warfare (4)
USS Iwo Jima (2)
USS San Antonio (1)
Uzbekistan (1)
V-22 Osprey (4)
Veterans (3)
Vietnam (1)
War & Warfare (363)
War & Warfare (40)
War Movies (4)
War Reporting (21)
Wardak Province (1)
Warriors (6)
Waziristan (1)
Weapons and Tactics (75)
West Point (1)
Winter Operations (1)
Women in Combat (21)
WTF? (1)
Yemen (1)

August 2022
July 2022
June 2022
May 2022
April 2022
March 2022
February 2022
January 2022
December 2021
November 2021
October 2021
September 2021
August 2021
July 2021
June 2021
May 2021
April 2021
March 2021
February 2021
January 2021
December 2020
November 2020
October 2020
September 2020
August 2020
July 2020
June 2020
May 2020
April 2020
March 2020
February 2020
January 2020
December 2019
November 2019
October 2019
September 2019
August 2019
July 2019
June 2019
May 2019
April 2019
March 2019
February 2019
January 2019
December 2018
November 2018
October 2018
September 2018
August 2018
July 2018
June 2018
May 2018
April 2018
March 2018
February 2018
January 2018
December 2017
November 2017
October 2017
September 2017
August 2017
July 2017
June 2017
May 2017
April 2017
March 2017
February 2017
January 2017
December 2016
November 2016
October 2016
September 2016
August 2016
July 2016
June 2016
May 2016
April 2016
March 2016
February 2016
January 2016
December 2015
November 2015
October 2015
September 2015
August 2015
July 2015
June 2015
May 2015
April 2015
March 2015
February 2015
January 2015
December 2014
November 2014
October 2014
September 2014
August 2014
July 2014
June 2014
May 2014
April 2014
March 2014
February 2014
January 2014
December 2013
November 2013
October 2013
September 2013
August 2013
July 2013
June 2013
May 2013
April 2013
March 2013
February 2013
January 2013
December 2012
November 2012
October 2012
September 2012
August 2012
July 2012
June 2012
May 2012
April 2012
March 2012
February 2012
January 2012
December 2011
November 2011
October 2011
September 2011
August 2011
July 2011
June 2011
May 2011
April 2011
March 2011
February 2011
January 2011
December 2010
November 2010
October 2010
September 2010
August 2010
July 2010
June 2010
May 2010
April 2010
March 2010
February 2010
January 2010
December 2009
November 2009
October 2009
September 2009
August 2009
July 2009
June 2009
May 2009
April 2009
March 2009
February 2009
January 2009
December 2008
November 2008
October 2008
September 2008
August 2008
July 2008
June 2008
May 2008
April 2008
March 2008
February 2008
January 2008
December 2007
November 2007
October 2007
September 2007
August 2007
July 2007
June 2007
May 2007
April 2007
March 2007
February 2007
January 2007
December 2006
November 2006
October 2006
September 2006
August 2006
July 2006
June 2006
May 2006

about · archives · contact · register

Copyright © 2006-2022 Captain's Journal. All rights reserved.