18.5" thin barrel vs heavy barrel - potential accuracy differences?
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Thread: 18.5" thin barrel vs heavy barrel - potential accuracy differences?

  1. #1
    Newbie Astute Observer's Avatar
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    18.5" thin barrel vs heavy barrel - potential accuracy differences?

    Hey all;

    New to the forum here - registered a while back but haven't posted.

    Anyways, I just got a new goodie in the mail - I have both a light and heavy profile barrel in 18.5", with two receivers; one standard quad rail, the other keymod.

    18.5" thin barrel vs heavy barrel - potential accuracy differences?-2015-05-04-23.54.43-1.jpg

    One optic is pretty good, the other is so-so. I've got a Millett 1-6 DMR on the heavy barrel, but a Trijicon 1-4x Accupoint on the light.

    My intent is to replace the Millett with something a little more suited to longer range shooting, and use that upper / barrel combo for a target rifle, and the lighter upper for either hunting, or perforating pumpkins and soda bottles.

    But I nonetheless got curious about the accuracy differences between the two. I'm going to use the same optic (the accupoint) and go shooting on the same day, with the same ammo, out the two different barrels. Planning to do so within a week.

    Curious if anybody's done an A/B comparison of the two barrels yet. I'm really hoping that the light barrel doesn't whip too much, especially as it's rather long at 18.5" (I'm in Canada, min length for non-restricted).

    Cheers and happy to have joined the forum!

  2. #2
    Carbineman Rockworx's Avatar
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    I'm getting great accuracy from my 7.62x39 light barrel, even after it heats up.
    "Up yours!" - Abraham Linclon

  3. #3
    XCR Guru Bravo's Avatar
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    Mine is supposed to be a light barrel. I would say for a fact that it is, but I've never had a heavy barrel close by to compare it to, so I'm assuming that what I ordered was what I got.

    My light barrel will stay MOA when hot, and a little sub-MOA when I'm doing my part.

    Anything within 800 meters (I recently changed from yards to meters, so there!) is fair game, and in immenent danger of being shot.
    Do you really think we want laws to be observed? We want them broken. There's no way to rule innocent men. The only power any government has is the power to crack down on criminals. When there aren't enough criminals, one makes them. One declares so many things to be crimes that it becomes impossible for men to live without breaking laws. Who wants law-abiding citizens? Pass the kind of laws that can’t be observed, enforced, or objectively interpreted - and you create a nation of law-breakers.

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  5. #4
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    OK, I finally got to the range today! Lots of good data. I fired four different ammo types out of both barrels, and came up with significantly different results.

    So with both a heavy and light profile barrel in my posession, I decided to do an A/B comparison. This is the first sort of comparison I've ever done, please offer any constructive criticism of my methodology. Where there are sloppy methods used, I try to mention it up front.

    While I have two receivers, I fired both barrels out of the same receiver. I didn't want to switch the receiver or optic, as I wanted to minimize the variables and just test the barrels. Optic used was a Millett 1-6x DMS. The centre dot was about 1.5 MoA thick unfortunately, so not the best precision optic IMHO. The setup looked like this:



    Switching the barrel moved the point of impact about 14" off at 100m. I felt that's a pretty big difference for a rifle that has it's quick barrel change as the primary feature. However, I was quite pleased when I switched back, that the adjustment was the exact opposite - 14" back. So removing and replacing the same barrel returns to the exact same point of impact.

    So here's the final results right up front in a chart... Numbers are minutes of angle (MoA) that I shot with an XCR-M using a light barrel or heavy barrel, and the type of ammo used.



    I did a partial test like this in June 2014, and included those results as well. Since June 2014, Dave at Wolverine replaced my muzzle brake, gas block and gas tube on the heavy barrel in an effort to improve accuracy. It worked, if you compare the 2014 to 2015 data for the heavy barrel. Other factors that changed from 2014 - 2015 include the bipod. The 2015 test used a Tango Down bipod, while the 2014 used a FAB Defence T-Pod G2. Unfortunately I didn't have the same bipod available from last year. Of course the weather was different too. It was fairly windy today (gusting to 25 kmh I would estimate) while the day in June 2014 was relatively still. But all in all, I think the test is nonetheless revealing.

    So let's look at the data!

    The 2014 numbers show that the heavy barrel is indeed slightly more accurate than the light, even right out of the box. If you compare the results from 2014, the difference was about 1 MoA or so average for all ammo types. But for me, shooting 5-7 MoA is not satisfying at all, for either barrel.

    From 2014, the light barrel:


    7.62x51 Norinco. 4.75" group at 100m. Shot from light barrel June 2014.




    180 grain Winchester Super-X Power-Point. 6.8" group at 100m. Shot from light barrel June 2014.




    150 grain Remington Express Core-Lokt. 7.1" group at 100m. Shot from light barrel June 2014.



    From 2015, the light barrel:


    150 grain Remington Express Core-Lokt. 4.2" group at 100m. Shot from light barrel 23 May 2015.




    180 grain Winchester Super-X Power-Point. 4.25" group at 100m. Shot from light barrel 23 May 2015.




    7.62x51 Norinco. 5.3" group at 100m. Shot from light barrel 23 May 2015.




    150 grain Hornady American Whitetail. 3.6" group at 100m. Shot from light barrel 23 May 2015.



    From 2014, the heavy barrel:


    180 grain Winchester Super-X Power-Point. 4.75" group at 100m. Shot from heavy barrel June 2014.




    150 grain Remington Express Core-Lokt. 3.1" group at 100m. Shot from heavy barrel June 2014.


    From 2015, the heavy barrel (shot twice to confirm I didn't just get lucky):


    150 grain Remington Express Core-Lokt. 1.75" and 2.4" groups at 100m. Shot from heavy barrel 23 May 2015.




    180 grain Winchester Super-X Power-Point. 2.2" and 2.25" groups at 100m. Shot from heavy barrel 23 May 2015.




    7.62x51 Norinco. 2.6" and 1.8" groups at 100m. Shot from heavy barrel 23 May 2015.




    150 grain Hornady American Whitetail. 1.75" and 1.9" groups at 100m. Shot from heavy barrel 23 May 2015.
    I think I jerked the trigger on that one flier - I could have had a 1" group there potentially.

    All in all, I think that the heavy barrel is much more satisfying. The weight difference between the barrels was not worth the difference in performance. I also conclude that if you're having minute-of-pie-plate accuracy, your gas block might be loose in your receiver - that was the problem with my heavy barrel back in 2014, and might be the problem with my light barrel still.

    It was Dave at Wolverine Supplies up in Virden Manitoba Canada did an amazing job accurizing my heavy barrel. Given I'm a moderate shooter (always pass the Canadian Forces basic infantry standard), I can say I was still the weaker link in the chain when using the XCR-M with the heavy barrel. The work he did was to install a new compensator that didn't apply downward force on the barrel, and a tighter gas system that's really snug in the receiver. I feel satisified with this rifle's accuracy, and for me 2 - 2.5 MoA is reasonable for a battle rifle.

    Hope some of you found this helpful!
    Sean K., wombat338 and sprint like this.

  6. #5
    XCR Guru mjorin's Avatar
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    WOW! That was quite a review. Thank-you for the in depth write-up.
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  7. #6
    XCR Guru navalbeaver's Avatar
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    RA is doing that mod to older style Ms. If you haven't done that mod to both barrels, I'm going to go out on a limb and say its not a good comparison. RA modded my M with the new tube, brake and block relief. My groups shrunk by half after the mod. The difference is significant.

    You could swap the blocks (modded block on the light barre) and shoot without a brake (on both barrels) so you could take a couple more variables out of the test.

    Either way. Thanks for the posts. Its good to get some more data on the M.

  8. #7
    XCR Guru Bravo's Avatar
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    Exactly what Nate said. And I agree, that was a nice write-up!

    Some -Ms had accuracy problems (again, see what Nate has gone through). A good one is really good.

    Also, it looks to me like you've got Norinco ball and hunting rounds. If you're going to use it as a game-getter, more power to you! If you're not though, some target bullets should increase the accuracy. At least if yours turns out to be like mine.
    Do you really think we want laws to be observed? We want them broken. There's no way to rule innocent men. The only power any government has is the power to crack down on criminals. When there aren't enough criminals, one makes them. One declares so many things to be crimes that it becomes impossible for men to live without breaking laws. Who wants law-abiding citizens? Pass the kind of laws that can’t be observed, enforced, or objectively interpreted - and you create a nation of law-breakers.

  9. #8
    Carbineman Rockworx's Avatar
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    He's Canadian.... and we all shoot Norinco ammo. Cheap cheap! Some of the lots are good, some not. But like Bravo said, wonder what some Federal GM Match would shoot like?
    "Up yours!" - Abraham Linclon

  10. #9
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    Thanks for the advice - definitely understand that proper match ammo would tighten those groups up. I bought hunting rounds because I wanted to find a hunting bullet that was as accurate as possible, and sadly I'm on a bit of a budget lately - I didn't even think of match ammo this time.

    The Norinco stuff is just what Rockworx said: Super cheap, so I picked up a case for plinking. Honestly, with the heavy barrel and new gas system it's a 2.5 MOA rifle even with the Norinco, some of the cheapest ammo sold in Canada. That's not bad.

    I'm planning to get into handloading next winter, already saving brass. That should improve accuracy even further. I'll also try to get my gas system on the light barrel replaced, I think I now understand just what a difference that made and it seems clear that yes, I ended up comparing apples to oranges here with the old and new gas system.
    Last edited by Astute Observer; 05-24-2015 at 11:15 AM.

  11. #10
    XCR Guru Bravo's Avatar
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    My -M loves hand-loads.
    Actually, if I've ever fired a factory load in mine, I can't remember it off the top of my head.

    Try Lapua brass with the cases sized to the headspace of your chamber - for my barrel, that's .002 short of SAAMI spec (you've gotta measure your chambers!).
    Plug the primer pocket with a CCI large rifle benchrest primer, throw 44.4 grains of Varget in there, and cap it all off with a 175 Sierra MatchKing. That load keeps right about MOA at 900 yards for me, with my gas dial in the middle position.*

    The dies make a big difference. I've got my set-up down-pat now, but it wasn't cheap.

    I've got another 157 (4 pounds) of those casings in the REBEL 17 as I write this, so I can load up another 5 pounds of Varget worth when they're all done. Damned Varget shortage.....

    The only caveat on that round is that it's great on a rifle as-is, but I'm getting some serious pressure with it - with the gas dial on the lowest setting - with the can attached. I'm still experimenting with loads for that application.

    *even though that's a normal "book load", working up to that in your rifle is always a prudent idea. I'm not gonna take responsibility if you don't follow good loading procedures ;-)
    Astute Observer likes this.
    Do you really think we want laws to be observed? We want them broken. There's no way to rule innocent men. The only power any government has is the power to crack down on criminals. When there aren't enough criminals, one makes them. One declares so many things to be crimes that it becomes impossible for men to live without breaking laws. Who wants law-abiding citizens? Pass the kind of laws that can’t be observed, enforced, or objectively interpreted - and you create a nation of law-breakers.

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