Interesting results after grouping my standard and BFH BCM barrels - update 02 JUN

[Wake27]

I took an LPVO class a few weeks ago and developed some concerns about the new upper I was using. It had about 1k rounds through it, but I hadn't gotten a chance to shoot it further than 50m at that point. The 100m groups we started out with were around 2-3 MOA, which was a little disappointing but I assumed it was either me or the older PPU 75 gr BTHP ammo that I was shooting. As we pushed the distance, it got worse, to the point that I couldn't make consistent hits on steel at 200 and 300m. The instructors started watching me a lot closer and looking at the scope and mount to see what could be the problem. There were some theories that the 1.93 mount was making it hard for me to get a consistent cheek weld, but I wasn't convinced that was the case, especially after one of the instructors shot it on paper and got some very odd results. Knowing that it'd be a while before I could do it, I sent the upper and scope to Robb Jensen, since he assembled it for me. His target is below, and he stated that he was surprised that it didn't shoot better based off of his own BFH barrels.

Upper: BCM BFH
Optic: Razor HD 3-18x
Ammo: Hornady 75gr BTHP
Distance: 100 yards
Results: 1.99" five round group, 2.36" 11 round group.



I wanted to do some more testing on my own so after another week I finally got to the 100 yard range with my backup AR, a factory BCM ELW-F with original KMR. I think the round count is around 2,500 and while I've shot it at 100m before, its always been with an Aimpoint. I brought my primary lower with SD-E and swapped the uppers and Atlas bipod, but shot both optics. I also used a rear bag. Ideally I would've shot every type of ammo (I had several that I didn't get to) at 50 and 100 with both uppers using the Razor, but I ran out of time. All groups were five rounds. Here are my results.

Target 1
Upper: BCM ELW-F
Optic: Razor 1-6
Ammo: FGMM 69 gr.
Distance: 50 yards
Results: 3/4"



Target 2
Upper: BCM ELW-F
Optic: T-2
Ammo: PPU 75 gr.
Distance: 50 yards
Results: 1 1/4"
Note: Ignore group one, that was my first one of the day so I shot it at 25 yards



Target 3
Upper: BCM ELW-F
Optic: T-2
Ammo: Gold Dot 75 gr.
Distance: 50 yards
Results: 1 1/4" for both
Note: windage zero target



Target 4
Upper: BCM ELW-F
Optic: Razor 1-6
Ammo: FGMM 69 gr.
Distance: 50 yards
Results: 1 1/8" and 1 1/2"
Note: elevation zero target



______

Target 5
Upper: BCM BFH
Optic: Razor 1-6
Ammo: FGMM 69 gr.
Distance: 50 yards
Results: 1 7/8"



Target 6
Upper: BCM BFH
Optic: Razor 1-6
Ammo: FGMM 69 gr.
Distance: 100 yards
Results: 1 3/4"



Target 7
Upper: BCM BFH
Optic: Razor 1-6
Ammo: PPU 75 gr.
Distance: 50 yards
Results: 1 5/8"



Target 8
Upper: BCM BFH
Optic: Razor 1-6
Ammo: PPU 75 gr.
Distance: 100 yards
Results: 2"



I thought I had shot the PPU with the Razor on the ELW-F upper so that I'd have two direct comparisons, but apparently I only shot it with the Aimpoint which means that targets one and five are the only direct comparisons. The ELW-F shot a 3/4" group whereas the BFH shot a 1 7/8" group. Also, while not a direct comparison, targets two and seven are similar and interesting because the ELW-F with a T-2 shot a 1 1/4" group while the BFH with the Razor shot a 1 5/8" group. So in the one direct comparison, the ELW-F with double the round count outshot the BFH barrel by a singificant margin. It also shaved off almost half an inch in the next closest comparison, despite the significant difference in magnification. Finally, the groups from the ELW-F are on average tighter with one flier, whereas the groups from the BFH are all fairly dispersed.

I'm surprised at how well the BFH barrel shot, considering how it did at the class. I was actually expecting five MOA groups because of how it did on paper when the instructor shot it. So here are some questions that I have.

1. If I have an upper that will shoot 2" at 100 yards, how well does that translate to 4" at 200 and so on? I know its not a perfect science, but are there instances where it would suddenly open up considerably, like 6-8" at 200 yds?
2. What's the consensus on a newish BFH barrel grouping no better than 2 MOA? I wouldn't expect 1 MOA, especially knowing that BCM barrels are often out shot by DD and Colt, but I'm pretty happy with the performance of the standard barrel, just not the BFH.

[MistWolf]

1. If I have an upper that will shoot 2" at 100 yards, how well does that translate to 4" at 200 and so on? I know its not a perfect science, but are there instances where it would suddenly open up considerably, like 6-8" at 200 yds?.

You won't know how it will perform at any range until you actually test it at that range. In theory, a barrel/ammo combination that shoots 2 moa at 100 yards should shoot 2 moa at longer ranges. But that depends on what distance a bullet "goes to sleep", how much precession it has and how close to destabilizing it is at any given point. Many experienced shooters will tell that you can't tell how precise a rifle really is until you shoot for groups at two or three hundred yards.

In short yes, there are instances where groups will suddenly open up at longer ranges. You won't know what they are until you shoot.

[gaijin]

The “in theory” part goes along with; “in a perfect world/given ideal conditions”, re X-MOA being consistent at extended range.
We try and shoot load/rifle at 300 prone/benched to get some idea of what to expect.

[alx01]

@Wake27,

It's a very interesting result you got. I don't think this is all that unusual to have groups open up past 200-400m depending on the barrel wear. In my experience it is typically indicative of a normal wear (barrel throat erosion) but I would expect it to occur no sooner than 10-12k rounds for a CHF barrel. That's how you get stories of people going 40k rounds on a CHF barrels - they rarely shoot it beyond 100m at that round count. You shouldn't have any of those issues (shooting 8-10moa at 200) before 10k rounds on a non-abused quality barrel, period.

Maybe I have low standards for accuracy, but I think that 2-3 MOA out of a regular production barrel is pretty good.

I would personally check things in the following order:
- change ammo brand, try lighter a grain or a different bullet type.
- scope mount
- scope itself (reticle shift, etc.). Try doing a walkthrough on a target using turret adjustments: zero the scope, then select the initial point of aim and continue by adjusting in 1 moa increments left/right and up/down and repeat so you draw a square on a target using bullet holes.
- barrel nut
- check chrome lining for flaking/cracks, check throat erosion
- muzzle device torque
- change bolt - maybe that will tighten up a headspace (never seen this to be an issue personally, but heard somebody else mention that)

Keeps us posted. This is a very interesting case.

[Wake27]

@Wake27,

It's a very interesting result you got. I don't think this is all that unusual to have groups open up past 200-400m depending on the barrel wear. In my experience it is typically indicative of a normal wear (barrel throat erosion) but I would expect it to occur no sooner than 10-12k rounds for a CHF barrel. That's how you get stories of people going 40k rounds on a CHF barrels - they rarely shoot it beyond 100m at that round count. You shouldn't have any of those issues (shooting 8-10moa at 200) before 10k rounds on a non-abused quality barrel, period. I actually had an issue like this before, which may be why I'm overhtinking this. There's a long thread on it here, but the Noveske upper this one replaced had a barrel with defective chrome lining and that was a mess. It wouldn't even consistently group at 100 yards though, so this definitely isn't on that level.

Maybe I have low standards for accuracy, but I think that 2-3 MOA out of a regular production barrel is pretty good. This is what I meant by overthinking. There are so many claims of 1 MOA guns on the interwbz without any proof that maybe I just have higher expectations than I should. But, since the standard barrel consistently shot better, I don't know. I know every barrel is a law unto itself and I shouldn't necessarily complain about a consistent 2 MOA gun, but I really would've preferred 1.5 or so, and who knows how it groups beyond 100 (which is the real question now).

I would personally check things in the following order:
- change ammo brand, try lighter a grain or a different bullet type. I wanted to test more options, but I think there is a good bit up there already. Three different brands and two different bullet weights, though they are both heavier. That's why I made sure I shot the 69gr stuff early, I wanted to mix it up from the 75 gr.
- scope mount Robb used an entirely different scope and mount.
- scope itself (reticle shift, etc.). Try doing a walkthrough on a target using turret adjustments: zero the scope, then select the initial point of aim and continue by adjusting in 1 moa increments left/right and up/down and repeat so you draw a square on a target using bullet holes. See above. Also, I experienced no issues shooting my backup gun with the same scope and mount I used in the class, though I shot purely for groups and didn't make any adjustments. The Razor did take a beating in December, so this would be a good thing to test regardless of what the barrel is doing.
- barrel nut I assume Robb checked this, but don't really know so that's a good point.
- check chrome lining for flaking/cracks, check throat erosion The bore is a bit cloudy, but smooth and nothing like the Noveske that I mentioned above. That had visible cracking.
- muzzle device torque Also an assumption that Robb checked this, I'll have to ask.
- change bolt - maybe that will tighten up a headspace (never seen this to be an issue personally, but heard somebody else mention that) Didn't even consider this since I don't remember ever seeing anyone discuss it, but I know headspace can be an issue and would've been an easy thing to check with my back up right there.

Keeps us posted. This is a very interesting case.

Comments in bold, thanks for the ideas.

[Jolr]

You won't know how it will perform at any range until you actually test it at that range. In theory, a barrel/ammo combination that shoots 2 moa at 100 yards should shoot 2 moa at longer ranges. But that depends on what distance a bullet "goes to sleep", how much precession it has and how close to destabilizing it is at any given point. Many experienced shooters will tell that you can't tell how precise a rifle really is until you shoot for groups at two or three hundred yards.

In short yes, there are instances where groups will suddenly open up at longer ranges. You won't know what they are until you shoot.

Isn’t the point where a bullet “goes to sleep” the point where stabilization occurs and maximum accuracy is achieved?

[vicious_cb]

Isn’t the point where a bullet “goes to sleep” the point where stabilization occurs and maximum accuracy is achieved?

No, thats the wrong way to think about. Either bullet is stable or its not when it leaves the muzzle.

The "going to sleep" phenomenon is the bullet being initially disturbed by muzzle blast or barrel harmonics. Brian Litz showed that this initial "epicyclic swerve" both goes away rather quickly(definitely by 50 yards) and is insignificant.

In other words, group size increases linearly. Stop worry about "bullets going to sleep" because its not happening at 50 yards and its not significant enough to affect groups.

http://www.appliedballisticsllc.com/...clicSwerve.pdf

[Jolr]

No, thats the wrong way to think about. Either bullet is stable or its not when it leaves the muzzle.

The "going to sleep" phenomenon is the bullet being initially disturbed by muzzle blast or barrel harmonics. Brian Litz showed that this initial "epicyclic swerve" both goes away rather quickly(definitely by 50 yards) and is insignificant.

In other words, group size increases linearly. Stop worry about "bullets going to sleep" because its not happening at 50 yards and its not significant enough to affect groups.

http://www.appliedballisticsllc.com/...clicSwerve.pdf

I watched his video and came to the opposite conclusion. Over distance, the bullet displays less yaw and becomes more stable.

[Jolr]

The Army’s testing of m855 illustrated the phenomenon of the bullet going to sleep over distance. There is maximum yaw(instability) at the muzzle. Over distance the degree of yaw decreases. Bullets are least stable when they leave the muzzle

[Wake27]

You won't know how it will perform at any range until you actually test it at that range. In theory, a barrel/ammo combination that shoots 2 moa at 100 yards should shoot 2 moa at longer ranges. But that depends on what distance a bullet "goes to sleep", how much precession it has and how close to destabilizing it is at any given point. Many experienced shooters will tell that you can't tell how precise a rifle really is until you shoot for groups at two or three hundred yards.

In short yes, there are instances where groups will suddenly open up at longer ranges. You won't know what they are until you shoot.

The “in theory” part goes along with; “in a perfect world/given ideal conditions”, re X-MOA being consistent at extended range.
We try and shoot load/rifle at 300 prone/benched to get some idea of what to expect.

Yeah that’s what I kind of expected. Still, the groups that the instructor shot were at 100m and were very inconsistent. The first was 2-3 MOA, he made some adjustments to the scope, and then his follow up group was at least 5 MOA if I remember correctly. It was also way off, like double or triple the distance that it should have been based on the adjustment he made. We thought it was the scope until I sent it to Robb.


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