Nitty-Gritty Comparison: 6mm vs. 6.5mm out of an AR-10?

[BAC]

Hello! New here, decided I'd join up as often as I'm reading the rest of the site.

I wanted to pick the brains on the technical details of the 6mm and 6.5mm rounds, especially concerning ballistics.

Currently, I'm brainstorming/drafting up a DMR/SPR project, but with a twist: AR-10 platform, sending a 6-6.5mm package, but must maintain the ability to use existing .308 magazines. There are several 6mm and several 6.5mm chamberings that would work with the AR-10, and as always there's a catch 22. Folks like Robert Whitley and others have described it to me as this: 6mm rounds will win on accuracy, 6.5mm will win on power,energy). Either buck wind better,depending who you ask), both are more inherently accurate than the more common 5.56N and 7.62N choices, and both fall between the two in terms of energy delivered.

The question, condensed nicely into one sentence: how different, functionally, are 6mm and 6.5mm bullets in terms of accuracy and energy on target, given the limits of the AR-10 mag well? Are the differences more academic and less practical, or are they real and can be seen and demonstrated in actual use?

Appreciate the reading, and thanks in advance,


-B

 

[bfoosh006]

I'd take the 6.5x 308 over the 6mm any day of the week. Ooops, I mean the 260 Rem.

 

[MikeyG]

I'd take the 6.5x 308 over the 6mm any day of the week. Ooops, I mean the 260 Rem.

I'm definately not an expert but from what I heard you may have difficulty in seating the .260Rem projectiles out to the lands while still fitting in the .308 mag. 6.5 by 47 should fit the mag with bullets seated out.

What do you wnat to use you rifle for? Some priorities will be different when comparing a hunting rifle against a target rifle.

 

[BAC]

MikeyG, that's one of my bigger concerns with the longer 6.5mm rounds. One individual had pointed out to me that the 6XC and 6.5 Creedmore could both be loaded to 'chase the lands' as the throat starts wearing, but my understanding of the sharper shoulder angle of the 6.5x47 is that it would reduce the wear on the throat, so 'chasing the lands' might not be much an issue.

This would be a rifle to both compete with, train with at precision rifle courses, and hunt with. I know, there are compromises that would need to be made, hence my question in the first place. I realize there are differences between 6mm and 6.5mm bullets in overall performance, but I'm more concerned with whether or not those differences are purely academic or if they would have real-world impact on any one area of my shooting.

A thought crossed my mind that if I can get a couple barrels, one in 6.5mm and one in 6mm, I can use the 6.5x47 Lapua and 6-6.5x47 Lapua,Robert Whitley described it as a 6.5x47 literally necked down to a 6mm). Shoot 'em both, see which performs better, and keep it. Thoughts? Does this idea have merit, or should I just pick a single cartridge and stick with it?


-B

 

[MikeyG]

All my experience is with .308Win in single shot palma rifles. I can only pass on what I have heard.

Given your usage though I would look at which cartridge will fire projectiles heavy enough for the game you intend to hunt. As you will be firing a lot of rounds through competition and practice as well I would pick a smaller capacity case to get decent barrel life, i.e. a 6mmBR rather than a .243 Winchester. That is just my opinion though.

At the end of the day in a decent rifle pretty much any of the 6 or 6.5 mm projectiles can be made to shoot well. What it may come down to is your own requirements for recoil, barrell life, projectile weight, etc.

 

[tdking]

The question, condensed nicely into one sentence: how different, functionally, are 6mm and 6.5mm bullets in terms of accuracy and energy on target, given the limits of the AR-10 mag well? Are the differences more academic and less practical, or are they real and can be seen and demonstrated in actual use?
-B

B - It really depends on what you want to use the rifle for. Having said that, I currently have an AR-10 in 260 Rem which will shoot extremely well with magazine length 142 Sierras out to 1000 yards. I also have a T2K in 6XC which has comparable ballistics using 115 gr. bullets. I can't tell a lot of difference on paper targets. I have not shot the 6XC on metal plates so I can't compare them there.

I have shot the AR in tactical matches at ranges to 800 yds and the only complaint I heard was that the round was doing too much damage to the steel plates. FWIW, I have heard of some silhouette shooters who have said the same thing about the 243 Win on the ram targets at 500 meters. Since the 6XC will push 107 Sierras at essentially the same velocity as the 243 Win, I would expect it to perform similarly.

As for chasing the lands, that is a personal choice as well as Eric Strecker of Berger Bullets has proved that seating VLDs to the lands is not required to get excellent accuracy. I have shot VLDs upto 0.140 off the lands with 1/3 MOA accuracy in my bolt guns.

So if you are worried about chasing the lands, go with something shorter than the 260 Win,6.5 x 47 will give you 0.100 more room in the mag, same for a 6 x 47 or 6XC). Otherwise use the 260 Rem. Brass is cheaper, but you will use a few more grains of powder to get the same velocity.

If you use 115 6mm bullets, then the 6 x 47 or 6XC will not give up anything to the 260 Rem. or 6.5 x 47.

Just my $0.02 worth,

TDK