[Ringo]

First off - does ANYONE have chrono data on the 6.8 from comparable barrel lengths?

Had a convo with the FIL today (not 10 minutes ago) about what length I wanted to get my 6.8 barrel. When I said, "18, because the cartridge is optimized for that in the AR platform" he said that has to be bogus. Physics, he says, demands that you gain upwards of 70 fps per inch with barrels. And insists further that depending on the powder used for handloads still burns out to 18 inches, and thus you aren't getting full potential. He called BS on me about the gas system having any real effect on velocity and performance, and encouraged me to consider 20" or longer for the barrel. I told him I'd check into it, and I don't know enough about the physics and handloading yet to compile a valid counterpoint.

Anyone heard this argument before?

-R.

 

[LKirchoff68]

To tell you the truth, you would be just find getting a 16" barrel. I have an 18" Bison barrel, but there really isn't much of a noticeable difference, in fps, between the two lenghts. You may gain about 25-50 fps, it could even be less not entirely sure though, with an 18" over the 16". The 6.8 is best suited in shorter barrel lengths. With a 12.5" barrel you get a rifle that is much lighter and easier to manipulate than a 16" upper, while being every bit as effective. If I had enough money right now and a SBR lower, I would consider getting a 10" or 12.5" barrel to serve as my hunting rifle. That way, I can easily shoulder the rifle from inside the truck if a big enough buck presents itself. Also, I would have no problems walking around the ranch with a set of rattling horns all day long.

 

[ron169]

Check the reloading forum, there is a crap load of date with speeds with each barrel length. Check some of Paulo's stickied threads. Just tell your dad that the 6.8 was designed and constructed by the Army MArksmanship Unit, and it was designed with maximum performance through short barrel lengths. BEcause the of the fast burning powders that the 6.8 uses, the amount of velocity gain from a 16 to an 18 inch is lower than other rounds. Also, case powder capacity is also a limiting factor with slower burning powders that would excel with a longer barrel.

 

[HTR]

First off - does ANYONE have chrono data on the 6.8 from comparable barrel lengths?

Had a convo with the FIL today (not 10 minutes ago) about what length I wanted to get my 6.8 barrel. When I said, "18, because the cartridge is optimized for that in the AR platform" he said that has to be bogus. Physics, he says, demands that you gain upwards of 70 fps per inch with barrels. And insists further that depending on the powder used for handloads still burns out to 18 inches, and thus you aren't getting full potential. He called BS on me about the gas system having any real effect on velocity and performance, and encouraged me to consider 20" or longer for the barrel. I told him I'd check into it, and I don't know enough about the physics and handloading yet to compile a valid counterpoint.

Anyone heard this argument before?

-R.

Tell him to come to my ranch, stand behind me, and watch my chronograph, as I shoot the Barnes TTSX 110 grain load I cook up. It has 29.5 grains of RE10X and is loaded to 2.30". It produces 2700 FPS from my 16" barrel, and ....well, lookie there, it only produces 2736 FPS from my 20" ARP 12 twist, 3 groove.

He needs to incorporate a lot more of the tenets of physics into his opinion, especially when it is readily apparent that he has never fired a 6.8 SPC.

4 extra inches of barrel....36 FPS....doesn't add up, does it?

OTOH, when I shoot my SPH load general purpose hunting load, 30.8 grains of H322, then the 16" barrel gives me 2710 FPS, and the 20 ARP gives me 2830. Wow, that's exactly 30 FPS per inch. That is more in line with what to expect when you fire 6.8 SPC wit its high case efficiency, bore area to case ratio, and the capability to burn fast powders completely in the first 4-6 " of the barrel, with the remaining expanding gas producing propulsion, not so much heat and flame.

The general rule of thumb is, Physics does not move aside for the 6.8! This cartridge as optimized to produce a high % of its potential velocity in short barrels, so you get the bullet up to speed quickly in shorter barrels and gain a little less as the barrels get longer.....though you do still gain some.

The 85 grain barnes TSX goes 2975 in the 12" barrel
3150 in the 16" barrel
3200 in the 20" barrel

Again, right in the 30-40 FPS per inch zone. That is what you should expect. You can pass this along to your FIL, too.

 

[Ringo]

Thanks Ron. Got a little hasty with forgetting my search-fu discipline. Headed over to the reloading section now. I tried to tell him that it was designed for shorter rifles in the AR platform, but he still insisted physics say otherwise. So I'm sniffing out some proof. Thanks.

-R.

 

[joshuades]

I"d say he's right and wrong on that one. Because of the dimensions of the cartridge and bore, the bullets can be driven with faster powders. This in turn gets the velocity up faster. Now if you were to try driving a heavy bullet (say 130 or above that would fit) down the barrel with a slower powder, you would see gains with a longer barrel. Aside from those heavy bullets and hand loading them to your specific barrel, the normal loaded weights will not see much going from 16" to 18" and even less from 18" to 20". If you're wanting to squeeze the most velocity out of your rifle, then a 20" or longer would be optimal (I wouldn't go with more than a 22" in this calibre). Otherwise, 16" will get you the vast majority of velocity achievable (to the tune of about 98%) and you don't have all that extra barrel to deal with.

 

[NE-AR]

On a related note............

Ringo...take it from an old married guy who still remembers arguing with the father in law..........you can't win! He's still upset about you doing his daughter

Seriously, when I first found this place and started reading the info I was pretty convinced these guys had been smoking something other than pork but then I asked myself who has anything to gain by making this stuff up? Plus it would be easy to check out with just a rifle, ammo and chrono.

However your FIL isn't alone just looking around the web there are lots of people convinced that this is ballistic baloney. But if you stop to think about it you'll see it makes perfect sense. At least it does to me.

 

[Lastrites]

On a related note............

Ringo...take it from an old married guy who still remembers arguing with the father in law..........you can't win! He's still upset about you doing his daughter

.

As true as that may be, that is as funny as a line that I've read in some time! Darn near blew my drink.