18" vs 16" barrels for accuracy?

[Virtus985]

Anyone here familiar with 6.8spc and AR15 barrel lengths in general?

I'm building another AR, only this time it's in 6.8, planning on making it a DMR sort of rifle. I don't intend on shooting super long distance (nothing past 500m or so) but I want to get it as accurate as possible.

I was originally planning on getting an 18" or 20" barrel, but builders that make those are few and far between. By far the most common size is 16", and I've done some research and I found that the extra few inches of barrel only nets you a small increase in velocity.

Does that increase in velocity necessarily get you an increase in accuracy, or will the 18" just be 2 extra inches of barrel to carry around with no real benefit to accuracy?

[bloomautomatic]

The general rule of thumb is that longer barrels will be more accurate...HOWEVER many other factors come into play. If you reload, and if you're concerned about maximum accuracy that's really the only way to go, you should be able to work up a loads that produce similar accuracy with either length barrel.

Vince

[Virtus985]

I wasn't planning on reloading, just trying to get the best I could get out of an "out of the box" configuration. (What can I say, I'm lazy and cheap )

Thanks for the info though, rep sent!

[hzox221]

Not sure of the ballistics of the 6.8. But alot of 3 gun shooters prefer an 18" barrel over a 16" to allow for a rifle length gas system. An 18" barrel works better with a rifle length gas system than a 16". Another reason why the 18" bacame popular was as a trade off because of the weight of the optics. I would suggest a 20" barrel to up the PF.

[knight0334]

Between a 16" and 18", there will be virtually no accuracy difference so long as the projectile stays above the sonic barrier where turbulence causes problems. The 18" may offer 30-80fps more velocity, thus allowing a slightly greater range before hitting the sonic barrier, which that range will be well beyond even typical varmint ranges.

And since the AR platform uses either a scope or open sights that are in the same general position whether its a 16" or 18", the sight radius will not be different to offer any more, or less, accuracy.

Remember, longer barrels only offer velocity over shorter barrels. Accuracy only improves if the barrel length increases the sight radius. ...which with a scope, that is not even a factor because the sight radius is completely between the lenses of the scope.

[Virtus985]

Interesting...

Just found THIS, a model 1 sales 6.8 upper for pretty cheap, and it's a 24" fluted barrel, maybe too long? It would definitely give me the evil black sniper rifle look but the price makes me think too good to be true. I've heard good things about Model 1 and I've also heard to avoid them like the plague...

It has an ER Shaw barrel, which is what the more hi-end builders use, but who knows if that's any indication of its accuracy.

[GRIZ]

I don't think you'll find much diffrence in accuracy between a 16" and 18" barrel no matter what the caliber.

[Guns4Fun]

I don't know but i thought the whole idea behind the creation of the 6.8 was so that our soldiers could use the shorter barrel M4 14.5" and less and still get acceptable stopping power in combat. Again, the whole reason they wanted the 6.8 round was so the bullet would have a little more ooomf out of the shorter barrels and still penetrate glass, doors, buildings, bad guys, etc better than the 5.56 round. I thought the 6.5 grendel (sp) was what they were trying to push for longer distances out of an ar platform?


Anyhow, i can't see much difference between a 16" and 18" barrel. But then again, i havn't done a great deal of study on the 6.8 round.

[Virtus985]

Theres a huge debate over 6.8 vs 6.5, and a lot of chest thumping over charts and more charts on either side. Usually those charts are actually indexed to whoever made the chart's personal favorite, making either 6.8 or 6.5 look like complete ass on paper compared to the other.

You're right about the original intent of 6.8 being thumping harder out of short barrels. A rather fortuitous secondary characteristic of 6.8 was that it had better bullet drop ballistic characteristics than 7.62 NATO within 600 yards. Wind drift was a little worse 'cause of the smaller bullet, but overall it had a flatter trajectory.

6.5 on the other hand, is a more purpose made long-range cartridge with a much more unorthodox case dimension, and very good long range ballistics. However, as far as I've seen 6.5 barrels and accessories (bolts, etc) are very few and far between.

I picked 6.8 for my build because there are a lot more companies out there that deal with 6.8, and the ballistics are fine for the ranges I'm going to be shooting at. When I want to really reach out and touch someone and go for super long ranges, I'm not gonna dick around, I'll just go straight for .50 BMG.

The model 1 sales 6.8 upper price actually doesn't include a bolt or bolt carrier, which is why it's so low. Also their barrel twist rate is strange and the barrels are chrome lined instead of SS, which isn't what I'm looking for. Darn.

I guess I still havent solved my 16 vs 18 dilemma! hahaha

[Guns4Fun]

If you havn't already, check out this site on the 6.8. I have heard they know their stuff---but i've not joined there because 6.8 isn't my thing---yet.

http://68forums.com/index.php