Looking to do an urban defense rifle build chambered in 6.8 (for when the zombies come ). How much of a difference in terms of durability, longevity, and heat tolerance/cooling will the various barrel profiles make, or is the difference basically just the weight?

The difference in barrel profiles is going to affect how long it takes to heat them up, and when they heat up how much they change shape. However, once it heat soaks, a lighter barrel will cool off more quickly. The other thing it affects is barrel rigidity. IN general decreased rigidity with increase barrel harmonics, and decrease accuracy unless the harmonic leaves the muzzle as a nodal point for a given cartridge. 

 

There are two ways to mitigate this. Barrel fluting is one. It still heats up faster than a big fat bull barrel, but also cools down faster, and retains a lot of the regidity of a bull profile of equivalent diameter due to the fluting. Another are carbon fiber wrapped barrels. You get the rigidity of a bull barrel, and CF dissipates heat faster than steel. It is also light. Good luck sourcing one without dropping the cash on a whole custom rifle that might be finished sometime before you are dead. Maybe.  

 

Myself, I don't like true M4 cut barrels because the grenade launcher cut is just asking for stringing when it heats up. For 5.56 I like fluted HBAR barrels for accuracy, and standard mid weight barrels for run and gun. Both are compromises for their given job. For a bigger projectile like a 6.8 I'd probably go with a beefier mid-weight style profile like black hole weaponry uses, or go with a fluted SPR or HBAR profile. Wilson makes a lightwieght profile though. If you plan on carrying something a lot, lightness counts a lot. 

 

For real practical purposes I wouldn't touch 6.8. It's not bad ballistically, it's just very poorly supported. It needs scarce ammo, or if you want to make your own, scarce brass and bullets. If you want something oddball, 6mmx45 or 300 blackout are bit more sane. Both are formed off .223 brass and use more common bullets. 6mm is popular in benchrest and is shared with the fairly popular .243 cartridge. .300 use good old .30 caliber bullets that .308, .30-06, etc. use. so.. 

 

Part of me is tempted to make a 6mmx45 upper. 

The difference in barrel profiles is going to affect how long it takes to heat them up, and when they heat up how much they change shape. However, once it heat soaks, a lighter barrel will cool off more quickly. The other thing it affects is barrel rigidity. IN general decreased rigidity with increase barrel harmonics, and decrease accuracy unless the harmonic leaves the muzzle as a nodal point for a given cartridge. 

 

There are two ways to mitigate this. Barrel fluting is one. It still heats up faster than a big fat bull barrel, but also cools down faster, and retains a lot of the regidity of a bull profile of equivalent diameter due to the fluting. Another are carbon fiber wrapped barrels. You get the rigidity of a bull barrel, and CF dissipates heat faster than steel. It is also light. Good luck sourcing one without dropping the cash on a whole custom rifle that might be finished sometime before you are dead. Maybe.  

 

Myself, I don't like true M4 cut barrels because the grenade launcher cut is just asking for stringing when it heats up. For 5.56 I like fluted HBAR barrels for accuracy, and standard mid weight barrels for run and gun. Both are compromises for their given job. For a bigger projectile like a 6.8 I'd probably go with a beefier mid-weight style profile like black hole weaponry uses, or go with a fluted SPR or HBAR profile. Wilson makes a lightwieght profile though. If you plan on carrying something a lot, lightness counts a lot. 

 

For real practical purposes I wouldn't touch 6.8. It's not bad ballistically, it's just very poorly supported. It needs scarce ammo, or if you want to make your own, scarce brass and bullets. If you want something oddball, 6mmx45 or 300 blackout are bit more sane. Both are formed off .223 brass and use more common bullets. 6mm is popular in benchrest and is shared with the fairly popular .243 cartridge. .300 use good old .30 caliber bullets that .308, .30-06, etc. use. so.. 

 

Part of me is tempted to make a 6mmx45 upper. 

huh?  no problem reloading during the crisis and more ammo makers coming online for 6.8. 

huh? no problem reloading during the crisis and more ammo makers coming online for 6.8.

Last I checked in there were two sources of brass, and two bullets that are really appropriate.

 

Looking now, there are three sources of brass: hornady, ssa, and remington. Assuming ssa isn't just custom head stamp from one of the others.

 

6.5 Grendel and 300 blackout both have four.

 

It now has 12 whole bullets available for it. 6mm appropriate for 6x45 there are about 50, a bit more for .300 blackout. 6.5 Grendel bullets there are about .40.

 

 

That's still a TEENY, TINY number of sources compared to the others. It's still a curiosity for the most part, and that should be kept in mind when buying into it.

I was thinking light and fast (5 lbs of accessories and a bull barrel are practically useless inside a home), but was worried about the larger round overheating too quickly. Besides the typical pencil, HBAR, SOCOM, M4, etc. there's so many companies that make up their own names for things and it's hard to keep track of. I was thinking of a mid-weight rig and free floating it for accuracy, plus I figure that all that air and a ventilated hand guard would help with heat.

For 6.8, this is a well thought out barrel profile for a lighter gun. http://ar15performance.com/inc/sdetail/11562/14400

 

I have one of their really light profile .233 barrels. It shoots well for being as light as it is. As a business, they give a lot of attention to 6.8.

What is 5R? Five lands or something else?