6mm arc gas gun. How much jump is to much?

[Dave__th3__ss]

Just finished my 6mm arc build. I chose to run 70 grain nosler varmageddon bullets. With cfe223 (it’s what I have and easy to find)

Max OAL is 2.090 I loaded to 2.094 cause I had so much jump.

My jam is 2.230

Is that way to much jump for an ar?
If I increase the OAL is that going to cause pressure To go up or down?

I fired a few rounds today to check pressure and have no pressure signs. And cycled just fine. All the way to max load data of 33.0gr.

Now I’ll put my thermal on and go sight in and shoot for accuracy

 

[Dave__th3__ss]

33 views and no one has any answers?

 

[308sako]

It's a new cartridge with limited availability of components & dies... Hope like you do to have some shared experiences soon.

 

[Dave__th3__ss]

It's a new cartridge with limited availability of components & dies... Hope like you do to have some shared experiences soon.

it doesnt really have to be specific to the 6mm arc per say. mostly asking about jump in the ar platform.
but yes the arc being new there isnt alot of data on it for really anything.

 

[spife7980]

With an AR mag feeding you have to do what you have to do. Make them mag length and then test shorter from there, you cant go longer unless youre fine single feeding a thermal gun.

Generally as you make the round longer you increase the internal volume which makes pressure go down. Thats until you hit the lands and pressure can then spike. So it depends.

 

[larryh128]

Just finished my 6mm arc build. I chose to run 70 grain nosler varmageddon bullets. With cfe223 (it’s what I have and easy to find)

Max OAL is 2.090 I loaded to 2.094 cause I had so much jump.

My jam is 2.230

Is that way to much jump for an ar?
If I increase the OAL is that going to cause pressure To go up or down?

I fired a few rounds today to check pressure and have no pressure signs. And cycled just fine. All the way to max load data of 33.0gr.

Now I’ll put my thermal on and go sight in and shoot for accuracy

In my opinion there isn't a too much jump situation in an ar.

 

[Tx_Aggie]

With an AR mag feeding you have to do what you have to do. Make them mag length and then test shorter from there, you cant go longer unless youre fine single feeding a thermal gun.

Generally as you make the round longer you increase the internal volume which makes pressure go down. Thats until you hit the lands and pressure can then spike. So it depends.

I was typing essentially the same thing when @spife7980 posted this.

OP, Choosing jump tolerant bullets will go a long way towards simplifying load development in an AR.

 

[CK1.0]

Unless one is totally hung up on getting the absolute smallest groups on paper off a bench that their gun is capable of, IMO a bigger jump is pretty much always a good thing.

A bigger jump almost always means less pressure, barrel lasts longer, more consistent waterline at 500+, less finicky (less load adjustment needed over time), etc… all good stuff.

Every bullet is a little different, but if it shoots it shoots, and seems like most of these heavier bullets we all use now for longer ranges seem to like to jump.

I don’t bother testing anything closer than .060” off anymore…

All that said, when I think 6mm I think 105gr+… I don’t know if the same things apply running a 70gr bullet, so YMMV…

 

[2aBaC̶a̶]

The numbers dont matter. Load for reliability and precision.

 

[Rockdoc173]

If it shoots good and you like the velocity, go with it. It also depends what you're doing with it. Shooting varmints at 200 is different than shooting steel at 1k. However, I started and left mine at a Hybrid 105gr OAL of 2.260" because that's as far as I could go in the AR mags. At that length, I have 0.068" jump and a CBTO of 1.648". Your Nosler bullets have an OAL of 0.875" and my Hybrids are 1.256" so the Hybrids are obviously quite a bit longer. I think you could run the 70gr bullets much further out and pick up a bunch of velocity while closing that jump. The ARC was designed to get more out of an 18" AR barrel and be ballistically superior to everything else, since it was designed for the military that means more energy on soft targets father than those that came before it. You're not going to find that superiority with a 70gr bullet since it can't compete with higher weight, high BC bullets at long range. Again, it you're carpet bombing dog towns at a couple hundred yards it doesn't matter. In the end, I'm not sure there's a 'right' answer here, it comes down to what your goal is for the cartridge.

 

[Dave__th3__ss]

If it shoots good and you like the velocity, go with it. It also depends what you're doing with it. Shooting varmints at 200 is different than shooting steel at 1k. However, I started and left mine at a Hybrid 105gr OAL of 2.260" because that's as far as I could go in the AR mags. At that length, I have 0.068" jump and a CBTO of 1.648". Your Nosler bullets have an OAL of 0.875" and my Hybrids are 1.256" so the Hybrids are obviously quite a bit longer. I think you could run the 70gr bullets much further out and pick up a bunch of velocity while closing that jump. The ARC was designed to get more out of an 18" AR barrel and be ballistically superior to everything else, since it was designed for the military that means more energy on soft targets father than those that came before it. You're not going to find that superiority with a 70gr bullet since it can't compete with higher weight, high BC bullets at long range. Again, it you're carpet bombing dog towns at a couple hundred yards it doesn't matter. In the end, I'm not sure there's a 'right' answer here, it comes down to what your goal is for the cartridge.

It’s strictly a varmint rifle. 20” cmmg 1-7.5 barrel. I don’t plan on shooting past 200-300 tops.
I’ve considered running heavier 90 grain eldx
But I’d like to see 3000fps out of my rounds. More energy on target

 

[Rockdoc173]

Yea with that in mind I think you can push that bullet out and get you some added velocity

 

[alamo5000]

With an AR mag feeding you have to do what you have to do. Make them mag length and then test shorter from there, you cant go longer unless youre fine single feeding a thermal gun.

Generally as you make the round longer you increase the internal volume which makes pressure go down. Thats until you hit the lands and pressure can then spike. So it depends.

^^^Yep. What he said.

I have not shot the lightweight bullets and especially those specific ones the OP mentioned but just as an 'in general' I've shot bullets that hated jumping and I have shot bullets that LOVED jumping.

For a mag fed gun though start at the absolute longest you could possibly go while still fitting in a magazine and work your way in from there. I've had times when I was actually extremely surprised at how much jump some bullets like before they shine.

 

[r.tenorio671]

...the attached image of a Nosler 70 VG and Nosler 105 RDF shows you a relationship of their seating depths with similar CBTO measurements and their individual aspects to how much bullet is retained in the neck and intrusion into the powder stack. What CBTO measurement YOUR barrel's chamber will allow WILL BE DIFFERENT.