6mm ARC

[FRB6.5]

https://www.instagram.com/p/CBGV4ISpd_2/

In the comments Mark LaRue confirms they will be making stuff for 6mm ARC as well.

[SDguy]

Imagin CZ will be 2 years or more B4 they have a bolt action offering.

[MTN]

7.62x39 or 6.5G Cz's are easy enough to rebarrel. Below is my 18" 6 ARC.

As for why 6 ARC over the 6.5 Grendel, a 103-108gr class 6mm bullet at 2600fps (18") vs. a 120-125gr bullet at 2450fps. The ARC has slightly higher energy at the muzzle, and the higher BC bullets (.530's on the low end, up to over .6 on the high end) will retain it better down range. You also get a wider envelope of really useful bullets for the case capacity, IMO. 55gr thru 110 and all perform really well. With the 6.5 it's hard to find much below 90gr, and anything over 125 is really starting to ask a lot of the limited case capacity.

KIMG1821.JPG

[Old Bob]

I noticed that barrels for the 6mm ARC have different gas port locations. Proof & Odin barrels have rifle length + gas ports while BA & Faxon have standard gas port locations. Since Barrett used Proof barrels with + length gas ports during the development of the cartridge, there must be a reason why they went with that particular gas port location. There are members of this forum who have a lot more experience dealing with AR functionality than I do. Perhaps some of you AR gurus can enlighten me on gas port location theory for this new cartridge.

Emailed Wilson Combat. In their reply they said they didn't have any 6mm ARC components (barrels) yet but to keep checking back for new developments. My question to them about gas port location went unanswered. I figure they don't know yet themselves where it will be. I imagine they'll publicly announce when they have 6mm ARC stuff in stock.

[kmon]

I noticed that barrels for the 6mm ARC have different gas port locations. Proof & Odin barrels have rifle length + gas ports while BA & Faxon have standard gas port locations. Since Barrett used Proof barrels with + length gas ports during the development of the cartridge, there must be a reason why they went with that particular gas port location. There are members of this forum who have a lot more experience dealing with AR functionality than I do. Perhaps some of you AR gurus can enlighten me on gas port location theory for this new cartridge.

Emailed Wilson Combat. In their reply they said they didn't have any 6mm ARC components (barrels) yet but to keep checking back for new developments. My question to them about gas port location went unanswered. I figure they don't know yet themselves where it will be. I imagine they'll publicly announce when they have 6mm ARC stuff in stock.

I suspect they are using slower burning powder in the 6mmARC which yields higher pressure at the Gasport and muzzle than the Grendel. It would not surprise me if powders as slow as 6.5STAball powder can get good velocity in the ARC

Progressive powders burn the most efficient under pressure and to an extent the smaller diameter bullet/bore diameter used the less room for hot gasses to expand in and the more time slower powders can do their thing effectively. For instant you can use slower burning powders in the 243 for great results than in the 7mm-08 or 308 and you can use even faster powders to better use in a 358 Winchester than the 308 or 7mm-08. '

Another way to think of it is we start with X case volume with the bullet seated, as the bullet moves under pressure of burning powder and resulting gas the further down the barrel the more volume the powder has, now with a larger bullet that volume of the chamber increases faster as th bullet moves down the barrel resulting in a larger Expansion Chamber to hold those expanding gasses resulting in pressure dropping faster than a smaller chamber due to the smaller bullet.

With a gas gun to deal with higher pressure at the gas port you can use a smaller gas port for less gas transfer but the pressure of that gas will be the same basically between gas port sized until it hits the larger diameter gas tube and the rest of the gas system. Ot using an adjustable gas block it reduce the volume of gas getting through, or simply move the gas port further down the barrel to where the gas pressure is optimal for the system with standard port size. It is a balancing engineering game getting the gas volume and pressure correct for timing with too little, just right or too much gas.

Hope that does well enough explaining this, sometimes us engineers have trouble explaining things.

[A5BLASTER]

I suspect they are using slower burning powder in the 6mmARC which yields higher pressure at the Gasport and muzzle than the Grendel. It would not surprise me if powders as slow as 6.5STAball powder can get good velocity in the ARC

Progressive powders burn the most efficient under pressure and to an extent the smaller diameter bullet/bore diameter used the less room for hot gasses to expand in and the more time slower powders can do their thing effectively. For instant you can use slower burning powders in the 243 for great results than in the 7mm-08 or 308 and you can use even faster powders to better use in a 358 Winchester than the 308 or 7mm-08. '

Another way to think of it is we start with X case volume with the bullet seated, as the bullet moves under pressure of burning powder and resulting gas the further down the barrel the more volume the powder has, now with a larger bullet that volume of the chamber increases faster as th bullet moves down the barrel resulting in a larger Expansion Chamber to hold those expanding gasses resulting in pressure dropping faster than a smaller chamber due to the smaller bullet.

With a gas gun to deal with higher pressure at the gas port you can use a smaller gas port for less gas transfer but the pressure of that gas will be the same basically between gas port sized until it hits the larger diameter gas tube and the rest of the gas system. Ot using an adjustable gas block it reduce the volume of gas getting through, or simply move the gas port further down the barrel to where the gas pressure is optimal for the system with standard port size. It is a balancing engineering game getting the gas volume and pressure correct for timing with too little, just right or too much gas.

Hope that does well enough explaining this, sometimes us engineers have trouble explaining things.

Yep good job explaining it.

Tells me the reason the military unit went with longer gas lengths is to better work the gas pressure because they don't run adjustable gas.

My thinking is normal gas lengths will work just fine if one was to use a adjustable gas block.

BFT if your going to go with normal gas lengths, I might just go ahead and get one barrel just for test since all my ar grendels run on lowmass and adjustable gas systems, I could strio one down and throw the 6mm ARC barrel on it and see how it does with a normal gas length but useing adjustable gas.

[Old Bob]

I suspect they are using slower burning powder in the 6mmARC which yields higher pressure at the Gasport and muzzle than the Grendel. It would not surprise me if powders as slow as 6.5STAball powder can get good velocity in the ARC

Progressive powders burn the most efficient under pressure and to an extent the smaller diameter bullet/bore diameter used the less room for hot gasses to expand in and the more time slower powders can do their thing effectively. For instant you can use slower burning powders in the 243 for great results than in the 7mm-08 or 308 and you can use even faster powders to better use in a 358 Winchester than the 308 or 7mm-08. '

Another way to think of it is we start with X case volume with the bullet seated, as the bullet moves under pressure of burning powder and resulting gas the further down the barrel the more volume the powder has, now with a larger bullet that volume of the chamber increases faster as th bullet moves down the barrel resulting in a larger Expansion Chamber to hold those expanding gasses resulting in pressure dropping faster than a smaller chamber due to the smaller bullet.

With a gas gun to deal with higher pressure at the gas port you can use a smaller gas port for less gas transfer but the pressure of that gas will be the same basically between gas port sized until it hits the larger diameter gas tube and the rest of the gas system. Ot using an adjustable gas block it reduce the volume of gas getting through, or simply move the gas port further down the barrel to where the gas pressure is optimal for the system with standard port size. It is a balancing engineering game getting the gas volume and pressure correct for timing with too little, just right or too much gas.

Hope that does well enough explaining this, sometimes us engineers have trouble explaining things.

I think I get what you're telling me. If I want to shoot the same ammo that works well in a 6mm ARC AR with a plus length gas system in another AR with a shorter length gas system (a 16" barrel with a mid-length system for instance), I oughta have an adjustable gas block to keep from over-gassing that rifle.

I've been wondering about which powders would be the most efficient with the 6mm ARC. There is zip/nada reloading data for the cartridge right now but I suspect that info will be forthcoming in the days ahead as reloaders start experimenting - after barrels become more plentiful.

Hmmm... I have a 22 Nosler I built 2 years ago but never fired 'cause my Grendel has been taking up all my time. That might be a good place to start working on a 6mm ARC sometime in the future.

[SDguy]

7.62x39 or 6.5G Cz's are easy enough to rebarrel. Below is my 18" 6 ARC.

As for why 6 ARC over the 6.5 Grendel, a 103-108gr class 6mm bullet at 2600fps (18") vs. a 120-125gr bullet at 2450fps. The ARC has slightly higher energy at the muzzle, and the higher BC bullets (.530's on the low end, up to over .6 on the high end) will retain it better down range. You also get a wider envelope of really useful bullets for the case capacity, IMO. 55gr thru 110 and all perform really well. With the 6.5 it's hard to find much below 90gr, and anything over 125 is really starting to ask a lot of the limited case capacity.

[ATTACH=CONFIG]16287[/ATTACH]

Mind if I ask how or why you settled on 18 " & which barrel mfg you went with?
Both of my 24" 6.5 Grendel do surprisingly well w a full or compressed case of LVR pushing a 129 gr. Hornady FB Spire point. Would not mind giving a good 6mm ARC barrel a try i suppose. have to imagine I would appreciate the lesser recoil if nothing else.

[Bronz]

Looks interesting and good premise. Small bullet, can obtain faster velocity. Might be something the .mil is interested in. But still has the large case rim which makes the bolts so thin around the locking lugs. Which is probably a deal killer for the military.

[A5BLASTER]

Looks interesting and good premise. Small bullet, can obtain faster velocity. Might be something the .mil is interested in. But still has the large case rim which makes the bolts so thin around the locking lugs. Which is probably a deal killer for the military.

The round was developed for a military unit.

[Gtscotty]

Yep good job explaining it.

Tells me the reason the military unit went with longer gas lengths is to better work the gas pressure because they don't run adjustable gas.

My thinking is normal gas lengths will work just fine if one was to use a adjustable gas block.

BFT if your going to go with normal gas lengths, I might just go ahead and get one barrel just for test since all my ar grendels run on lowmass and adjustable gas systems, I could strio one down and throw the 6mm ARC barrel on it and see how it does with a normal gas length but useing adjustable gas.

I think you still want to go with a longer gas system if possible. It's not 100% port size, even with an adjustable gas block, you can still have timing issues that make the gun more finicky to tune. I'm betting that Proof has this this cartridge pretty well figured out, so whatever barrel I buy is going to have the same length gas system, which from their page is rifle for the 16", rifle +1 for the 18", and I'd assume you'd want to go rifle +2 for over 20".

[ricsmall]

Yes it needs the + gas system. My 224 Grendel does not have the+ gas, and with gas tuned way down and rifle weight buffer it is still hard on brass.

[A5BLASTER]

I think you still want to go with a longer gas system if possible. It's not 100% port size, even with an adjustable gas block, you can still have timing issues that make the gun more finicky to tune. I'm betting that Proof has this this cartridge pretty well figured out, so whatever barrel I buy is going to have the same length gas system, which from their page is rifle for the 16", rifle +1 for the 18", and I'd assume you'd want to go rifle +2 for over 20".

That's cool but I'm still going to test and see if normal gas lengths will work. If I do get one it will be a 12 inch.

[Gtscotty]

Yes it needs the + gas system. My 224 Grendel does not have the+ gas, and with gas tuned way down and rifle weight buffer it is still hard on brass.

Yeah, I have a mid-length .308 that I think would probably benefit from the higher dwell time of an intermediate or rifle length gas system. I'm definitely going long on the ARC, no reason not to.

That's cool but I'm still going to test and see if normal gas lengths will work. If I do get one it will be a 12 inch.

Cool, let us know how it works out.

[Lemonaid]

Gavin Tube has a short vid on the 6 ARC and CMMG, and his plans on testing. https://www.youtube.com/watch?v=VWm1HO5ooFo