6mm arc sbr/ pistol?

[Sincerd]

Been wondering the performance of a 6 Arc out of a short-barrel? Maybe12.5? With the velocity as it is did anyone have any idea how much velocity you would lose? Would it end up being a 6mm blackout basically?

Seems like most of the barrels are made in 18 in. The people have SBR Grendel's

 

[ormandj]

The free Hornady load data for 6mm ARC and 105-108gr bullets on their site has velocity info on the 12.5”. I am waiting on a 10.5” barrel, but once I get it will have some more info for that length.

 

[stilesg57]

No real world experience, but should be a bigger drop off than the Grendel due to the 6ARC case capacity being proportionally more over bore.

Blackout gets a complete powder burn in about 9” of barrel. Grendel needs a longer barrel to pull that off; 6ARC should need longer still.

 

[Ledzep]

 

[Sincerd]

Blackout gets a complete powder burn in about 9” of barrel. Grendel needs a longer barrel to pull that off; 6ARC should need longer still.

That is a myth and has been disproven time and time again. Somehow it's repeated in every blackout thread I've ever seen though. 95% of modern powders burn within the first 3 in of barrel. If its saying complete powder burn, 223 will do the same. If it's saying you stop gaining muzzle velocity that's just wrong. Maybe what they're trying to say is you don't get a big Fireball from the expanding gases?

But again, most (if not all) modern powders (pistol - 308 type)burn within the first 3 inches of barrel but containing the blast and harnessing the energy for a longer period of time gives you more muzzle velocity. With Blackout you see substantial velocity gains well past 16"(2400 fps). About 25 fps per inch which which is about the same 223. Any of the blackout Test with different barrel lengths on YouTube can show you this. Jrb or Johnny's reloading bench did a good series on 300 blk comparing 16" to sub 10" barrels.

 

[stilesg57]

The first 50 hits on google alone would make me ask for your sourcing on that...

However, I’m the first to agree that “conventional wisdom” is often interchangeable with “shared delusion.”

That said, from personal experience alone the blast and fireball from more overbore cartridges out of the same barrel length absolutely increases. Some of that translates into added velocity, which is why I usually see 50-35fps/inch from 16-20 for .223 and 25-20fps/inch from 16-20 for 300blk.

I’ve got or have had .223 barrels in:

10.5
12.5
14.5
16
18
20

and 300blk barrels in:
9
10.5
16

and 6.5G barrels in
12.5
18
20 (bolt)
24 (bolt)

so I’m not making it up. The smaller bore over more powder (or over the same powder) has less volume for gasses to expand the shorter the barrel. If that’s not technically correct because the powder is actually consumed way earlier (would truly be interested in some sourcing on that), it still doesn’t change the advice. But I thought that was technically correct too, ha.

 

[Sincerd]

The first 50 hits on google alone would make me ask for your sourcing on that...

However, I’m the first to agree that “conventional wisdom” is often interchangeable with “shared delusion.”

That said, from personal experience alone the blast and fireball from more overbore cartridges out of the same barrel length absolutely increases. Some of that translates into added velocity, which is why I usually see 50-35fps/inch from 16-20 for .223 and 25-20fps/inch from 16-20 for 300blk.

View attachment 7397936

I’ve got or have had .223 barrels in:

10.5
12.5
14.5
16
18
20

and 300blk barrels in:
9
10.5
16

and 6.5G barrels in
12.5
18
20 (bolt)
24 (bolt)

so I’m not making it up. The smaller bore over more powder (or over the same powder) has less volume for gasses to expand the shorter the barrel. If that’s not technically correct because the powder is actually consumed way earlier (would truly be interested in some sourcing on that), it still doesn’t change the advice. But I thought that was technically correct too, ha.

Mix and mash up from jrb,

Suppressed 5.56 SBR Velocity Tests--41 Factory Loads from 10.5", 11.5", 12.5" & 14.5" ALL DONE! - AR15.COM

Firearm Discussion and Resources from AR-15, AK-47, Handguns and more! Buy, Sell, and Trade your Firearms and Gear.

www.ar15.com

And several threads like it

Reloading books

 

[stilesg57]

Cool I’ll look into those — will be the first to admit if I’m wrong.

...does the advice change though? Go longer barrel the more overbore the cartridge? That I’m pretty confident of from experience alone.

 

[Sincerd]

Would make sense with less pressure or...

Less holding back the larger pressure.

I'm not an expert. More of a hobbies but I do read good deal and I've tried alot with 300 blackout.

An example would be. I couldn't get it MCX piston rifle to cycle. I found out that it was not the powder charge or the amount of gas. Seating the bullet too deep caused the powder to burn itself out before it got to the gas block. Thought that was interesting.

That and the relationship with volume and amount of powder. You can get more velocity from 7 grains of powder with the bullet right on top of the powder than you can with 8 grains of powder with a small gap between.

 

[ormandj]

Cool I’ll look into those — will be the first to admit if I’m wrong.

...does the advice change though? Go longer barrel the more overbore the cartridge? That I’m pretty confident of from experience alone.

That advice is still not correct. Go with the length you need to achieve your goals. I'm putting together a 10.5" 6mm ARC because I'm fine with the velocity reduction; I'm not expecting to make first round 1000yd hits with it, and I want the shorter barrel. Nothing "bad" is going to happen, I'm just giving up velocity. I shoot suppressed with a sacrificial brake, and it's a Sandman S, so erosion isn't a huge ordeal.

What is true - the more overbore you are, the more velocity you start losing per inch as you go shorter and shorter. You just need to determine what velocity you need and work out your minimum barrel length with that cartridge - if it fits your desired length, awesome, if not - pick another cartridge.

 

[Nbleak21]

That advice is still not correct. Go with the length you need to achieve your goals. I'm putting together a 10.5" 6mm ARC because I'm fine with the velocity reduction...

what is your intent behind the 6mm in a short barrel? Just because? Round compatibility? Velocity?

 

[stilesg57]

That advice is still not correct. Go with the length you need to achieve your goals. I'm putting together a 10.5" 6mm ARC because I'm fine with the velocity reduction; I'm not expecting to make first round 1000yd hits with it, and I want the shorter barrel. Nothing "bad" is going to happen, I'm just giving up velocity. I shoot suppressed with a sacrificial brake, and it's a Sandman S, so erosion isn't a huge ordeal.

What is true - the more overbore you are, the more velocity you start losing per inch as you go shorter and shorter. You just need to determine what velocity you need and work out your minimum barrel length with that cartridge - if it fits your desired length, awesome, if not - pick another cartridge.

Question then: what's the scenario where you'd be better served by 6ARC than by 6.5 Grendel? I am an admitted Grendel whore, but I see the ARC's only advantage in longer barrels at 400+ yards (and practically quite a bit further than that).

All I can come up with is that you want slightly less recoil (albeit you're putting less energy and mass on target), you're already committed to the 6ARC cartridge in chambered rifles/pistols and other equipment (doubtful), or you reload and have a ton of 6mm bullets on hand and not 6.5mms.

 

[ormandj]

Question then: what's the scenario where you'd be better served by 6ARC than by 6.5 Grendel? I am an admitted Grendel whore, but I see the ARC's only advantage in longer barrels at 400+ yards (and practically quite a bit further than that).

All I can come up with is that you want slightly less recoil (albeit you're putting less energy and mass on target), you're already committed to the 6ARC cartridge in chambered rifles/pistols and other equipment (doubtful), or you reload and have a ton of 6mm bullets on hand and not 6.5mms.

Your “doubtful” guess was correct. 18” ar15 6mm ARC as well as a lightweight mini action bolt rifle. My other stuff is 6.5mm Creedmoor and larger. I saw no reason to add another intermediate cartridge to the stable. I agree the Grendel would be very nice, but don’t feel like I’m losing much with the ARC at short range and it’s one less cartridge to reload for me.

 

[ormandj]

what is your intent behind the 6mm in a short barrel? Just because? Round compatibility? Velocity?

It’s the intermediate cartridge I choose for my AR15 rifles, so made no sense to do something else right now. The SBR (suppressed) will just be a handy hunting rifle for nothing larger than medium game out to 300 or so, for which this cartridge is fine. I didn’t want to tote around the 18” in the field when it wasn’t needed. Less weight and length when hiking is definitely appreciated!

 

[Milemarker28]

I ordered a 12.5 barrel from Mark over at PF in 6arc for my daughter to hunt with this year (she’s 10). Excited to see how 103’s and 90’s will perform.

 

[SBRSarge]

Old thread, but this is relevant…

12” Monster 6mm ARC Barrel- five shot group averages
ELD-X 2343 fps and 2344 fps w/ five shot group sizes of 1.34 and 2.52.

TAP LE load 2347 fps and 2335 fps w/ group size 2.5

All in all, not pin-point precision. Out of my 18” Odin barrel the -X was .88 and .53 moa. Still, for intermediate hunting distances a good short barrel will be fine. And keep in mind, my 12” has a 1-8x with course crosshair reticle while the 18” wears a 2.5-20x NightForce with a fine dot for an aiming point. I used max magnification on each scope to shoot the groups

I have to put a better scope on the 12” and shoot some more to see how it stacks up.

Also see this article…

6MM ARC: Long Range in a Small Package with the Lantac 6ARC

Read: 6MM ARC: Long Range in a Small Package with the Lantac 6ARC from Dave Merrill on January 6, 2021 for Recoil.

www.recoilweb.com

 

[ormandj]

The barrel length shouldn’t have an impact on accuracy. My 10.5” is five-shot sub-MOA even with factory eld-m, but it’s a Bartlein.

 

[Shiner Southpaw]

bookmarked.

 

[Nikon147]

A friend of mine has an SBR 6ARC (12 or 13 inch can't remember which). His intent was a lightweight hunting rifle that would be easy to carry and be effective to 3-400 yards.

He rings steel out past 800 on his local range. Not sure how effective it is at that range, but it rings the steel with factory Ammo.

If you're looking for super long range, the longer barrel is probably a better choice, but it's a pretty capable cartridge even in an SBR.

 

[ormandj]

A friend of mine has an SBR 6ARC (12 or 13 inch can't remember which). His intent was a lightweight hunting rifle that would be easy to carry and be effective to 3-400 yards.

He rings steel out past 800 on his local range. Not sure how effective it is at that range, but it rings the steel with factory Ammo.

If you're looking for super long range, the longer barrel is probably a better choice, but it's a pretty capable cartridge even in an SBR.

I ring steel at 700 with my 10.5” with good consistency. 1000 is a fair bit more challenging if there’s any wind, but I still hit steel more often than not at that range. Flip side, there is very little energy left at that point, so I’d definitely rule out long range hunting anything but varmints.

 

[6Arc904]

Guys I finished my 12.5" 6 ARC pistol this past weekend.
Shot it out to 850 yards.
1 shot hits out to 600 yards.
Took a little more effort to make hits out to 850 but it shot pretty good even at that distance.
Factory Hornady 105, 106 and 108 grain ammo
12.5" Ballistic Advantage barrel 1/7 twist
mid length gas system
17 Design lower/Anderson upper
PSA 6.5 G bcg

Can't wait til Saturday to shoot it again!!!