6ARC & 6.5 Grendel for 400-1000YD range shooting

Chief_Rick · Feb 14, 2025

While going through some storage containers last night I discovered a complete Bushmaster lower (~2010 production), a stripped flat-top upper, and a 12.5" free-float handguard. It was almost like Christmas!

The Bushmaster lower was on a 20" HBAR rifle, so I'm not sure what spring/buffer is in it.

I also have an unopened Magpul PRS stock and CMC Tactical trigger group (single stage, 3.5 lb flat trigger) on the shelf.

If you were to build either of the subject rounds for a strictly range rifle, what length barrel/gas system would you use?

No competitions or hunting planned. I have a 6.8 SPC and 308 for that, if I wanted.

I reload but have nothing for either round.

I'm leaning to the 6ARC but have no experience with either.

TonyTheTiger · Feb 14, 2025

No hunting, no carrying of the rifle and basically a bench gun for use out to 1k? 6ARC in as long as barrel as you can stand. Might as well max out the specs for the one very narrow use case you have in mind.

That said, this is probably thread #147 on exactly this subject.

LRRPF52 · Feb 14, 2025

Check your upper to see if it’s a Bushmaster flat top vs an actual M4 upper.

Bushmaster used uppers that didn’t have M4 feed ramps for the longest time. Can’t remember if they ever switched over.

The extended feed ramps aren’t really necessary for 5.56 SEMI auto use, but for larger high performance intermediate cartridges, I think extended feed ramps are necessary due to the larger case diameter and shoulders that will ride up the ramps.

I actually de-edge, blend, and polish my feed ramps, making them a bit wider and more accommodating for these larger cases.

For a range rifle shooting 400-1000yd, your 6mm bullets have higher BCs for weight, but 6mm ARC is kinda lacking in factory ammo options compared to Grendel, which has 134 different loads, last I counted.

Does your range have paper, steel, or both?

6mm ARC is very close to 6 BR, so it flies flat and stays supersonic for a long time.

With the higher BC 123gr loads in Grendel, spit from 20-26” barrels, you can be right there with a 6mm in the wind.

With the 6mm ARC shoulder being set back .030”, it makes kind of a pivot point that might make feeding a little more difficult.

I don’t remember problems with 6mm AR in feeding, which is just Grendel necked down to 6mm before the ARC came out.

I haven’t had feeding issues with Grendel over the years.

Both of them are very easy to self-spot.

Your Bushmaster lower likely has the rifle RET, rifle buffer, and rifle spring. Magpul PRS will be plug-and-play in that regard. Just follow the directions.

If you’re on a medium budget, I would go with a Criterion barrel from PF/Samurai, maybe even do a 26”. Get a Rexus Ultrabolt.

Pros to 6mm are the mid-range .5xx BC 103-109gr class of bullets.

Cons are ammo availability and slower speeds when comparing weight-to-weight. BCs generally overcome the speed though.

Pros to 6.5 Grendel are 134 factory loads with a ton of different target bullets, faster velocity per bullet weight class.

Cons are 123gr BCs range from .468 to .527 G1 in the cup and core, but Fort Scott TUI is .571 turned solid copper, factory ammo available as well.

Chief_Rick · 2025-02-22T21:10:18-0500

LRRPF52 said:
Check your upper to see if it’s a Bushmaster flat top vs an actual M4 upper.

Bushmaster used uppers that didn’t have M4 feed ramps for the longest time. Can’t remember if they ever switched over.

The extended feed ramps aren’t really necessary for 5.56 SEMI auto use, but for larger high performance intermediate cartridges, I think extended feed ramps are necessary due to the larger case diameter and shoulders that will ride up the ramps.

I actually de-edge, blend, and polish my feed ramps, making them a bit wider and more accommodating for these larger cases.

For a range rifle shooting 400-1000yd, your 6mm bullets have higher BCs for weight, but 6mm ARC is kinda lacking in factory ammo options compared to Grendel, which has 134 different loads, last I counted.

Does your range have paper, steel, or both?

6mm ARC is very close to 6 BR, so it flies flat and stays supersonic for a long time.

With the higher BC 123gr loads in Grendel, spit from 20-26” barrels, you can be right there with a 6mm in the wind.

With the 6mm ARC shoulder being set back .030”, it makes kind of a pivot point that might make feeding a little more difficult.

I don’t remember problems with 6mm AR in feeding, which is just Grendel necked down to 6mm before the ARC came out.

I haven’t had feeding issues with Grendel over the years.

Both of them are very easy to self-spot.

Your Bushmaster lower likely has the rifle RET, rifle buffer, and rifle spring. Magpul PRS will be plug-and-play in that regard. Just follow the directions.

If you’re on a medium budget, I would go with a Criterion barrel from PF/Samurai, maybe even do a 26”. Get a Rexus Ultrabolt.

Pros to 6mm are the mid-range .5xx BC 103-109gr class of bullets.

Cons are ammo availability and slower speeds when comparing weight-to-weight. BCs generally overcome the speed though.

Pros to 6.5 Grendel are 134 factory loads with a ton of different target bullets, faster velocity per bullet weight class.

Cons are 123gr BCs range from .468 to .527 G1 in the cup and core, but Fort Scott TUI is .571 turned solid copper, factory ammo available as well.

Thank you for the knowledge share!

My stripped upper is not a Bushmaster, but I don't recall where I got it from.

Our range has steel set up to 1000 yds but we can set up paper targets or additional steel targets if we want.

The gun would be run suppressed.

I am trying to figure out what barrel length would be optimum for both calibers. What is the point of diminishing/no return?

JFR · 2025-02-22T21:23:49-0500

Post #2 nailed it. 6 ARC. Keep the rifle buffer and run 20 inch or longer barrel with rifle +1 or more gas length.

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6ARC & 6.5 Grendel for 400-1000YD range shooting

Chief_Rick

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TonyTheTiger

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LRRPF52

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Chief_Rick

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JFR

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