What say you OB?

Purpose would be "because I can, for now".

Depends on caliber but if the purpose is "because I can" then I personally would go with the 6". Some 6" is shorter than others.

If using 5.56 I would not go any shorter than 10.5".

Really depends on the caliber.

5.56

Horse with No Name said:

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5.56

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As stated by another poster, I wouldn't go below 10.5" for a 5.56. That said, plenty of people do go shorter.

In 5.56 10.5 or 11.5 are "optimal". Shorter and you lose too much velocity. Longer and you lose the practicality of a pistol - might as well go 14.5 + pin/weld or 16" instead.

DatTallArchitect said:

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Horse with No Name said:

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5.56

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As stated by another poster, I wouldn't go below 10.5" for a 5.56. That said, plenty of people do go shorter.

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Is there a rule of thumb regarding caliber/length?

Bigger projectile for shorter barrel or some such?

I've heard most spec ops are moving to 11.5 for 5.56.

I've got a 10.5

Horse with No Name said:

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DatTallArchitect said:

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Horse with No Name said:

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5.56

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As stated by another poster, I wouldn't go below 10.5" for a 5.56. That said, plenty of people do go shorter.

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Is there a rule of thumb regarding caliber/length?

Bigger projectile for shorter barrel or some such?

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There isn't a general rule. Some cartridges are designed to use faster burning powders than others. .300 blackout and 6.8 SPC were both designed for a shorter barrel length. There's also a factor of speed and rifling. There are others that can speak better to this, but (from my understanding) you want the bullet to get a full turn before leaving the barrel in order for it to stabilize.

Really depends on how far you are going to shoot. 10.3 or 11.5 are the recommended lengths. 11.5 would be 125FPS + faster

GarandThumb gives a pretty detailed description

9" is truly optimal for the 300blk. To get max velocity out of the 6.8 you still have to be 16-18", but it still performs well out of shorter. I think most of the common rounds outside of the blackout you're balancing between performance and convenience under 16". Lots of 12.5" 6.8's and Grendels.

When I was researching the 6.8 for my build, I remember seeing that it was designed for a shorter barrel than 16". It was developed for the Navy Seals, and they would definitely want a shorter barrel.

The 6.5 Grendel was designed for a 22" barrel, but, like you said, there are a lot of 12.5" SBRs and pistols out there.

I think we're both right here to an extent. Again, I'm strictly speaking in terms of max velocity, which I'm pretty certain is achieved at 16" or 18" (the google give me mixed results between 16-18"). It does however perform VERY well with minimal lost velocity out of a shorter barrel.

I'll yield to any true experts on the topic, just my understanding. I've got a 16" 6.8 and would love to add a 12.5" to the collection at some point.

DatTallArchitect said:

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The 6.5 Grendel was designed for a 22" barrel, but, like you said, there are a lot of 12.5" SBRs and pistols out there.

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My 12" Grendel is less than 300 fps slower MV than my 20" shooting factory 123gr loads.

Short Grendels are fine.

Didn't say they weren't. Only stated they were designed for a longer barrel.

Yesterday said:

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Depends on caliber but if the purpose is "because I can" then I personally would go with the 6". Some 6" is shorter than others.

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Tell me about it...

DatTallArchitect said:

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Didn't say they weren't. Only stated they were designed for a longer barrel.

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Just sharing some data. When the general rule of thumb is 50 fps less per inch, losing only 284 fps in 8" is remarkably good for a round "not designed" for short barrels.

Had a 10 inch.223 barrel for my T/C Contender. Loudest gun I ever shot. Blew flame 2 feet. Hated it. Sold it several years ago and bought a Super 14 in 7mm T/CU.

After only ever shooting rifle length 556, I was shocked out how much louder a 10.5 inch barrel was. Not sure if it is actually a lot louder or it's just because you're a few inches closer to the end of the barrel, but either way it's maybe something to think about.