What usually happens when I come into extra money has happened again....a build is on my brain. I'm thinking I want to transfer my 9" .300 onto a pistol lower, and take the SBR lower put something different on it. I already have a 5.56 SBR, and love the .300 so far, so I was thinking maybe another .300 but with a longer barrel? Let the 9" be strictly for subsonic and the other be for super AND sub? I might also indulge myself in an ACOG (TA31C), so would a 14.5" make sense? Or keep it shorter, like 12"?

IMO, no 14.5" defeats the purpose of .300 blk, I shoot it suppressed 95% and the other 5% is mainly to show the difference in sound.

But if you love the round, go for it!

I’ve settled on 12” being my favorite rifle length.

14” is just too long and heavy for not much benefit. My 12” 556 guns get used while the other lengths collect dust. I’ll grab a IWI X95 before I grab a 14” out of the safe.

I’ve got a lower waiting on a stamp now that will be a 12” 300 blk. I’m a big fan of the round.

quote:

Originally posted by David W:
14.5" defeats the purpose of .300 blk,

What purpose is that?

(And please don't tell me nonsensical internet revisionist history of "designed for short barrels".)

Correct me if I’m mistaken but most of the velocity gain with 300BO occurs in the first 9 inches. Anything longer is a marginal increase according to the length vs velocity tests I’ve seen online. So why carry additional barrel weight/length when it doesn’t seem to provide any benefit?

quote:

Originally posted by kilyung:
Correct me if I’m mistaken but most of the velocity gain with 300BO occurs in the first 9 inches. Anything longer is a marginal increase according to the length vs velocity tests I’ve seen online. So why carry additional barrel weight/length when it doesn’t seem to provide any benefit?

Winner winner chicken dinner.

12"-14" barrels...6.5 grendel is what I would go for in those lengths. Hits like a hammer out of a small package while having an advantage over 5.56 and 300 blackout.

Your signature line compliments your post so well!!!

I was thinking 9” was the optimal length for .300 as well, but I wasn’t sure if that was for subsonic or all .300s.

Had no issues with 9” 300blk, supersonic or subsonic. Suppressed and unsuppressed. Worked fine. Built a 7.5” 300blk. No issues either. Waiting for my Sandman-K for this.

Built a 12” 6.5 Grendel to shoot suppressed. This is just plain awesome.

quote:

Originally posted by kilyung:
Correct me if I’m mistaken but most of the velocity gain with 300BO occurs in the first 9 inches. Anything longer is a marginal increase according to the length vs velocity tests I’ve seen online. So why carry additional barrel weight/length when it doesn’t seem to provide any benefit?

The same can be said for virtually all bottle neck rifle cartridges -- from 223 to the biggest magnums. And yet we have long barrels for many applications, such as long distance shooting. And applications where one wants to minimize bullet drop and wind deflection. And applications where one wants to maximize down range velocity to increase energy on target.

Originally posted on arfcom, there's a muzzle velocity chart for 110 grain Barnes 300blk for barrels of 4" to 24". Max MV is 2,563 at 24". MV is still increasing by 17 fps per inch for the last two inches. At 9" MV is 2116, which is 82.6% of maximum. At 16" MV is 2397 fps, which is 93.5% of maximum.

Maybe MV doesn't mean much to the user, so let's talk ft-lbs of energy on target. Max energy is 1604 ft-lbs at 24". At 9" energy is 1,094 ft-lbs at muzzle, or 68.2% of max. At 16" energy is 1,403 ft-lbs at muzzle, or 85.5% of max.

Longer barrels aren't just for paper punching competitors. Those who use their rifles for things like hunting, self defense, and tactical matches can all benefit from longer barrels.

quote:

Originally posted by AllenInWV:
Your signature line compliments your post so well!!!

I was thinking 9” was the optimal length for .300 as well, but I wasn’t sure if that was for subsonic or all .300s.

"Optimal" analysis requires a defined goal -- metrics so one can compare good to not-so-good.

From what I recall, virtually every factory load with bullets of 150 grains or less will be supersonic with a 9" barrel.

I was using "optimal" as the length of barrel needed for all the powder to burn. I had it in my head that a longer barrel might let an unsuppressed, supersonic 300 go farther.

quote:

Originally posted by StayFrosty

I don't have any data on 16" subsonic but there isn't enough case volume...

I'm going to call myself out on this. There's plenty of volume in the case, I wasn't thinking clearly when I said that. The rest of it

quote:

...to match the performance out of a 9" barrel..."

does apply for subsonic loads. If the projectile leaves the barrel at <1100 feet per second, having a longer barrel simply extends the effective range by the increase of its own length unless there's some magic rifling or barrel harmonic that the 16" offers that the shorter lengths don't. Perhaps it may be more accurate, I wouldn't know as I don't own/haven't tested any 16" 300 Blackouts.

My apologies for the misstatement. I wouldn't want my mistake to be taken as fact.

While I'm cluttering your thread I suppose I'll share my experience lobbing 300 Blackout downrange. I've consistently hit 4" steel plates at 200 meters using subsonic 220gr bullets. I would never use it for hunting or self defense but if it was supersonic ammo it'd be adequate for either out of an 8.2" barrel.

A quick search revealed the following data.

https://www.bonefroggunclub.co...-110gr-hornady-80891

I don't know what your definition of lobbing is but according to that a 110gr Hornady GMX TAP out of an 8.5" barrel has 519 ft lbs of energy at 300 yards, velocity of 1457 and drop of 29.1".

To put that in perspective, a 110/125gr 300 Blackout at 300 yards has almost identical velocity and energy of a .357 SIG at the muzzle. Anyone here wants to get shot by a .357 SIG at point blank range haha?

Don't quote me on this but the 16" was almost doubled the length of the 8.2" barrel when I built mine and I recall the effective range increased by 10, maybe 15% and anything past 300 yards I was going to use my 6.5 Grendel or .308 anyways.

I pulled this up quickly so take it for what it's worth.

quote:

The US Military rates the Max Effective Range of the M4 as 500 meters for a point target.

If the max effective range of the M4 with M855 at 2900 fps is 500 meters, that has 100 inches of drop, 41 inches drift, and 291 ft/lbs of energy:

16 inch 300 AAC BLACKOUT 125 grain at 2220 fps has:
100 inches drop at 440 meters
41 inches drift at 484 meters
291 ft/lbs of energy at 700 meters.

The military tends to go by hit probability rather than energy. If we use the drift and drop range as being correlated with hit probability, and discount the energy advantage of 300 BLK, we get 462 meters.

Using M4 military hit-probability standards, the max effective range of 300 AAC Blackout from a 16 inch barrel is 460 meters.

From a 9 inch barrel (2049 fps):
100 inches drop at 410 meters
41 inches drift at 470 meters
291 ft/lbs of energy at 625 meters.

440 meters for a 9 inch.

300 BLK from a 9 inch barrel has the same energy at the muzzle as a 14.5 inch barrel M4, and about 5% more energy at 440 meters.

http://forum.snipershide.com/t...-aac-blackout.74998/



ETA: Apparently I just edited over my original post, whoops! This information is probably more useful anyways.

BTW if you are having trouble deciding I challenge you to find a sexier 16" 300 Blackout than the 8-9" varieties. Here's mine.



Didn't really notice until I took this picture but the 9" Specwar 762 is longer than my barrel. Now I kinda want to find a shorter suppressor haha!

This message has been edited. Last edited by: StayFrosty,

quote:

Originally posted by AllenInWV:
I was using "optimal" as the length of barrel needed for all the powder to burn. I had it in my head that a longer barrel might let an unsuppressed, supersonic 300 go farther.

Length of barrel for complete powder burn is not a well defined metric. There are differences depending on powder burn rates and charge weights. Anecdotal analysis can be made from observing muzzle flash from various barrel lengths and loads. To do this correctly, video analysis and consistent lighting is necessary.

Distance of powder burn within the barrel isn't the final determination of MV. Pressure in the barrel doesn't magically drop to zero once the powder has burned. The real factor is how much pressure is built up in the barrel due to the burn, and how long will the pressure be greater than the friction of the bullet in the barrel. As long as the pressure is greater than friction, the bullet's speed will continue to increase.

"Go further" is rather nebulous. Do you intend to point the barrel at an up angle of 40 degrees then pace off the distance for when it hits ground again? The quantitative analysis is the bullet drop, bullet drift, and energy on target numbers for the resulting MV of a particular barrel length and load, for desired target distances.

When I assembled the 9" SBR, everything I read on the 'net was a 9" barrel was the sweet spot where additional length didn't yield any great rewards....at least when it came to subsonic ammo. It was my understanding that I could expect decent performance (another nebulous term.....I'm not well-versed in terminal ballistics, etc.) out to around 100-150m. I was thinking that perhaps a longer (12-14.5") barrel and lighter/supersonic (say in the 124?gr) ammo might allow that envelope to expand out to 200-300m.

And no, I'm not trying to lob them in. LOL

I have an 8" 300 bo suppressed, and I wouldn't want it much longer. I really don't see the point in carbine length for this caliber. There are such better options.

The 30-221 (aka 300-221, aka 300/221, aka 221/30, aka 300 Whisper, aka 300 Fireball, aka 300 Blackout ) was "designed" to not damage break action single shots, which the full length 30-223 and the loads it used were doing. Basically they made a "short" version which had a couple up sides for them, none related to barrel length.

Any design theories you read about in reference to efficiency in short barrels, AR's, SBR's, suppressors... was all thought up by someone well after the fact. Most of it like 30 years later.

quote:

Originally posted by cas:
Any design theories you read about in reference to efficiency in short barrels, AR's, SBR's, suppressors... was all thought up by someone well after the fact. Most of it like 30 years later.

A google search used to bring up a certain blackout "development history" on the first page. The history made it sound like a certain AAC person magically designed/invented the blackout virtually from scratch, with emphasis on making the cartridge work uber well in shorter barrels, suppressed, unsuppressed, subsonic, supersonic, and a large variety of bullet weights and types. All while using standard mags and bolts of an M4. And having more muzzle energy than a 223, especially for shorter barrels.

It seems that "development history" sales pitch has gone away, or maybe my google-fu needs a wax-on tuneup from Mr. Miyagi.

As you stated, the basics of the blackout cartridge had been worked out 3 decades prior to its SAAMI standardization. The blackout just differed by a few thousandths of an inch here and there, and it didn't have a trademark registered name,

Wildcatters have known for a long time (decades?) what happens to chamberings for a given powder capacity as bore diameters are increased or decreased. Any undergrad physics student who's taken fluid dynamics can describe the same thing in formulas. Any plumber with decent piping skills can also describe the same, although maybe in more of "work done" basis.

But it's way more fun to try to describe on the webz how the 300blk does things that no catridge before it ever could. Sales pitches can be very powerful.

The 'design" phase went like this.

Step 1. Take an empty .221 Fireball case.
Step 2. Run a .270" expander through it.
Step 3. Run a .308" expander through it.

Tah-da! Magic!



Step 7. Change IHMSA rules on how targets are set, remove need for heavy .30 bullets.
Step 8. Forget about 30/221 for 20+ years.
Step 9. Re-invent round as needed, add "your name here".

Back to the topic at hand... are you just punching paper, or might you shoot something with it?

If you're shooting 220's, 240's, they're unstable and will tumble. Usually, hopefully.

If you're shooting lighter bullets, you'll want them to expand. They need speed to expand. A longer barrel will make them go faster, even if just a little. Everything is relative. 100, 150 or more fps one way or the other doesn't mean much on the higher end. 2800fps, 2950 doesn't matter much. But when you're down in the very bottom of a bullet's operating envelope, down in the mid teens, that 100fps can mean the difference between a bullet expanding, working like it's supposed to, or ice picking a .308" hole in your target.