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Posted: 10/3/2021 8:38:36 AM EDT
[#1]
.300 BLK was designed for an optimal barrel length of 16". It performs quite well in shorter barrels (far better, relatively, than 5.56.)
My SBR wears a suppressed 10.5" barrel. I like it, but probably should have gone with 8.5" instead. I may actually do a barrel swap some day, just to gain the 2". 
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Posted: 10/3/2021 8:58:09 AM EDT
[#2]
7.5 is just fine, but in my opinion any barrel length can work for most applications.
There is a 3.75, 7.5, 8.5, 10.5 , and 2 16 inch, in this pic  10.5 bolt straight pull AR.  ST300P MK 2 7.5 Deer Hunting gun 
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Posted: 10/3/2021 11:06:46 AM EDT
[#3]
300blk uses pistol powder that burns complete around 9 inches. A 300 black in 8 or 9 or 10 inch barrel gets much closer to it's full ballistics capacity than a 5.56/223. In a 16" barrel muzzle energy for 5.56 and 300blk are similar. But at 100 yards that 300 black carrys that energy downrange much better.
My deer gun, 9.5" barrel. 
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Posted: 10/3/2021 11:14:47 AM EDT
[#4]
300blk is probably the best centerfire rifle cartridge you can get for super short barrels. 7.5 inch is fine for 300blk…..I have a pinned Faxon 7.5 with a Law folder for a backpack 300.
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Posted: 10/3/2021 11:40:51 AM EDT
[#5]
300 does better in shorter barrels than 5.56, but part of it also has to do with shooting a fatter heavier bullet. A 308 diameter bullet has an 89.08% larger frontal area compared to a .224 diameter bullet. But, "damage" is also a function of velocity. 350 Legend (.355" diameter bullet) has a 33% larger frontal area than a .308 bullet.
Velocities for each are going to depend on bullet weight, but at 9", 5.56 runs somewhere in the 2300-2500 FPS velocity range. 300 Blk at 9", again depending on bullet, can have a much wider range as it is loaded with a broader range of bullet weights, up to very heavy subsonics. A 110 grain bullet in 300 and a 9" barrel runs somewhere in the 2100 FPS range, typically. a 55 grain bullet at 2500 FPS produces 763 ft-lbs of energy. A 110 grain bullet at 2100 FPS makes 1,077 ft-lbs of energy. A couple of guys on YouTube did a "350 Legend by the Inch" video. Starting with a 25" barrel, fire 5 rounds, average velocity, cut off an inch, rinse repeat. Shooting 147 grain bullets, at 9" 350 Legend averaged 2033 FPS. A 147 grain bullet at 2033 FPS makes 1349 ft-lbs of energy. Clearly, 350 Legend wins in a short barrel for bullet diameter and energy. Where it loses is in range. The 300 Blk will carry more energy at longer ranges due to the bullet's BC. So, for a short truck gun, the question is, do you want caliber and energy for short range engagements, or do want to be able to shoot out past a few hundred yards too? |
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Posted: 10/3/2021 11:56:11 AM EDT
[#6]
8" barrel 300Blk checking in. I think 7-9" is the perfect length.
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Posted: 10/3/2021 1:00:02 PM EDT
[#7]
Will be far better than your 5.56 in a 7.5
Another inch or two wouldn't hurt, but might not fit your purpose as well. |
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Posted: 10/3/2021 2:34:07 PM EDT
[#8]
.300 blk is a great choice for shorter barrel lengths and will be fine unsuppressed.
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Posted: 10/3/2021 9:41:52 PM EDT
[#9]
I agree with the others. 300 Blk Out performs great out of short barrels. When I built a PDW/truck gun, I used a 4.75" KAK Industries barrel and upper.
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Posted: 10/3/2021 9:45:26 PM EDT
[#10]
8" 300 BLK checking in.
Plenty of energy and I don't feel like it is overly long, even with a can on it. I think anywhere between 8-10" is probably best for your application. I'd probably not go much shorter. |
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Posted: 10/3/2021 11:17:24 PM EDT
[#11]
I’ve used 8-10.5” barrels. If for mostly sub use, I prefer the shorter barrels, if mostly supers, I like the 10.5
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Posted: 10/4/2021 8:14:28 PM EDT
[#12]
Quote HistoryQuoted: I’ve used 8-10.5” barrels. If for mostly sub use, I prefer the shorter barrels, if mostly supers, I like the 10.5 After having the debate that this thread lays out for a long time, I too came to the same conclusion. I spent some time playing around for awhile, then gave up on 300BLK subs, as they are basically 45ACP rounds with a crappy ballistic coefficient. I decided to stick with supers. I want a short gun, that can reach out to 200 yards, at the max, and be suppressed well. 10" plus or minus barrel, 300blk, supers, 125ish grain, is the sweet spot. |
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Posted: 10/5/2021 1:09:48 PM EDT
[#13]
Had a 16 inch barrel and a 10 inch barrel. Only lost around 250 FPS dropping down to a 10 inch. Sold the 16 inch upper. BUT, if your not vested in 300 BO, suggest you look at the 300 Ham'R for a little extra umph. More expensive though. You can build a 300 BO fairly cheap these days.
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Posted: 10/6/2021 9:35:41 AM EDT
[#14]
Quote HistoryQuoted: After having the debate that this thread lays out for a long time, I too came to the same conclusion. I spent some time playing around for awhile, then gave up on 300BLK subs, as they are basically 45ACP rounds with a crappy ballistic coefficient. I decided to stick with supers. I want a short gun, that can reach out to 200 yards, at the max, and be suppressed well. 10" plus or minus barrel, 300blk, supers, 125ish grain, is the sweet spot. Actually 300blk subs (220 grain SMK for example) has a great ballistic coefficient. If it starts out at 1050fps it will still have 1030fps at 100 yards. Then there is the sectional density! I shoot both a each has their place. As a note, I keep a 8.5 inch 300 in the car loaded with 110 grain tac-tx. |
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Posted: 10/6/2021 9:45:52 AM EDT
[#15]
Quote HistoryQuoted: Actually 300blk subs (220 grain SMK for example) has a great ballistic coefficient. If it starts out at 1050fps it will still have 1030fps at 100 yards. Then there is the sectional density! I shoot both a each has their place. As a note, I keep a 8.5 inch 300 in the car loaded with 110 grain tac-tx. I'd assume that was just a typo. I think he meant to say what you said, but maybe not. Good call on Tac-Tx. Best bullet there is for 300 BLK. 
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Posted: 10/6/2021 9:51:53 AM EDT
[#16]
 The Best 300BLK Barrel Length & All Things 300 Blackout Answered Watch this. Since you aren’t suppressing it, a 16” barrel does yield higher velocities than 10.5 and is about the same length as a 10.5 with 7” can. |
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Posted: 10/6/2021 9:35:27 PM EDT
[#17]
For an unsuppressed truck gun a 300blk is hard to beat and what I use.
The main reason is the 300 will have far less blast than a 5.56, and if I need to use my truck gun there is a strong chance it will be at night in low light conditions. |
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Posted: 10/8/2021 8:11:13 AM EDT
[#18]
Kind of fitting to see this thread because I had the same questions--I answered them on my own from reading here and last weekend, I swapped my 7.5" 5.56 barrel for a 7" 300 BO barrel.
I used my 10" AR500 gong to sight in at 50 yds. The 5.56 with M193 Federal barely caused it to move and the 300 with 110g Hornady black box swings it almost 4 inches. Totally unscientific and completely opinionated, but to me it sure looks like there is considerably more energy deposited on target. My little AR pistol is my truck gun. I'm using a Sylvan Arms (knockoff LAW) folder. It lives in a deceptively small backpack with a couple of 20rd Pmags, and so far, it's absolutely perfect for my needs. Fast to deploy, hits harder than I expected it to, and when firing it's nowhere near as concussive as the 5.56 was. I can't wait for my next pig hunt to see what it can really do. |
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Posted: 10/8/2021 11:10:45 AM EDT
[#19]
300blk uses pistol powder that burns complete around 9 inches. If that were really the case, muzzle velocity wouldn't be any greater from a 16" barrel than a 9" barrel, and we know that is not the case. |
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Posted: 10/8/2021 11:14:38 AM EDT
[#20]
Quote HistoryQuoted: If that were really the case, muzzle velocity wouldn't be any greater from a 16" barrel than a 9" barrel, and we know that is not the case. If you look at Quickload, there are plenty of 300 BLK powders that completely burn in 9". Not all of them...but some, absolutely. Pressure is still (for subs) over 12kpsi or so, even completely burned, and that pressure will continue to accelerate the projectile, regardless of whether powder is actively burning or not. Powder burning generates additional pressure. Doesn't mean the pressure that is already there cannot further accelerate the projectile. |
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Posted: 10/8/2021 12:32:35 PM EDT
[#21]
Quote HistoryQuoted: Kind of fitting to see this thread because I had the same questions--I answered them on my own from reading here and last weekend, I swapped my 7.5" 5.56 barrel for a 7" 300 BO barrel. I used my 10" AR500 gong to sight in at 50 yds. The 5.56 with M193 Federal barely caused it to move and the 300 with 110g Hornady black box swings it almost 4 inches. Totally unscientific and completely opinionated, but to me it sure looks like there is considerably more energy deposited on target. My little AR pistol is my truck gun. I'm using a Sylvan Arms (knockoff LAW) folder. It lives in a deceptively small backpack with a couple of 20rd Pmags, and so far, it's absolutely perfect for my needs. Fast to deploy, hits harder than I expected it to, and when firing it's nowhere near as concussive as the 5.56 was. I can't wait for my next pig hunt to see what it can really do. I bet that is around a 40% increase in energy on target, out of slightly shorter barrel, with less blast to boot. I have a 7.5" 300blk on Law Folder for a backpack weapon. 20 round mag in the weapon with a 30 round mag in the bag. It is a pretty small and powerful package. |
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Posted: 10/8/2021 8:56:03 PM EDT
[#22]
Quote HistoryQuoted: If you look at Quickload, there are plenty of 300 BLK powders that completely burn in 9". Not all of them...but some, absolutely. Pressure is still (for subs) over 12kpsi or so, even completely burned, and that pressure will continue to accelerate the projectile, regardless of whether powder is actively burning or not. Powder burning generates additional pressure. Doesn't mean the pressure that is already there cannot further accelerate the projectile. I understand completely, and you are not wrong. Then again there are those who argue that all is burned when the bullet leaves the case and those who will argue that 100% burn is never achieved, but that isn't my point. Not everybody knows how the whole process of combustion works. They just want a gun that will do what they need. Just one example that comes to mind, pretend you are new to the AR and looking for information on 300 BLK and you read "all the powder is burned in a 9" barrel" comments. Maybe you need a little more velocity, to kill deer or whatever, you may not consider a longer barrel because someone at ARFCOM said "all the powder is burned in a 9" barrel" See what I mean? It is kinda misleading at best. The only cartridges I have ever found to shoot slower in a carbine than a pistol are 22 Short and 45 ACP. There may be others but point is for the most part, more barrel = more speed, especially when working in the 8 - 16 inch range. Whether we agree to disagree, just wanted to clarify a little. IGY6 |
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Posted: 10/8/2021 9:00:22 PM EDT
[#23]
Quote HistoryQuoted: I understand completely, and you are not wrong. Then again there are those who argue that all is burned when the bullet leaves the case and those who will argue that 100% burn is never achieved, but that isn't my point. Not everybody knows how the whole process of combustion works. They just want a gun that will do what they need. Just one example that comes to mind, pretend you are new to the AR and looking for information on 300 BLK and you read "all the powder is burned in a 9" barrel" comments. Maybe you need a little more velocity, to kill deer or whatever, you may not consider a longer barrel because someone at ARFCOM said "all the powder is burned in a 9" barrel" See what I mean? It is kinda misleading at best. The only cartridges I have ever found to shoot slower in a carbine than a pistol are 22 Short and 45 ACP. There may be others but point is for the most part, more barrel = more speed, especially when working in the 8 - 16 inch range. Whether we agree to disagree, just wanted to clarify a little. IGY6 Not really sure what point you're trying to make. You said you agree. Then you said we will agree to disagree? 
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Posted: 10/8/2021 9:07:47 PM EDT
[#24]
Quote HistoryQuoted: Not really sure what point you're trying to make. You said you agree. Then you said we will agree to disagree? Seems pretty clear to me, but CHEERS! |
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Posted: 10/9/2021 3:44:05 PM EDT
[#25]
I might suggest you all read or re-read Hatcher's Notebook.
It provides insight as to why the Thompson M1 had the barrel length it had. True that 300 blk/Whisper was not around then nor most of the more common canister grade powders used today, but the science is still valid. |
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Posted: 10/10/2021 1:52:54 AM EDT
[#26]
Quote HistoryQuoted: I might suggest you all read or re-read Hatcher's Notebook. It provides insight as to why the Thompson M1 had the barrel length it had. True that 300 blk/Whisper was not around then nor most of the more common canister grade powders used today, but the science is still valid. I'm interested in your point, but I don't want to go have to track down and read a book to know what it is. |
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Posted: 10/13/2021 8:50:31 PM EDT
[#27]
When Hatcher was put in charge of all U.S. Arsenals he did a lot of field studies. Also at the start of WW-1, he was lamenting the fact that almost all of the army's machine guns (at the time that meant belt-fed) were still along our southern border after the completion of the Spanish-American war. In his opinion the war (WW1) caught us with our pants down.
One test case was shooting a 1903 Springfield 30-06 with service grade ball ammo straight up. To time the total time of flight and estimate total altitude reached and how the projectiles would land. One question was would the bullets tumble on the way down. Another was using 30-06 shot at 100lb kegs of smokeless power to see it it would detonate. But back on point - IIRC (last time I think I read it was 2006) He took an 03 action and fitted a 20-22" 45 barrel chambered for 45acp Shots were chronographed then the barrel cut down - repeat until down to like 1.5" Rounds fired from the long length were considerably slower than those fired from shorter lengths. I seem to recall the ideal (max velocity) length for the ammo tested ended up being 10.1". The 45 acp stocked by the army was the standard issue 230gr ball ammo issued for use in the 1911 pistol. Now consider that many loads for the 300 B.O. use pistol or shotgun powders, also the bullet weights are often close to 45acp. Granted the bore cylindrical volume is greater for 45acp but for less than full loads I don't think its a major factor. FYI https://en.wikipedia.org/wiki/Julian_Hatcher |
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Posted: 10/14/2021 7:31:19 PM EDT
[#28]
Actually, 45 ACP, 9mm, and 40 S&W reach max velocity somewhere in the 15" to 18" barrel length range. Of course, this does vary slightly from load to load. A key thing to consider, however, is the velocity gain from around 10" on is minimal.
A "perfect" length barrel is not one that stops exactly when the powder burns out completely. It would be the point where pressure no longer accelerates the bullet, which will be some distance past the point of complete combustion. |
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Posted: 10/14/2021 8:03:11 PM EDT
[#29]
Actually, 45 ACP, 9mm, and 40 S&W reach max velocity somewhere in the 15" to 18" barrel length range. I can't say about the others, but I had a 16" Marlin Camp Carbine in 45 ACP and it shot slower than my 5" 1911. Was shooting Titegroup so fast powder and all. There might be something to do with blowback vs locked as well, but the chrony was consistent |
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Posted: 10/15/2021 1:38:16 AM EDT
[#30]
Quoted: I can't say about the others, but I had a 16" Marlin Camp Carbine in 45 ACP and it shot slower than my 5" 1911. Was shooting Titegroup so fast powder and all. There might be something to do with blowback vs locked as well, but the chrony was consistent That surprises me. How much slower was it. |
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Posted: 10/15/2021 11:18:58 AM EDT
[#31]
Quote HistoryQuoted: That surprises me. How much slower was it. Around 50 fps if memory serves. It has been a while and I sold the MCC shortly thereafter so I cannot duplicate the experiment. |
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Posted: 10/16/2021 5:14:40 PM EDT
[#32]
http://www.ballisticsbytheinch.com/45auto2.html
We're talking on average and commercial vs handloads, so there is some variance. |
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