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Posted: 11/19/2003 6:29:43 AM EDT
Here's my question.

Anybody know what the shortest length barrel will allow complete powder burn for factory ammo. Yeah, I know there will be some variation according to ammo.

A while back when I was reading about the Steyr scout rifle concept, I ran across something that stated that the reason that a 19 inch barrel was chosen was that it was the shortest (translated lightest) length barrel that would allow complete powder burn of a .308. This may or may not be true, but it got me to wondering about .223.

Since the velocity of 11.5, 14.5 and 16 inch barrels is lower than 20 inch the short barrels and carbine lengths couldn't represent complete burn. I am not aware that 24 inch barrels offer any increase in velocity and if the powder is all spent way before the muzzle, it might actually slow down the projectile.

So I'm guessing that the minimum complete powder burn length is somewhere between 16 and 20 inches.

Any ideas? Feel free to call BS on the whole post. Not sure I have any idea what I'm talking about.

Link Posted: 11/19/2003 12:20:06 PM EDT
No takers?
Link Posted: 11/19/2003 12:25:42 PM EDT
Complete powder burn occurs in like 7 inches or something like that. This has nothing to do with velocity. You are talking about the point where the hot gasses pressure is outweighed by barrel friction and the bullet begins to slow. 24" barrels produce velocity higher than 20" I think in .223 the number is like 28 or 30" for the bullet to stop accelerating. After the powder burns 100% there is still pressure in the barrel. The larger the space between the bolt face and the projectile the lower the pressure will be. The question you are asking is when does that pressure get so low that barrel fiction is greater than the pressure pushing the projectile. The numbers I just used are off the top of my head and are probably wrong. I have not discussed this in a long time but it should give you an idea that powder burn has nothing to do with barrel length selection. Just use velocity as the criteria. The difference in 14.5-16 is a bit more than 16-18 and there is little improvement to 20. 16-18 probably gives the most fps per inch but I have not done the calculations. If a 14.5" barrel will allow projectile fragmentation at 150 yards in soft tissue, thats good enough for me and I prefer the shortest barrel possible. Your needs may be different.
Link Posted: 11/19/2003 12:43:00 PM EDT
"If a 14.5" barrel will allow projectile fragmentation at 150 yards in soft tissue, thats good enough for me" DevL You're off by about a third. 14.5" reliably fragments to about 100 yards.
Link Posted: 11/19/2003 12:44:19 PM EDT
I believe that the longer AR barrels do, in fact, provide more velocity, but with diminishing returns. Smokeless powder only burns efficently when under high pressure. As the bullet procedes farther down the barrel the pressure falls off and the powder burn rate slows down. If you look at a pressure curve vs. time, it is heavily slanted toward the time period right after ignition. It peaks quickly and then falls off slowly. The longer you make the barrel, the more of the pressure "under the curve" you can make use of, but you get less and less for your effort as you go. I'm not an internal ballistics expert but my first guess would be that the powder charge is mostly consumed by the peak of the pressure curve. The pressure curve falls because the volume is expanding (as the bullet travels down the barrel) and there is no more powder buring to maintain pressure. This probably happens below the legal barrel length limit. I think that the choice of barrel length has more to do with trade-offs between velocity and portability than whether or not all the powder is used.
Link Posted: 11/19/2003 1:55:18 PM EDT
Originally Posted By Beowulf: "If a 14.5" barrel will allow projectile fragmentation at 150 yards in soft tissue, thats good enough for me" DevL You're off by about a third. 14.5" reliably fragments to about 100 yards.
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No I am not off, MK 262 ammo fragments to 150 yards from a 14.5" barrel. NATO pressure 75 grain Hornady OTM also fragments at 150 yards.
Link Posted: 11/19/2003 5:42:24 PM EDT
-------------------------------------------------------------------------------- Originally Posted By Beowulf: "If a 14.5" barrel will allow projectile fragmentation at 150 yards in soft tissue, thats good enough for me" DevL You're off by about a third. 14.5" reliably fragments to about 100 yards. -------------------------------------------------------------------------------- No I am not off, MK 262 ammo fragments to 150 yards from a 14.5" barrel. NATO pressure 75 grain Hornady OTM also fragments at 150 yards.
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OK I get it, trick bullets. I stand corrected.
Link Posted: 11/22/2003 10:24:46 AM EDT
Appreciate the commentary. So if I decide to build an upper for "precision work" is a 20inch barrell adequate for shots out to 300 yards using 75gr bullets. What twist? Any suggestion on such a setup?
Link Posted: 11/22/2003 2:04:10 PM EDT
You need to look at the energy released not the powder burn. A firearm has to waste alot of energy to get the projectile out of the barrel. If there was only enough energy to get the projectile out of the barrel it would fall straight on the ground. The perfect projectile should leave the barrel micro seconds before the energy peaks. Once the energy passes the acme point the bullet will start to slow. The eye cant see it though.
Link Posted: 11/22/2003 3:09:31 PM EDT
It is probably around the 10” mark for .223. I base this statement on viewing people firing different lengths of AR-15 barrels. The only barrels where I have seen incomplete burn from are the 7” pistol barrels. Carbine barrels, while producing more flash than 20”+ barrels, do not show incomplete burn. It is pretty obvious when there is incomplete burn; there is a fire ball that is BRIGHT in the day time and very inconsistent from shot to shot. I have only observed this on rifle-caliber pistols and sawed-off shotguns. As for how long increased barrel length increases velocity: It is still increasing with length at 26”, but I don’t know of the exact cut-off point. According to Dr. Fackler: from a 20” barrel, M193 will DRAMATICLY fragment out to 100m, and SPLIT into two pieces at the cannelure out to 200m. A 14.5” barrel will reduce both of these ranges by 100m; the bullet will SPLIT out to 100m and probably only fragment dramatically at CQB ranges. Of course, other bullet types/weights will perform differently.
Link Posted: 11/23/2003 7:56:29 PM EDT
If you want a SUB MOA shooter you can select almost any barrel length. I have a 14.5" barrel that can do sub MOA at 100 yards and have shot an 18" barreled SPR that can shoot 77 grain ammo sub MOA at 300 yards. The thing you are considering with barrel length is not accuracy but velocity. A longer barrel is always faster. How fast do you want the bullet to go? I dont see 300 yard frag range even for a 20" barrel but you would be getting close. You would certainly see over 200 with an 18" barrel though. Id suggest a twist rate of 1/8 to 1/7 if you plan to use heavy match bullets.
Link Posted: 11/24/2003 9:39:49 AM EDT
[Last Edit: 11/24/2003 10:32:01 AM EDT by blikbok]
I've got the numbers and a pretty graph, but I need someone to host the pic. I used Bushmaster's chronograph data for 55gr and 62gr FMJ from 10, 11.5, 14.5, 16, 20, 24, and 26" barrels. I think there is a typo in their data, because there is an odd jump with the 24" barrel. The graph definately flattens out between 20 and 16". I think the Armalite guys knew something when they made it 20" for 55gr ammo. With the 77gr, I think the case can be made for 16-18" being optimum. If I recall correctly, the point where a .22LR is no longer accelerating with a longer barrel, but slowing from drag, is around 14". So it has to be longer for the AR.
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