Yet another AR barrel length thread...

[Bart B.]

What's the rifling pitch angle incremental change to 22 caliber bullets across 8, 9, 10, 11 and 12 inch twists? About 1.6 degrees from 8 to 12; insignificant angle change per inch is about .4 degree.

Note that the more resistance to bullet entrance into and travel in the barrel, pressure goes up more.

 

[Mauser lover]

I don't know, but I'm not really worried about that. I'm just trying to decide what length.

 

[IndianaBoy]

It depends entirely on what you want to do with it.

Walking varminter? 16

Shot solely from a bench? 20


I second the poster who suggested an HBAR contour. There is no reason to go thicker than .750 from the gas block forward. I had a BCM SAM-R stainless 20" barrel and it was a tack driver. Heavy though.

 

[bfoosh006]

Here is my data for 100+ OEM rounds from a 16" and 20" AR.

http://www.ar15.com/forums/t_3_16/6...--16-Middie-and-20-Rifle------223---5-56.html

 

[ponchh]

Yeah, I know, this has probably already been covered at least a zillion times, but I couldn't find what I was looking for.

So, I'll start with my question. What barrel length should I get?

Okay, now the details. I am pretty much dead set on a bull barrel (because I want to, that's why!). I understand that all other things being equal a shorter barrel is more accurate, because it is stiffer. Also, I am looking at a .223/5.56 (not quite sure which yet, I'll be handloading, so I'm not worried about a 5.56 being inaccurate).

So, my specific quandary, I can't find any good data about what velocity difference is between a 16 inch and a 20 inch. This is what I am really interested in. If you have data for an 18 or a 24 inch, that is welcome too, but I'm really wanting the 16 and 20 inch data.

I want this info because if it isn't a big velocity difference, I'll get a 16 inch because it is lighter (and cheaper often too!)

Thanks all!

There is a article in Guns and Ammo 2/1/16
Patrick Sweeney started with a 26" stainless Oak Armament 1:8 barrel. Tested 19 loads at 20 lengths by cutting it off one inch after every string. Its real good read. Mostly with the heaviest bullets velocity was lost with the longer barrel.

Middle weight bullet example 62gr TSX
16" 2,960
17" 2,989
18" 3,027
19" 3,058
20" 3,082

26" 3,126
7" 2,220
Give me a bullet weight and I will look it up.

 

[Mauser lover]

What the heaviest bullet you have data for?

That is what I am most interested in, but I will probably also do some lighter weight shooting.

 

[TN Outlaw]

So with 62gr there was 122fps avg diff in the 16" and 20" bbl when all else is the same. So around 100fps with everything identical but bbl length. I havent looked but prob less when heavier bullets are used but idk that for sure.

Then you factor in chamber specs from one gun vs another, bbl steel, type of cut rifling (assuming same rate of twist) SD of your ammo, and you could end up with the 20" bbl being 200fps faster or 50fps slower. Who knows when real world velocity is applied?

Either way, its more than likely not enough velocity diff to base a decision on imho, other than something fun to talk about. Id put alot more stock into which length fit what you would be doing with the rifle and which one fit you best but thats just me. I wish you luck getting a shooter whichever you choose.

 

[ponchh]

Heaviest bullet 77gr SMK

26". 2,730
25". 2,670
24". 2,684
23". 2,675
22". 2,699
21". 2,671
20". 2,633
19". 2,615
18". 2,601
17". 2,583
16". 2,567

10". 2,117

22 thru 26 inch those numbers are not a miss print.


77gr black hills
16". 2,669
17". 2,713
18". 2,750
19". 2,798
20". 2,836

26". 2,817
10". 2,328

 

[Gtscotty]

The rifleshooter link BartB posted is an interesting read as they have taken barrels in several chamberings and progressively cut them down inch by inch recording the velocities along the way. This approach gives you a better answer than comparing different barrels of different lengths that might have variances leading to faster or slower velocities peculiar to the individual barrel.

I developed a cheap load specifically for plinking out to 400 yds with my iron sighted 20" AR comprised of a 55gr Hornady SP over 25.8gr Tac in an LC 15 case. Chronoed in my 20" AR this load averages 3,022 fps, in my 16", 2,953 fps, both FN barreled PSA rifles. I would have expected more than 69 fps difference, and probably would have seen more if I were progressively cutting down one barrel, but with two different barrels there are more factors at play with the numbers than just length.

 

[MistWolf]

 

[hdwhit]

Bart B wrote:
So, you think my post 16 link to the Rifle Shooters tests showing a test as good as possible with the only variable is barrel length for several types of ammo is, in your opinion, not reliable nor meaningful at all?

What I think is that the test you linked to proved my point with precision.

Mauser lover wrote:
So, my specific quandary, I can't find any good data about what velocity difference is between a 16 inch and a 20 inch.

My response was, "Nor are you likely to find any that is realiable [sic] or meaningful."

Four types of ammunition were tested and the decrease (or paradoxically, increase) from shortening the barrel varied from a decrease of 84 fps per inch to an increase of 46 fps per inch (with overall drops from 20 to 16.5 inch varying between 72 and 119 fps). There was no discernible pattern to velocity changes with changes in the barrel length. Velocities did trend down as barrel length shortened, but the decrease was neither uniformly proportional to the length of the remaining barrel, the amount of barrel removed nor was it proportional to the velocity of the cartridge.

If Mauser lover were going to purchase and shoot Black Hills 68-grain Heavy Match ammunition, the Rifle Shooter test would tell him to expect a loss of about 72 fps between a 20 inch barrel and a 16.5 inch barrel. But what it he is going to purchase and shoot Wolf brand? Or PMC? Or Aguilla? Would the loss likewise be 72 fps? Or the 119 fps seen with the Federal XM193? Or some entirely different figure?

So, No, apart from showing that shortening a rifle's barrel by an inch either lowers velocity by as much as 84 fps or raises it as much 46 fps (although shortening a barrel from 20 to 16.5 tended, on average, to reduce velocity by something on the order of 20 to 34 fps per inch), I don't think the test or its results were responsive to Mauser lover's question.

 

[Corn-Picker]

I did a curve fit on some of the data in this thread, another way to represent the average data is to say that each inch of barrel length gives you approximately 1.2% more muzzle energy, e.g. a bullet from a 26 inch barrel yields about 12% more energy than from a 16 inch barrel (this relationship breaks down at shorter barrel lengths). Could be the difference between ~1200 ft-lbs and ~1350 ft-lbs at the muzzle, which may or may not make a difference depending on the application.

 

[Mauser lover]

Hmmm... 12% But, yeah, the graph is not a very smooth curve, and the margin for error in these pretty limited test samples is pretty big.

I can turn a 12% bigger prairie dog into pink mist with the longer barrel...

I'll probably end up going with a 20 inch or so... unless the right deal comes along in a 16, 18 or 22 inch upper. Or a really, really good deal comes along with a 24 or longer.

 

[CoalTrain49]

You're set on a bull barrel, but you're worried about a few ounces with 16 vs. 20" tube?

IMO, 20" or 22" is a good balance for a useful tool that achieves good velocity with all loads.

That's why 20" is a very popular length.

I'm having a hard time understanding the premise of the velocity considerations with barrel length. If the OP reloads you can go up and down the velocity scale by changing the charge and bullet weight. Velocity would be my last consideration when deciding on a barrel profile, maybe wouldn't even be a consideration unless it was something unusual like a SBR.

I have a rifle with a 20 bull barrel. No joy anywhere except shooting supported from a bench or prone. That's all it does.