BARREL LENGTH AND THE PRECISION RIFLE

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10 years 8 months ago #36440 by OleCowboy
BARREL LENGTH AND THE PRECISION RIFLE
Why shorter barrels may often be better
by
Eugene Nielsen

There’s a growing trend to shorter barrels on tactical precision rifles. In years past, a 24- to 26-inch barrel was practically a given. Accepted wisdom was that it was necessary to sacrifice a little maneuverability to gain a more complete powder burn and significantly reduced flash signature. Today, it’s not uncommon to rifles with significlly shorter barrrels.

Attitudes are are changing. The desire for more manueverable rifles for the urban setting has led a growing number of manufacturer's to come out with shorter-barreled precision rifles. This brings up an obvious question -- how short is too short? What sacrifices, if any, are made by going to a shorter barrel?

To answer these questions, we must first start by taking a look at the subject of internal ballistics. Internal ballistics is a very complex subject. There are many factors which affect the internal performance of a given cartridge and bullet. Factors affecting internal performance include the powder chamber capacity; load density; amount and burning characteristics of the propellant powder; temperature of the propellant prior to ignition; uniformity and speed of ignition; diameter, weight and bearing length of the bullet; and the length of the barrel and its interior dimensions.

Longer barrels give the powder more time to work on propelling the bullet. For this reason longer barrels generally provide higher velocities, everything else being equal. However, the gas pressure behind the bullet diminishes as the bullet moves down the bore. Given a long enough barrel, there will eventually be a point in which the bore friction and air pressure in front of the bullet will equal the gas pressure behind it. At this point, the velocity of the bullet will start to decrease.




Shorter barrels are capable of surprising long-range accurancy. San Fernando (California) PD Special Response Team long rifle marksman Chris Colelli, shown here, fired a three-shot group at 700 yards that measured just under two inches center-to-center. Colleli also fired a sub-1/4- inch group at 200 yards. The target is now framed and hanging in his lieutenant's office.
There isn't any clear-cut answer as to how much velocity will be lost per inch of barrel length reduction. The amount of loss is closely tied to the expansion ratio. As previously noted, the type and amount of powder, as well as the weight and bearing length of the bullet, also play a major part. Rifles with high expansion ratios (smaller calibers) tend to lose less velocity than rifles with low expansion ratios (larger calibers).

Tactical Operations about in the April 2000 issue of S.W.A.T., typifies the trend to rifles with shorter barrels. Tac Ops considers a barrel of length of 18 to 20 inches to be optimal for the urban environment, with 18 inches the preferred length.

During the development of the Tango 51, Tac Ops took a standard 26-inch barrel and cut it down to 18 inches in one-inch increments. Between 10 to 20 rounds were fired at each invrement. They found that a 20-inch barrel provides for a complete propellant burn and no velocity loss when using Federal Match 168-grain BTHP, a cartridge that has become something of a law enforcement standard. Going to an 18-inch barrel only resulted in a loss of 32 feet per second (fps).

Shorter barreled rifles are more versatile, being equally suitable for both urban and rural operations. According to Tac Ops, there isn't any need to go to the 26-inch barrel unless you want to go to a heavier bullet or push the round to higher velocity using more powder or use a slower burning powder. The Los Angeles County Sheriff's Department's Special Enforcement Bureau (SEB) performed tests similar to those conducted by Tac Ops and came to similar conclusions.

Read the rest: www.tacticaloperations.com/SWATbarrel/

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10 years 8 months ago #36441 by Libertarian623
It really gives one some things to think about when specing out a new rifle. I think overwhelmingly that the urban environment of the last two wars has pushed this trend forward. Also the militarization of law enforcement is a factor. It is amazing to watch someone with a 18" barrel take it out to 800 meters. You wouldn't believe how bad I want to cut down mine (24") but right now it is my goto competition gun and I can't sacrifice the speed. But when I get a 260 or 300 rsaum AR-10, out will come the hack saw so to speak. And I may take it down to 18 or even 16 with a can. That LAR=8 is destined to end up as a short barrel rifle (BTW are now legal in Michigan)> Anyway good topic Cowboy , Leb would like to hear your thoughts on the subject.

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10 years 8 months ago #36442 by SOC
Shorter = stiffer but trades velocity.

There is an interesting possible exception.

This has to do with bullet exit timing.

When a round is fired a sound/compression wave travels down the barrel hits the muzzle and "bounces" back and forth between the muzzle and receiver many times faster than the bullet.

When the compression wave hits the muzzle the opening widens and can allow gas to escape around one side nudging the bullet.

A longer barrel will give you wider window for optimum exit timing.

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10 years 8 months ago #36447 by OleCowboy
I have been playing with doing a build for several years now. I have the AR10T and not only is it heavy but its LONG.

In thinking on my future build I keep looking at ways to reduce weight and the bigge is at the bbl, by going shorter and fluting.

So this has brought up the question where is my 'X', X being the point at which ease of use in length and optimal bbl length cross or where is the best length for me and my use with minimal loss of fps.

Bbls seem to come in the following sizes: 16 -26 in increments of 2 in. My 'T' has a 20 in bbl. My research tells me that a 18" configuration is optimal for my uses. I no longer play at the out limits of the .308, in fact most of my use will be at less than 200 yds and unless I go BIG scope I am not going past 300 yds. This is driven by where I live in the Texas Hill Country. Yet, with the right optics and some practice I fell at 18" bbl will take me to the limit and let me hit the "pie plate"

I am not into target shooting therefore my world is the size of a pie plate, if I hit game or OpFor within that radius around the heart or head, shoulder its going down, its meat on the table or another bad actor bites the dust.

I am going 18" on my build and my bet is that all but the best shooter will ever know the difference between 18 and 20.

As the article says, you lose 30 fps and the bad actor or game will never know the difference.

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10 years 8 months ago #36448 by Siscowet
Discussion about shorter barrels cannot be one dimensional. A 16 inch medium or light barrel and a 16 inch bull barrel are not created equal. Which would be better, a 22 inch light weight barrel, or an 18 inch bull barrel? Length and width need to be factored in, especially as a barrel heats up. Which is the better heat sink?

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10 years 8 months ago #36449 by OleCowboy
You are spot on about one dimensional, cannot approach singularly...

That is why I am going with fluting, STRAIGHT fluting, none of the swirls etc stuff. Fluting has advantages of faster cooling due to more surface area. In addition it will increase the rigidity of the bbl vs a bbl of the same weight not fluted.

While some folks deny it and use the argument that fluting the bbl post rifling removes and creates new stresses that in fact is a false argument as any quality bbl manuf is going to flute at the bbl blank stage then rifle...
The following user(s) said Thank You: jtallen83, Libertarian623

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10 years 8 months ago #36451 by bipe215
If you've never read the "Secrets of the Houston Warehouse" article you need to if you're interested in precision shooting. Several guys would shoot in perfect conditions at night
(Illegally) in a giant warehouse from a heavy bench, under perfect conditions. They were fanatical about their reloads and discovered that the two most important factors were neck tension and barrel length. They claimed that 21 3/4 was the optimal length. It's long but a great read.

www.angelfire.com/ma3/max357/houston.html

Steve
The following user(s) said Thank You: jtallen83, Libertarian623

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10 years 8 months ago #36456 by SOC
The difference between a fluted barrel and one of a thinner contour are very minimal.

The most extreme fluting rairly results in more than 10% additional heat loss or 10% more rigidity over a conventional contour.

The fluting also must be very drastic to get measurable results. The more drastic the more difficult it is to do right and the more expensive it gets.

I eventually plan to rebarrel my AR-10TU and go from 24" to 26" but should shed at least a lb. When I do I'll be going here.

proofresearch.com

I may also be looking at putting a heavy contour 16-18" barrel on my AR-10 carbine.

Proof Research has a 1/2 moa guarantee as well.

Now that weather is warming up I should see about visiting these guys along with Nemo (300 Win Mag ARs) Noreen (Bad News 338 Lapua ARs) Barker Machine Works (maker of 375 Chey M24 style rifles) Allan Precision (makes Ackly styles versions of everything from 270 to 50 BMG) and Cooper Arms (for fans of high grade custom hunting/bolt action rifles). Man I have some cool neighbors here in MT.

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10 years 8 months ago #36460 by Libertarian623
Bipe215- that was a great read, had not had a chance to read that before . Really puts things in a different perspective.

SOC- I can't wait to hear about those visits. Maybe I eed to move to Montana, I like Wyoming alot. Correction I like the western mountain area of Wyoming. When you visit I hope you get to shoot some . I am really interested in the NEMO.

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10 years 8 months ago #36462 by Siscowet
Bipe215 that is a great article. I read it originally. Amazing the groups they shot. The other point is if an individual will notice the difference in barrel length effect on performance. Leb, Lib, TxLongshot, SOC and other skilled distance shooters may see a clear difference. My 65 year old eyes would probably let me down in that regard. Another debate is barrel preparation. RRA, FN and others swear by Cryogenically treated barrels, Armalite does not. Does it make for a measurable difference?

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