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Case Capacity Verses Barrel Lenght?

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Old 03-28-2006, 08:25 PM
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Posts: 38
Case Capacity Verses Barrel Lenght?

I'm just wondering just how much effect barrel length has on velocity for a given capacity.

I know the general rule is 30-40fps per inch, but I dont think its that simple.

I've got a 243 that was 22" and I cut it back to 19". At 22' my hunting load was 100g bullet at 2850 fps. When I cut it down to 19" that same load went 2750 so I lost 100fps.

But I've since re-worked my loads and am now getting 3000fps out of the 19" barrel which is what most handbooks list for a 24" bbl.

I'm now thinking of rebarreling to 7mm08/284 or perhaps 7mmWSM but again I want to go with a 19" barrel.

What I'm not sure of is whether the larger capacity cases (284/wsm) will have a more significant velocity loss as they need the extra barrel length to acheive full combustion????

I see most of you guys that are serious "long Range" shooter are using 28-30" bbls. Is there really that much to gain from the extra 6" over a 22"?



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Old 03-28-2006, 10:14 PM
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Location: centre,alabama
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Re: Case Capacity Verses Barrel Lenght?

Longer barrels=more velocity...
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Old 03-28-2006, 10:15 PM
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Re: Case Capacity Verses Barrel Lenght?

the 243 is not realy an "overbore" case meaning that its pretty effecient in the use of its powder from what I have seen in the time I have been reloading the larger the case (more volume) for a set bullet diameter the slower the powder has to be and the slower the powder use the more that load will benifit from a longer barrel as it give the powder enough time to make its pressure over the longer curve.
lets look at the 308 vs the 30-378 wby we all know that the 308 is a very effecient caliber as its onfte had to get any more noticable velocity out of a 26" barrel than a 22" unless you get into heavy for caliber bullets and that takes slower powder and thats where the longer barrel helps
now you can take a 308 case fill it to the top with IMR-4350 and crush a bullet say 165gr down in their and it'll shoot with no problems. But if you do the same thing with the 30-378Wby your going to end up with a terrable mess.
likewise I personaly woulden't see any benifit in chambering a gun for the big Wby case unless the barrel was going to be in the 26" plus range otherwise your going to be close the same velocity class that a 300wby can get burning 20grs less powder.
so if your building a 308 to shoot he 175gr Match Kings fast enough to get to 1000yds then all you need is a 20" , yea a 24 will give you a little more veocity and a 26 even a little more but in a varmint type profile you would be talking about a pound of weight between the 26" and the 20" but if your gonna try to shoo the 220gr Match kings out as fast as possible then you'll benifit a little more from the 26" tube due to the slower powder used.
I reload the 308 for a 15" Encore pistol and I can generaly beat any factory loads shot out of a 24" barrel

So if you look at the guys shooting big mag case rounds they are probably shooting very slow powders and they need that extra barrel to get all the good from all that powder
Si Vis Pacem Parabellum
Molon Labe
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Old 03-28-2006, 10:56 PM
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Posts: 38
Re: Case Capacity Verses Barrel Lenght?

JDJones..thanks for the comments. What you say makes sense

It looks like the 284 and WSM both use moderately fast powders so I assume they will not benefir from longer barrels as much.

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Old 03-29-2006, 08:55 AM
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Re: Case Capacity Verses Barrel Lenght?


The amount of FPS gained per inch of barrel has as much to do with bore diameter as it does case capacity.

FOr instance, if you take a barrel and chamber it for my 257 Allen Mag and take another barrel of the exact same length, say 30" and chamber it for the 338 RUM you will get X amount of velocity for each round.

If you cut an inch off of each barrel and retested the velocity average, you would find the 338 RUM would loose only around 18 to 20 fps at most. My 257 Allen Mag with very similiar case capacity would loose over 40 fps per inch. In fact exactly 42 fps from actual tests.

Basically, even though both rounds have nearly the same case capacity, the smaller bored 257 AM will have twice the velocity drop as the 338 RUM.

Why, because of expansion ratios and the burn rate of powder needed for each round to perform properly.

Its not really the case capacity that determines the velocity potential of a round in a certain barrel length, its actually is the bore diameter.

Even though both of these rounds have similiar case capacities, the smaller bore of the 257 AM requires EXTREMELY slow burning powder to perform to it top levels. Powders such as AA8700, H-870, H-US869 and WC-872 are what makes this little pill run hard. All three have basically the same burn characteristics as Wet Coal!! [img]/ubbthreads/images/graemlins/grin.gif[/img]

But if you look at the 338 RUM, with a 200 gr bullet, the highest velocity potential reported by Hodgdons comes by using H-4350, relatively quick burning powder.

With a 250 gr pill Hodgdons lists Retumbo and H-1000 as the fastest powders in this chambering, still in comparision to the powders used in the 257 AM, Retumbo and 1000 are very quick burning.

This brings us to why bore diameter effects velocity potential so much. Simply put, the round that can use faster burning powder in a larger bore will produce more velocity in a shorter barrel then a round needing a slower burning powder.

Also, since the faster burning powder burns faster, there will be less muzzle pressure when the bullet exits the muzzle. This results in less FPS loss per inch of barrel loss.

You will also see much greater velocity drop the shorter you make the barrel. In your case you are taking a relatively shot barrel and making it VERY short. A long barrel will loose much less velocity per inch of barrel loss then a shot barrel would. That is why you are seeing the +30 fps velocity loss with your 243, but as you say, with some creative loading your over what the 22" barrel was getting you.

When your comparing a 243 Win to a round such as my 7mm Allen Mag which I recommend at least 28" of barrel length, you are comparing a case capacity of around 43 gr to one of 105 gr.

In this extreme comparision, there is a basic need for inches of barrel simply because of the powder used and the time needed to keep the bullet in the barrel under pressure to produce the high velocity.

This is an extreme case comparision though.

There are other factors with a longer barrel as well. In the large rounds, moving the muzzle farther away from you greatly reduces the muzzle blast the shooter is subjected to. Your 19" 243 will have a muzzle blast at least as great as my 7mm AM with a 30" barrel, at least as far as the what the shooter is subjected to. This makes the larger calibers easier to shoot.

I personally feel a longer barreled rifle with its added weight sets more solidly in the bags or on a bipod, to a certain point that is. Short barreled rifles seem very lively to me as far as stability. Not saying they are not extremely accurate because they are but its a personal preference thing again.

As far as rebarreling up to one of the larger rounds you list. The 7-08 will work fine in a 19" barrel, the 284 and 7mm WSM will still generate more velocity in the shorter barrel compared to the 7-08 but not by as large a margin as they would in a longer barrel.

I have used the 7mm-08 alot in 15" handgun barrels where a 140 gr Ballistic Tip can be launched at 2600 fps. That same load in a 24" barrel will produce just shy of 2900 fps for a difference of 300 fps. I would say that is significant.

A 243 with a 100 gr bullet will have roughly a 400 fps velocity spread in the same comparision showing the smaller bore will increase the velocity spread.

A 308 Win will have less then 200 fps spread comparing a 15 and 24" barrel velocities.

A 338 Federal would be in the 150 fps spread range with these two barrel lengths.

Simply put, the larger the bore diameter for a given ase capacity, the less effect barrel length will have on velocity. Conversly, the smaller the bore diameter for a give case capacity, the larger the effect will be on velocity.

Have a good day,

Kirby Allen(50)
Kirby Allen(50)

Allen Precision Shooting
Home of the Allen Magnum, Allen Xpress and Allen Tactical Wildcats and the Painkiller Muzzle brakes.

Farther, Faster and Flatter then ever before.

Web Page: www.apsrifles.com

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Old 03-29-2006, 01:38 PM
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Join Date: Dec 2005
Posts: 11
Re: Case Capacity Verses Barrel Lenght?

How does a muzzle brake affect the velocity? You are adding a couple inches; do you get any added velocity?
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Old 03-29-2006, 07:56 PM
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Posts: 2,856
Re: Case Capacity Verses Barrel Lenght?

no you won't get any added velocity becasue the gasses are stopping the pushing action of the bullet and being redirected out to the sides of the brake , thats what make the brake work.
I have heard a couple guys say that their brake desgine boost velocity , but I personaly can't see how this would be true and still have an effective brake
Si Vis Pacem Parabellum
Molon Labe
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