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