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Extra velocity with 30" barrel

 
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  #1  
Old 02-04-2008, 10:56 PM
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Extra velocity with 30" barrel

This forum contains tons of great info, been enjoying it greatly. I've been reading lots of info about the different 338 caliber cartridges and their velocities. I haven't found much info on the extra velocity I can expect with a 30" barrel for a 338 Lapua. My reloading book gives info with a 26" barrel. Velocity listed for the 300gr SMK was 2750, I ordered my barrel with a 30" finished length hoping to get close to 2900fps. I remember reading another post where a guy had a simular question, but I haven't found it again. Is my 2900fps goal reasonable with a 30" barrel? I notice most achieving these velocities are using improved cartridges. Thanks again.

Lucas Beitner
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Old 02-04-2008, 11:57 PM
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Yes, you should be able to get 2900fps with a 30" barrel.

W.
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  #3  
Old 02-05-2008, 12:29 AM
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Me thinks the Lapua brass would make more difference than the extra bbl length.

Having said that, it seems that 2730 out of a RUM is pretty fair, though not tops but at 1500 yards its a dead elk. 2900 would be better. 3300 would be
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Old 02-05-2008, 02:49 AM
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Thanks for the info guys.

I'm not concerned with having enough energy to kill and Elk at a certian range. I would like to do some target practice at 1760yds. A couple hundread fps at the muzzle sure helps the trajectory and wind bucking ability at such a distance. I would like to take an elk with this rifle, however, I wouldn't be shooting one at 1500yds anyway. I don't have enough experience shooting over 1000yds to take such a shot.
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Old 02-05-2008, 07:30 AM
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The numbers I see and what I expect is about 20 fps per inch of barrel from 26 to 30 inches and perhaps 15 fps per inch from 30 to 34 inches of lenght. This incorporates the idea that you are matching the bullet and powder to the barrel lenght so that you achieve maximum results. Perhaps you will get more, but if so, then just be happy with it. So with a longer barrel you probably want to be using a slower powder to make more use of the barrel.

There are point of diminishing returns where you do not get much more velocity no matter what you do.

If you go over to Dan Lilja's web site you will find his experiment and measurements with successive cutting off of a barrel.
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Old 02-05-2008, 11:43 AM
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I am certainly not questioning BB on his comments because he probably knows as much about internal ballistics as I do. I will however offer some information from some tests I have done in the past on this manner.

I tested this on my 257 Allen Magnum, 338 Allen Magnum and 7mm Allen Magnum. The 257 has roughtly the same capacity as the Lapua and the other two have larger capacities then your Lapua so they would be more dependant on barrel length then the Lapua.

First test was with my 257 Allen Magnum which is a 338 RUM case necked down and fireformed to sharper shoulder angle and less body taper. Case capacity is nearly identical to the 300 RUM or 338 Lapua.

The best barrel I used was a Lilja 30" 1-10 that had originally been chambered in 257 STW. I had to set the barrel back for the slightly shorter but much fatter Allen Magnum so we started at 29" of barrel length.

The test load was:

105.0 gr WC872
100 gr Ballistic Tip
Fed-215
3.610" OAL

In the 29" barrel, here are the performance numbers for 10 shots:

Average velocity...................4098 fps
Extreme Spread....................44 fps

I curt the barrel down to 26" and retested the same loads:

Average velocity....................3972
Extreme Spread.....................48 fps

In this test, the 257 Allen Magnum would gain roughly 42 fps per inch of barrel length from 26 to 29".

Second test was the 7mm Allen Magnum which is a 338 Lapua necked down to 7mm and fireformed to a sharper shoulder angle and less body taper for max capacity from that parent case.

The load used was:

105.0 gr WC872
200 gr ULD RBBT
Fed-215
3.620" OAL

I chambered a 30" Lilja 1-9 6 groove barrel for my 7mm. after the barrel break in, I measured the velocity of this load. This was an average of 15 shots in 45 degree temps with around 35% humidity.

Average velocity..................3322 fps
Extreme spread....................27 fps

I then cut the muzzle down so that the barrel finished at 27" and waited for a day as close to the shooting conditions listed above. The actual conditions were 42 degrees and 30% humidity.

Average velocity..................3245 fps
Extreme Spread...................33 fps

From this test out of the same barrel with same load in as close to the same conditions as I could get, the velocity gain from 27" to 30" was just over 25 fps per inch.

I also tested my 338 Allen Magnum. Original barrel was a 40" Lilja 1-10. Standard load in that rifle is:

147.0 gr H-50BMG
265 gr AT RBBT
Fed-215
4.550" OAL

In the 40" barrel for 10 shots

Average velocity..................3544 fps
Extreme Spread...................12 fps

After that barrel had a good number of rounds down it, I cut off 5" of the chamber end and refitted to the receiver. Out of the 35" barrel length, this same load produced:

Averge velocity....................3481 fps
Extreme Spread....................14 fps

Basically, only gaining just over 12 fps per inch of barrel over 35".

Interestingly enough, in my lightweight 338 AM with this same load and a 32" barrel, its average velocity is 3456 fps. This is only 8.5 fps per inch slower then the 35" barrel but I am sure this is due to the different barrel. With same barrel, I would expect this to be slightly more then 12 fps, probably around 15 fps per inch from 35 to 32" in barrel length.

With your 338 Lapua, if you use the same load, I would expect the change in velocity from 26 to 30" would be around 10-12 fps at most, so your looking at around 40-50 fps increase. The 338 bore is an amazingly efficent bore diameter so its not wedded to very long barrels.

You will get more benefits of a longer, heavier, more stable rifle then you will the increased velocity.

I generally recommend a 26-27" barrel for a packing long range rifle in the Lapua. For a sit and shoot rifle, 28 to 30 is great.

All my test show is that velocity per inch of barrel length is most greatly determined by two things, case capacity and bore diameter. For any given case capacity, the smaller the bore, the more velocity will be gained per inch of barrel. The larger the bore diameter the fewer FPS will be gained per inch of barrel.

Kirby Allen(50)
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  #7  
Old 02-05-2008, 01:42 PM
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Kirby, thanks for the info, that is good stuff. Kirby, will a regular 338 Lapua Magnum, with a 30" barrel, get 2900fps?

W.
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