Impact of Barrel Length on Powder Charge and Speed

Discussion in 'Reloading Forum (All Calibers)' started by MVW, Apr 26, 2012.

  1. MVW

    MVW

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    Gents: As a practical matter I understand many of the signs of over pressure - case expansion at the web, primer flattening and the sort. That said, I'd like to understand what I can expect from a 30" barrel.

    In looking three sources of load data for Varget behind a 175 GN bullets, I find the following reload data:

    Sierra 5th edition gives 36.3 - 41.7 (speeds 2,200 - 2,500)
    Hodgdon gives 42.0 to 45.0 (speeds 2,583 - 2,690)
    Lyman gives 40.0 to 45.2 (speeds 2,459 - 2,708)

    This is quite a spread in charge recommendations and speeds and in fact the top end for the Sierra book doesn't even reach the bottom of the Hodgdon range.

    I assume some of this due to variations in barrel length, primers, and brass, but still it is a lot.

    I have loaded 44 GN of Varget and reach speeds of ~2,750, which is rather greater than any of the above. However, again without significant pressure signs. 45 GN would really be flying.

    Can anyone explain to me what is going on with the reloads and how the 30" barrel factors into my reloading. Do I need to get a copy of QuickLoad to sort this out?

    With gun powder I am conservative. I don't mind working near max, but I want to make sure there are not any surprizes lurking.

    Thanks, Mike
     
  2. watercam

    watercam Site $$ Contributor

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    Every rifle (and pressure barrel) is different. Load data is for reference - you load what shoots in your rifle. Bear in mind that ambient temps. are a big factor. Loads worked up in the winter may be over pressure in the summer. As far as velocity, add roughly 25 fps per inch of barrel length. Should work out up to 30"-32" barrel length. With the .308 the velocity advantages fall off a bit much longer than that. However, if you are shooting irons the extra sight radius is always helpful. Bear in mind that the weight of your rig will increase. If you are concerned about shooting at 1000 yds. with what you have you should be fine with 175s @ 2750 fps..
    Best of luck!
     
  3. bayou shooter

    bayou shooter F-TR competitor

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    MVW; I use Varget exclusively for my match rifles. My F-T/R rig in .308 sports a 32 inch barrel and I use 180gr bullets.

    I trust the powder manufacturer's data over any bullet manufacturer's data. Hodgdon has a pretty extensive website and the range you provided is indeed the one I used during load development. Since the powder is not the product of the bullet manufacturers, they have to be extremely conservative with their max loads; they do not have any control over the product so they may be out of date. Hodgdon even has loads listed on their canisters. When I get my brace of kegs, I check to make sure it has not changed. By contrast, if I want to know what a bullet will do at the target, I will not ask the powder manufacturer for that information.


    My load is slightly under Hodgdon's max and my muzzle velocity is around 2850FPS. This is the reason I have long barrels, because I want as much muzzle velocity without having to go into overmax loads. The longer barrel allows the hot pressurized gases to accelerate the bullet further. As long as the pressure behind the bullet exceeds the resistance of the barrel, the bullet will accelerate and in a .308 it would require several feet of barrel length before the pressure behind the bullet falls below the resistance of the bullet through the barrel.
     
  4. Keith Glasscock

    Keith Glasscock Team Benchmark Site $$ Contributor

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    Sierra data is with Federal brass.
    Hodgdon uses Winchester.

    Both are correct for their respective cases. Winchester simply holds more powder and makes less initial pressure. Also remember that seating depth will make huge changes in pressure and velocity for a given charge.

    With Win brass, a 175 comes out of a 30" barrel at ~2800-2825 with a good hot load of Varget. I like ~44.5-44.7 grains of Varget in that case. It makes something just shy of 2800 in my barrel. (it was only 2650 or so when the barrel was new - it sped up).

    What I have found is that the barrel length makes a little more difference in speed with lighter bullets than heavies. 155's speed up 100 fps for 4" but 200's only get a 50 fps speed up.

    You will have to work up your loads no matter what your barrel length. If you are using Lapua brass, the best advice I've heard is to reduce the charges by half a grain from Winchester. If that holds true, you might find a node anywhere between 44 and 45 grains. The velocity doesn't matter so long as it is at least 2600.
     
  5. XTR

    XTR Site $$ Contributor

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    When I went from a Rock Creek 24" to a Krieger 30" I gained about 50 FPS with the heavies.

    In WW brass I could run more but I hit a node in the 44.0 to 44.5 range so I load 44.4 and let it ride.

    My 175SMKs are about 2780 and my 185 LRBTs are about 2740.
     
  6. MVW

    MVW

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    Thanks for the suggestions. This is helpful. Yes I am using Lapua brass now. I did notice that things run a little hotter than with WIN brass, which I assumed was because of the smaller case volume on the Lapua.

    Mike
     

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