juanvaldez
CGN Ultra frequent flyer
- Location
- Sudbury, Ontario
Not my gun, I just did the plumbing
The "Hulk"
- Thread starter juanvaldez
- Start date
Not my gun, I just did the plumbing
rnbra-shooter
CGN Ultra frequent flyer
- Location
- New Brunswick
It would be the same as any other .308 (nice looking rifle BTW; is it a free-floating action with the stock clamping onto the first inch or so of the barrel?)
A .308 with a 30" barrel will typically shoot 155 match bullets at around 3000fps. The same load fired from this rifle's 26.5" barrel would probably give a muzzle velocity of 2930fps. At that speed, my current favourite 155 (Berger 155.5 Fullbore) will drop about 27MOA from 100 to 1000 yards, and a 10mph crosswind will push it sideways 8.9MOA at 1000 yards
Another very popular long range bullet choice would be a Berger 185BT, from this barrel you might expect 2685fps. It will drop about 29 MOA from 100 to 1000, and be pushed 7.8MOA sideways by a 10mph wind.
If you like looking at these scenarios I can highly recommend this online ballistics calculator:
http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi
Awesome Thanks. I just saved it as a favorite so I can check it out. I've always run shorter barreled 308s but something I've been trying to process is the crossover point where a shorter barrel with a hotter loader (due to the lack of pressure) equals what a longer barrel does naturally. It's an interesting concept to me. Thanks for the info.
rnbra-shooter
CGN Ultra frequent flyer
- Location
- New Brunswick
Peak pressure in most centrefire rifle cartridges occurs when then bullet has moved 1.5" to 2.5" down the barrel. After that, the pressure is falling. So unless you're talking about a barrel shorter than that, "shorter barrel with a hotter loader (due to the lack of pressure)" is not going to happen (if I understand correctly what you have written?)
Having said that, the choice of a .308 Win rifle barrel at 16", or 20", or 24" or 26" or 30" or 34" is not nearly as big a deal as most people seem to think. The longer barrels will give more speed, but not as much as many people might assume; also, the benefits of a bit more speed are often less than people might assume. Often the selection of a barrel length will be more influenced by factors other than muzzle velocity (for example, how handy/toteable the rifle is, how heavy, how much muzzle blast and how close to the face, how far away from my eye would I like to mount a front sight so I can focus on it, how stiff the barrel is, etc).
Interesting. So I guess what I'm curious about then is whether or not someone with a 24" and up barrel length can load a lapua casing with 46.75 grains of Varget with a 175gr SMK and still get the casing out of the chamber or have the primer not fall out. I'm guessing not but it remains to be seen. My personal theory is that the longer the barrel, the more time the bullet stays in it and the more the pressure causing over pressure. Or better yet, with a 24" barrel or longer, how many grains of Varget does it take to make a 175gr bullet do 2800fps? If I had the budget for a new 30cal barrel, I'd start at 28"s and load rounds to get to that velocity and then get it cut back inch by inch to see if there is any difference.
rnbra-shooter
CGN Ultra frequent flyer
- Location
- New Brunswick
The peak pressure of your proposed load (yikes it sounds way too hot BTW) will not be affected by barrel length.
The peak pressure will be affected by bore and groove diameter, surface roughness, throat length and a bunch of other things. But whatever the peak pressure is, it will be achieved when the bullet is an inch or two down the barrel. After that time the pressure is falling, so by definition it is less than peak pressure. You could cut the barrel down to 10" in length, which will have no effect at all on what happens to the bullet and to the chamber pressure during the first inch or two of bullet travel.
From time to time people do the "cut off an inch at a time and chrono" test; they are interesting to read. Usually it is found that when dealing with a 30-ish inch long barrel, chopping an inch off lowers the muzzle velocity by 15fps or 20fps. The shorter a barrel gets, the more speed you lose per inch.
If you have an internal ballistics program (e.g. QuickLoad) or knows somebody who does, you can get a pretty good idea of the velocity profile down the barrel - how fast the bullet is travelling after 5 inches of travel, what the pressure is, how long it took to get there, then the same things at 10", 20" etc
Your question of how many grains of Varget do I need to get a 175 going 2800fps in a 24"+ barrel is a difficult one to answer and for what it's worth you are coming at it from the wrong angle.
If you want to know what speeds you might typically expect to get with a 175 from a 24" barrel, look at some loading manuals or ask some shooters. Or, extrapolate it from data you already have/know.
If you choose to shoot a 175 Sierra (good bullet, BTW) and if you choose to use Varget and a 24"-26" barrel (also good choices), you will get what you end up getting. What's important is that you get a nice accurate uniform load (which in a .308 typically happens at max or near max loads). Whether that ends up being 2650fps or 2800fps is *much* less important than whether you can get it shooting well.
FWIW I don't expect you will reach 2800fps with 175s in a 24" barrel. I would expect to get no more than about 2870fps from a 30" barrel (this is based on multiplying a "known hot" speed for a 155 of 3050fps by Sqrt[155/175] - this is a reasonably-decent scaling rule). You may well lose 120fps in a 24" bbl compared to a 30" (6 inches shorter, times 20fps per inch). My expectation is that your load will max out somewhere in the 2750fps region.
The peak pressure will be affected by bore and groove diameter, surface roughness, throat length and a bunch of other things. But whatever the peak pressure is, it will be achieved when the bullet is an inch or two down the barrel. After that time the pressure is falling, so by definition it is less than peak pressure. You could cut the barrel down to 10" in length, which will have no effect at all on what happens to the bullet and to the chamber pressure during the first inch or two of bullet travel.
From time to time people do the "cut off an inch at a time and chrono" test; they are interesting to read. Usually it is found that when dealing with a 30-ish inch long barrel, chopping an inch off lowers the muzzle velocity by 15fps or 20fps. The shorter a barrel gets, the more speed you lose per inch.
If you have an internal ballistics program (e.g. QuickLoad) or knows somebody who does, you can get a pretty good idea of the velocity profile down the barrel - how fast the bullet is travelling after 5 inches of travel, what the pressure is, how long it took to get there, then the same things at 10", 20" etc
Your question of how many grains of Varget do I need to get a 175 going 2800fps in a 24"+ barrel is a difficult one to answer and for what it's worth you are coming at it from the wrong angle.
If you want to know what speeds you might typically expect to get with a 175 from a 24" barrel, look at some loading manuals or ask some shooters. Or, extrapolate it from data you already have/know.
If you choose to shoot a 175 Sierra (good bullet, BTW) and if you choose to use Varget and a 24"-26" barrel (also good choices), you will get what you end up getting. What's important is that you get a nice accurate uniform load (which in a .308 typically happens at max or near max loads). Whether that ends up being 2650fps or 2800fps is *much* less important than whether you can get it shooting well.
FWIW I don't expect you will reach 2800fps with 175s in a 24" barrel. I would expect to get no more than about 2870fps from a 30" barrel (this is based on multiplying a "known hot" speed for a 155 of 3050fps by Sqrt[155/175] - this is a reasonably-decent scaling rule). You may well lose 120fps in a 24" bbl compared to a 30" (6 inches shorter, times 20fps per inch). My expectation is that your load will max out somewhere in the 2750fps region.
But now based on those numbers, I shouldn't be able to achieve 2800fps from a 21 3/4" barrel. This is why the calculating game is so tricky. I agree with you on varying barrels but going about testing different barrels and performance based on a set length of barrel would be a good start. If the barrels are the same length and diameter, harmonics should be close so all it should boil down to is friction which would result in velocity loss, maybe pressure also depending on how smooth the bore is. Something I found through running Gun Juice in my guns was an increase in velocity which maybe adds to the equation of how I'm able to achieve an 1/8" 5 shot group at 100yds with as high a velocity as I'm getting.
I think it would be interesting to do a barrel maker comparison. I'm going to look into that further.
I think it would be interesting to do a barrel maker comparison. I'm going to look into that further.
I assume you're burning n550 to get these numbers?
No. Varget. and it's been confirmed through cronograph. It wasn't just my gun either. There were about 10 308s all made at the same time with the same components that all liked the same load. Some of the guys took their loads down to the next harmonic node but I didn't bother screw'n with success.
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Good guess Daniel. My rifle shoots 175s over 44.8 gr of Varget at 2760 fps.
Is that at the upper end of pressure too? Have you ever ventured higher in powder and found the next accuracy harmonic node?
