Quick question, if you're at max powder charge, and the velocity isn't what you were hoping for (given all else is good, throat, bore, etc etc) do you try a faster or slower powder to boost velocity? (25" barrel .308 168gr)
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Powder burn speed, pressure, and velocity question
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Hmmm??? Am not sure what you're actually referring to regarding "max powder charge"??? You talking about max fill on the case or max pressure (like pressure signs)?
You can:
- seat the bullet longer and add more powder
- try a different powder
- get a different bullet (e.g. 150 gr)
- do all the above
Look at the factory load data and find a powder that provides what you are looking for or step up to a bigger cartridge. Loading above book max isn't a game I'm interested in playing.
OP’s question is actually something I’ve wondered also.
Slower means you get more MV at a given bullet weight, but you often run out of powder room or don’t get full combustion, right? Whereas a too-fast powder will give you full combustion but will over-pressure before you reach the MV you could attain with the right powder?
Slower means you get more MV at a given bullet weight, but you often run out of powder room or don’t get full combustion, right? Whereas a too-fast powder will give you full combustion but will over-pressure before you reach the MV you could attain with the right powder?
I know very little about interior ballistics but I believe we have an expert @Ledzep who frequents these forums. Perhaps he can help.
what is your current load, barrel, etc. A lot more info is needed so you can get started on the right path
It’s a combination of the volume behind the bullet in the chamber and then the rate at which the bullet is accelerating down the barrel as its fired creating more volume behind the bullet.
A lighter bullet can be more easily accelerated and thus create volume behind the bullet quicker, thus a faster burning powder is more ideal for that relationship and creating the right amount of combustion gas pressure.
A heavier bullet is slower to accelerate at the same pressure so you need a slower powder to properly fill the empty space as the bullet accelerates other wise you drop pressure.
Too much fast powder behind a heavy bullet means you make more pressure for the volume available.
Too slow of a powder and a light bullet means you are just bleeding potential velocity.
Pressure too high is the critical part to avoid. Everything else comes after.
I think of it this way;
X axis is time, y is pressure. I’m looking for maximum shaded area under the pressure graph line without exceeding max allowable. x (time) maxes out when the bullet leaves the muzzle. Playing with different powder burn rates, bullet weight (heavier accelerates slower and thus relieves pressure more slowly), bullet bearing surface area (friction changes), and barrel length are the basics. Actual bore dimensions, rifling details, raw energy available in various powders by volume/temp/humidity etc. are details that are rarely properly quantified by the hobbyist. Usually the hobbyist just cranks up the pressure until brass begins showing early signs of catastrophic
failure and then brags about it online.
I think of it this way;
X axis is time, y is pressure. I’m looking for maximum shaded area under the pressure graph line without exceeding max allowable. x (time) maxes out when the bullet leaves the muzzle. Playing with different powder burn rates, bullet weight (heavier accelerates slower and thus relieves pressure more slowly), bullet bearing surface area (friction changes), and barrel length are the basics. Actual bore dimensions, rifling details, raw energy available in various powders by volume/temp/humidity etc. are details that are rarely properly quantified by the hobbyist. Usually the hobbyist just cranks up the pressure until brass begins showing early signs of catastrophic
failure and then brags about it online.
If your goal is to boost velocity in a 308 with the 168 - 175 bullet weights, Alliant 2000 MR will boost velocities about 100 FPS above most other powders without pressure concerns .
I get 2750 FPS with a 175 bullet with 46.5 grs. of 2000 MR out of a 24" Hart barrel. the only downside is 2000 MR is a ball powder that is temperature sensitive and it is unobtanium right now. I've shot this load out to 1000 yards with good accuracy.
A second powder that will boost velocities is CFE 223. It is similar to 2000 MR. I tried it, I preferred 2000 MR.
CFE 223 is available if you want to test a pound of it.
I get 2750 FPS with a 175 bullet with 46.5 grs. of 2000 MR out of a 24" Hart barrel. the only downside is 2000 MR is a ball powder that is temperature sensitive and it is unobtanium right now. I've shot this load out to 1000 yards with good accuracy.
A second powder that will boost velocities is CFE 223. It is similar to 2000 MR. I tried it, I preferred 2000 MR.
CFE 223 is available if you want to test a pound of it.
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It depends. To make a complicated story simple, there is energy content, burn rate, and capacity. Without knowing pressure, it's hard to draw many meaningful conclusions.
There are single based and double based propellants. Double base will usually have higher energy content, but are also usually more temp sensitive.
Burn rates are relative, and the published lists are not always exactly correct for every application (cartridge+bullet combinations can change relative burn rates and burn behavior). Burn rate is usually independent of energy content.
So with that being said, if you are talking about the same type/family of powders, let's say single base spherical propellant, and you are filling the case completely full of powder, somehow you know you're not achieving max pressure, and are slower than you want to be... Odds are you could bump to a slightly faster burning powder and be able to run up to the pressure limit to get more velocity. You could also go to a double-based powder of the same/similar or slightly faster burn rate and likely get more velocity.
Conversely, if you are using a fast burn rate powder and pressuring out with 80% case fill, going to a slower burning powder will allow you to put more powder in the case and likely get more velocity.
There's a bit more nuance to it all than just that, but that's a general picture.
I got pressure signs really fast with CFE223 in .308 with Sierra 175TMK Work that stuff up carefully.
Gotcha. Great explanations here. I'm shooting Varget (I prefer to stay in the 'extreme' powders from hodgdons) currently, with Lapua brass and federal small rifle primers, and I cant find my data for 42.3 but for 42.2 behind a berger 168 I'm getting 2521 average off of my Magnetospeed at 80 degree temps in the shade.
Bergers reloading manual states 2685 for 42.5 grains of Varget. I'm going for 2625fps (shooting a ssg69 w/ factory scope with bdc set for a 168 projo at 2625fps-just experimenting with this scope) and with 42.5 averaging 2521 I dont think I'm going to make it with .3 more grains of powder.
Would a magnum primer help? The reason I'm confused is that I have always read that steyr barrels were usually pretty quick due to them being on the slightly tighter side of the spec. Is that not right?
Bergers reloading manual states 2685 for 42.5 grains of Varget. I'm going for 2625fps (shooting a ssg69 w/ factory scope with bdc set for a 168 projo at 2625fps-just experimenting with this scope) and with 42.5 averaging 2521 I dont think I'm going to make it with .3 more grains of powder.
Would a magnum primer help? The reason I'm confused is that I have always read that steyr barrels were usually pretty quick due to them being on the slightly tighter side of the spec. Is that not right?
Your rifle/components/seating depth/neck tension/etc. may not reach 2625 with the same load as published
Check some other manuals for powder weights.
You will see there can be a pretty large difference from each. What is Bergers max charge weight? Are you close to it?
With a 168 I think you should be able to increase your charge weight- Carefully and in small steps. .2-.3gr increments, watching for pressure signs.
I would not run a magnum primer with Varget.
Check some other manuals for powder weights.
You will see there can be a pretty large difference from each. What is Bergers max charge weight? Are you close to it?
With a 168 I think you should be able to increase your charge weight- Carefully and in small steps. .2-.3gr increments, watching for pressure signs.
I would not run a magnum primer with Varget.
Yea, in Hornadys manual its calling for 44 GRAINS for 2600fps... WAYYYYYYYYYYY different than Bergers manual lol.
I have three lots of Varget. The slowest lot is 100 FPS behind the fastest lot. If I combine equal amounts of both lots inside a case the velocity falls right down the middle. So maybe you have a slow lot.
Are you seeing any kind of pressure, because you shouldn't be anywhere near max. Hodgdon shows 2737 with a barnes 168 and 45 gr of varget. The berger data is no good.
Thinking way back, my standard load was 43gr of varget with a 175 smk.
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