Handloads pressure question

M

model14

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Ref: Hodgen Load Recipe Data
Why does 47.0 grains of Varget pushing a 140 grain bullet in a .308 cartridge yield 50,000 CUP, while only 42.2 grains of Varget pushing the same weight bullet in a .260 cartridge also yields 50,000 CUP (49,800 to be exact)? Is it because the throat is smaller, causing the pressure to peak higher even though less powder is used in the same volume case? Seems logical. I also notice that just 33 grains of Varget in the .243 and pushing a much lighter 100 grain bullet also yields 50,000 CUP. Same affect as above, I suppose.

If you look at Varget in the .338 Federal (same case as above), 47.0 grains, pushing a much heavier 185 grain bullet yields 54,700 CUP. This throat is larger than the .308, but the bullet is 45 grains heavier. So in this case can I conclude that the inertia and friction of the heavier bullet is more of a factor than the larger thoat? That is the pressure is peaking higher than the .308 (even though the throat is larger) because of the weight and friction effects.
I wonder what weight .338 bullet would give the exact pressure profile as the 140 grain .308 bullet for the same weight of powder charge?

With 47.0 grains of Varget in the .358 Win cartridge (same as .308 case)pushing a a 180 grain bullet, the pressure peak is only 42,500 CUP. It takes 51 grains to get close to 50,000 (48,200 actually). So in this case, I guess the larger throat diameter is winning out over the higher inertia and more friction of the heavier bullet, as compared to the 140 grain .308. What is interesting is that jumping all the way to a 220 grain bullet and keeping the Varget load about the same, does not change the peak pressure. You would think it would increase a lot in going from 180 to 220 grain using the approximate same powder load.

This is most interesting to me, hopefully it is to some of you.
Richard
 
I'd suggest buying and reading a good modern reloading manual. It can and will explain this sort of thing much better and more easily than most of us can. Sierra's manual does a good job, but Speer and Hornady do a good job as well.
 
I'm sorry Larry, but the manuals you reference don't explain the particular situations I am talking about. Believe me, I have read most all of them. If you can give me a particular quote from them that talks about pressure variations with neck diameter and bullet weight, as I discussed, I would certainly appreciate it. Maybe you have an opinion and can better educate me.
Thanks,
Richard
 
An analogy

Lets say we take two air tanks(empty),one big and one small, and put air into both for the same amount of time, then check the pressure in both tanks. The small tank has the most pressure in it.

Peak pressures happen at "some" distance down the barrel. Lets say 3 inches. Now which caliber has the biggest "tank"?

Yes there are other factors but volume is the biggest factor in what you are asking, I think. I hope this helps. -Mike Ezell
 
Mike,
I hadn't thought about bringing the barrel volume into the picture. Makes sense and more clearly explains the higher pressures of the smaller calibers. How would you explain the .338 Federal data?
Richard
 
I see it the same way,Richard...just a much heavier bullet. Lots of other variables come into this equation,to be precise, but suffice it to say, if you are pushing a 140 grain bullet at a given speed from a given ,case but of different calibers ,the larger will do it with less pressure. Change the weight, and then the extra weight will have to be figured into it. --Mike
 
You have to compare bullets of similar sectional density (SD) and bore volume. If you're old enough to have or have read about Homer Powley's "Computer for Handloaders", in reality a slide rule based on his calculations it might be clearer.

This computer was designed so that a handloader could figure out what the best powder would be for a cartridge with little or no data like a new wildcat. The powders and charges were designed to give pressures of about 45 kpsi, and since it was developed in the 1950's was based on the then available IMR powders.

Anyway to use this "computer" you needed to know the case capacity (in grains of water), bore diameter, bullet weight, and SD of the bullet of choice. It gave a charge weight, the powder to use, and then knowing caliber and barrel length velocity could be estimated. It clearly shows that not only case capacity, which is close to the same in the .243, 260, 7mm-08, .308, .338 Fed, and .358 Win, but sectional density had to be considered.

Obviously a 100 gr .243 bullet has higher SD (0.241) than a 100 gr bullet in 6.5 (0.206) or larger caliber. To have about the same SD a 6.5 mm bullet has to weigh nearly 120 gr, a 7 mm bullet has to weigh nearly 139 gr, a .30 cal bullet nearly 165 gr, a .338 bullet nearly 200 gr, and a .358 bullet around 220 gr. The larger diameter bullets have more base area (area of a circle increases as the square of the diameter, i.e., double the diameter and the area increases four times) for the powder gases to push against, and the larger bore has more volume for the gases to expand in.

If you have a garden hose with no nozzle on it and the faucet opened all the way the water doesn't come out very forcefully (less pressure). If you install a nozzle that has a small orifice the water comes out much more forcefully (more pressure) although the volume of the hose (case) and initial water pressure (charge) haven't changed the pressure has increased while the volume of water or powder gases exiting the hose/nozzle has decreased. If you have enough water pressure and a small enough nozzle you can bulge the hose. This probably isn't the best analogy, but it's the best one my feeble old brain can come up with this morning.
 
Larry,
In spite of your feeble brain, that is a great explanation. :D Thanks. It just never entered my mind before to use sectional density when comparing powder loads versus pressure.

Richard
 
In the first issues of Rifle magazine, Proffesor leonard Brownell did
a phenominal study of pressure vs secional density. It involved only
one caliber. Very worthwhile reading if you can find a copy
 
Richard,
This conversation is way over my head,but I'll tell you what.I've got a dead deer hangin' out in the shop,I'll ask him what he thinks.:)
Joel
p.s. Naw,he dosn't know either.
 
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