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> .260, 7mm08, .308
.260, 7mm08, .308
@ 07:38 am (GMT) |
RussellHello,I have been reading the reloading book and studying different loads rounds basically every thing ammo related, and I have found something I am hoping somebody can explain to me. A 260, 7mm08 and 308 all share the same basic case just necked differently, according to data I found online they all are very similar case capacities. How come a 260 can drive a 140gr bullet at 2750fps yet a 7mm08 can drive the same weight bullet (but with a worse bc) at 2860 fps with the same amount of case capacity, also a 308 can drive a even heavier 150gr at 2820, I couldn't find any data for 308 140gr in Nathans database. The above loads are all factory remmington corelokt. Is this somthing to do with the free bore you mention in the book does the 260 not have it?. I am really just curious as the physics don't stack up. Also if free bore works so well on the 308 why do we not use it on all cartridges, it seams a simple way to run higher velocity without brass damage or am I understanding it incorrectly. Thanks guys I'm hoping for some real complex scientific answers to this to get me thinking but I'm sure it's somthing simple I'm not understanding. |
Replies
@ 02:32 pm (GMT) |
Warwick MarflittRe: .260, 7mm08, .308Russell. Is the same power used in all three calibres?I MIGHT BE TOTALLY WRONG....Using what I know about hydraulics.. It's to do with the pressure applied to the differing diameters of the bases of the different size bullets and the friction of its surface area. As the diameter gets bigger the bullets surface area increases, giving the expanding burning powders gasses pressure more area to push on. Because Gasses and liquids behave in a similar way. The powder burn rates create pressure that applies force on the bullet.? The twist rate and the bullets base area difference of the calibres will cause different velocities because of the bore expansion ratio. This is the speed at which the volume (space) behind the bullet increases as it travels out the bore The gas of the burning powder has to expand and increase in volume to fill this increasing space . As it travels out the barrel. There's more space behind the 308 to fill than behind the 260. This is why we have different burn rate powders. To keep the pressure increasing behind the bullet so that it keeps accelerating and reaches a good speed Have a look at the hydraulic page. Links at the bottom. The amount of force a hydraulic cylinder can generate is equal to the hydraulic pressure times the “effective area” of the cylinder ................"Calibre of the bullet"........... I'm sure that I've confused everyone enough for the week. I'll find a hole to hide in and let you all be. http://www.enerpac.com/en/basic-hydraulics |
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@ 02:56 pm (GMT) |
Warwick MarflittRe: .260, 7mm08, .308Here's a diagram I drew last week thinking about Bore expansion ratio. Nathan delete or add correction if I'm wrong with this. I don't want to post incorrect or misleading information. Thanks.
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@ 03:41 pm (GMT) |
Warwick MarflittRe: .260, 7mm08, .308Same case volume on different Cal size bore
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@ 04:10 pm (GMT) |
Paul LevermanRe: .260, 7mm08, .308Referring to P.O. Ackley (Handbook for Shooters & Reloaders, 1962, p. 170):"Now there is a very concrete and mathematical reason for our inability to exceed certain velocities at a given pressure. The equation for Pressure is: P equals F, divided by A; in which P is pressure; F is Force and A is area. It will be seen that if we increase A (area) then P will fall off instantly, provided F is constant. All of this is unchangeable." Pretty much what Warwick said. |
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@ 04:22 pm (GMT) |
Paul LevermanRe: .260, 7mm08, .308But then again, you have to read the original post more carefully.From what I read, there are too many unknowns to draw solid conclusions. Are the powders burn rates and pressure curves the same? Are the bearing surfaces on the bullets of equal length? Is bullet jump to the rifling a factor? What is the twist rate of each of the barrels? Is the allowable pressure for each of the cartridges within a reasonable proximity? Barrel length would also come into it, either increasing or decreasing the overall length of the combustion chamber (Warwick's notes). Beyond me, that's for sure. |
@ 06:39 pm (GMT) |
Nathan FosterRe: .260, 7mm08, .308Hi Russel, the answer is simple and has been explained above.The bigger the hole through the bore, the greater the gas expansion area and the faster the bullet can be driven. A .308 drives a 130gr bullet at 3000fps with hand loads and can sometimes drive the same at 3050 and in some cases up to 3100fps. 100fps per 10 grains bullet weight gives you 2900fps to 2950fps with a 140gr bullet. If you want complex math or science that picks up from where Einstein finished, go to youtube and type in Bruce Cathie. There you will find lectures from a New Zealander who is either completely mad or totally brilliant. |
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@ 07:27 pm (GMT) |
Thomas KitchenRe: .260, 7mm08, .308Hi Russell and welcome backas I barely passed 5th form science at kamo eh bro I like keeping things simple. Imagine you have two 1.5litre coke bottles filled with water, on one you drill small hole through the lid on the other you leave the lid off. Now if you apply same amount of pressure to both bottles the one with the biggest opening will empty/ expel its pressure the quickest. This is at the most basic end of the equation as other people have eluded to once you get into powder burn rates and barrel lengths things get more complicated. As for the free bore question you may get velocity but on the other hand you can lose accuracy. |
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@ 03:35 am (GMT) |
RussellRe: .260, 7mm08, .308Thanks for the replies guys, got the teqnical replies I was after, pretty well explained really quite simple thanks. So on this information, I am thinking that is why larger bore rifles would suit shorter barrels better than small bore as the area required to fully expand and propell the bullet would appear quicker.Thanks again something else to ponder. Yea Thomas I only just got the 5th form maths and science at Kamo then I left so my physics etc is probably lacking. Cheers |
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