The point was that velocities are similar with the same projectiles, with a bit more capacity going to the 284 if given the freebore. Just wondering what the draw to it would be over the 284 - if you preferred the short action mag, why not a 7 SAUM or WSM or SS?
- Thread starter swampbuck
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The point was that velocities are similar with the same projectiles, with a bit more capacity going to the 284 if given the freebore. Just wondering what the draw to it would be over the 284 - if you preferred the short action mag, why not a 7 SAUM or WSM or SS?
Gotcha...
Never really paid attention to such stuff so I was not aware they were very similar with slight differences.
If that's the case though, then the 6.5PRC would be the same as your 284 just in a necked down version.
Either way, what drew me to the 7PRC was how great a cartridge the 6.5PRC has turned out to be, that if more energy was needed, the 7 could be a good wildcat option. I guess it just interests me as a hunting option. I suppose the SAUM would work too. Although the 7PRC interests me from a long action.
Wait, I’ve got a question on this: doesn’t the pressure increase in the 7mm relative to the .308 as it moves down the barrel? Thus offsetting the reduction in area and keeping force relatively constant?
If you have the same chamber pressure, then yes you get more force because of the increased area right as it leaves the chamber. But by the 20th inch of say a 22 inch barrel, gasses from the same amount of pressure at the chamber now have to fill an empty cylinder that’s
20*3.1416*(0.284/2)^2 = 1.267 in3
Vs.
20*3.1416*(0.308/2)^2 = 1.490 in3
That 0.223 in3 difference is 17.6% less space for the same amount of gas that created the equal pressure at the chamber. Doesn’t that therefore up the pressure on the 7mm for the last 2” of barrel?
Caveat: I know enough math to be dangerous but not enough to be good. Finance, not engineering
Wait, I’ve got a question on this: doesn’t the pressure increase in the 7mm relative to the .308 as it moves down the barrel? Thus offsetting the reduction in area and keeping force relatively constant?
If you have the same chamber pressure, then yes you get more force because of the increased area right as it leaves the chamber. But by the 20th inch of say a 22 inch barrel, gasses from the same amount of pressure at the chamber now have to fill an empty cylinder that’s
20*3.1416*(0.284/2)^2 = 1.267 in3
Vs.
20*3.1416*(0.308/2)^2 = 1.490 in3
That 0.223 in3 difference is 17.6% less space for the same amount of gas that created the equal pressure at the chamber. Doesn’t that therefore up the pressure on the 7mm for the last 2” of barrel?
Caveat: I know enough math to be dangerous but not enough to be good. Finance, not engineering
I think what you’re asking is if the 7-08 will equal out with a .308 due to a higher pressure at the end of the barrel but there are other factors at play, and all other factors equal, unless the propellant is still burning, higher muzzle pressures and temperatures for the same charge weight mean a cartridge that didn’t effectively use all of the available energy within the case. That usually shows up as more noise and a bigger muzzle flash.
With Fuels or propellants, you have the added component of heat energy that greatly aids in doing work to the bullet so there are many other factors that play into it and make it a horrible mess to calculate (why quickload exists). Similar to an engine, Combustion Can be put into three categories; constant volume (all the bang and pressure rise happens without the bullet or piston moving at all), constant pressure (the bang continues the whole time the bullet or piston is traveling), or a mixed system ( some combination of a Big Bang that happens quickly over small initial volume increases and then continues to burn for a little while after). Most combustion in engines or barrels (same thing from a thermodynamic stand point, just the piston isn’t as reusable) happens in Mixed conditions. gasoline engines and pistol powders in short barrels are primarily constant volume combustion with lesser contributions from some fuel burn after the initial pop. Diesel engines and some long burning rifle powders are primarily constant pressure combustion where the fuel continues to burn a decent way down the bore keeping pressure nearly constant until the exhaust valve opens (bullet leaves the muzzle).
A rifle cartridge sitting in the chamber can be thought of as the squish volume of an engines cylinder head. And the length of the barrel (minus the case length) can be thought of as the swept volume the piston travels through so we can effectively calculate the compression ratio (Realistically expansion ratio) of our rifle “engine”. We know that thermodynamic efficiency of any engine is greater when there is a higher compression ratio and with a gun (or a diesel) we don’t have to worry about that pesky knock or ping condition you get with gas engines when you crank the CR up too high. With a bolt action, we don’t have to worry about how much work it will take to compress the charge in the next cylinder over so there is no harmful side effect to a higher compression/expansion ratio.
given that .308 and 7-08 have almost the same case capacity(squish volume) and we’re looking at the same powder type, the .308s larger bore area means a higher barrel or swept volume (as you already correctly calculated) and by default a higher expansion ratio(ER=((chamber volume+barrel volume)/chamber volume))).the .308 will allow the same combustion gases at the same pressure and temperature to expand closer to atmospheric pressure meaning that more work is done to the bullet and the gas temperature leaving the muzzle will be lower ( part of the reason for it’s good barrel life) because the higher expansion ratio allowed the gases to expand more and cool more. The .308 can do more work on the same weight bullet over a given distance ( barrel length). In essence, with the example of a 150 grain .308 and 150 grain 7-08, and using the same powder charge, the 7-08 would need an additional 3.6 inches (~23.6”)of barrel to get the same amount of work out of the same powder charge as a 20” .308. Again though, the 7-08 should eat the .308‘s lunch downrange due to higher BC despite the slightly lower MV, but that’s only useful if you’re shooting beyond 6-700 yards. Theres enough variation in barrel speed from the same manufacturer that it may be difficult to test so its probably best to just have three or four rifles chambered for each cartridge just to make sure your bases are covered.
For me, it would be the availability and lower cost of ammo. I cant find SAUM ammo, I have to have it made and it's $90/box. The 7 PRC should have availability from Hornady just like the 300 & 6.5 at half the cost.
Wait, I’ve got a question on this: doesn’t the pressure increase in the 7mm relative to the .308 as it moves down the barrel? Thus offsetting the reduction in area and keeping force relatively constant?
If you have the same chamber pressure, then yes you get more force because of the increased area right as it leaves the chamber. But by the 20th inch of say a 22 inch barrel, gasses from the same amount of pressure at the chamber now have to fill an empty cylinder that’s
20*3.1416*(0.284/2)^2 = 1.267 in3
Vs.
20*3.1416*(0.308/2)^2 = 1.490 in3
That 0.223 in3 difference is 17.6% less space for the same amount of gas that created the equal pressure at the chamber. Doesn’t that therefore up the pressure on the 7mm for the last 2” of barrel?
Caveat: I know enough math to be dangerous but not enough to be good. Finance, not engineering
You didn’t ask, but I got bored and ran some numbers on your question. I considered the firing event to be constant volume, both rifles have the same barrel length and same initial chamber pressure, I did use the correct case capacities for each cartridge since there was a small difference. I’m considering the heat energy to be zero for both cartridges and analyzing this like a super high pressure air rifle (wrong but equally wrong for both). By dividing the barrel into 1/16” increments we can make a bunch of discreet systems and get close to a realistic model of how the pressure gets used to push the bullet. Since I negated the heat energy component, It’s not a perfect model but it’s good enough for comparison.
The 7mm does keep slightly higher pressures for slightly longer, but at each interval, the larger area of the .308 causes a higher force, meaning that the .308 will outpace the 7mm until it leaves the bore and all that pesky drag catches up to it. I checked the overall sum of the work done to each bullet over the length of the barrel and at all barrel lengths the .308 did more work with the available pressurized volume (case full of boom) than the 7 mm. With a 16” barrel the .308 did about 10% more work and with a 36” barrel, the .308 still did about 8% more work. As a final note, it looks like you’d need either need an additional 5900 psi (10% increase in pressure:yikes!) or a 28” 7mm barrel to do more work to the same weight bullet as done by a 20” .308.
what did I learn from this? firearms are more fun than work, internal ballistics is every bit as complicated as I thought it was, short barreled.308s are awesome, and most importantly nobody cares because we all have our favorite cartridge and we cling to them like our lives depend on it. Time well wasted.
When I was a young child, a family friend we hunted with let me look inside his gun safe. I was astounded at how many were in there! I asked him, how do you have so many guns? I'm from a family who usually has a bird gun, a deer gun, and MAYBE a handgun, so I had never seen anything like it. He told me never to sell a gun, and eventualy you have a huge colection.
I took that one to heart.
Oh very cool, thanks man. Really, I enjoyed the hell out of this post and the earlier one (esp the barrel length equalizer calc — may save a generic version of that formula in an excel file for firearm calcs...)
Fascinating: so there is an effect but not nearly as much as the rear frontal area advantage of the .308, very cool. Yep, .308s really do rock in shorties, ha.
Time well wasted indeed!
Oh, that barrel calc is proportional, right? So I can take the area of a projectile and given charge will produce the equivalent of 3.6” more barrel needed for 7mm to reach .308 velocity out of a 20”, right?
Edit: for example, I’d need a 32” barrel to get a mythical 150gr bullet in a .243 to the same velocity as a 150gr .308 with the same charge, right? Not counting minute case volume differences between a .308 and a necked 6mm version (aka the .243 Win) of course?
Edit: for example, I’d need a 32” barrel to get a mythical 150gr bullet in a .243 to the same velocity as a 150gr .308 with the same charge, right? Not counting minute case volume differences between a .308 and a necked 6mm version (aka the .243 Win) of course?
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It’s probably closer to a 38-40 inch barrel to get the same energy usage from the same powder charge in a .243” bore, total work doesn’t quite scale linearly with barrel volume because there is an exponential factor within the isentropic expansion equation that’s why you see an 18% increase in area built only a ~9% increase in total work. The exponent shows up As the ratio of specific heats of the combustion gas, shown as K. My guess is with a barrel that long the bearing surface friction might start playing a bigger part. So .243 will usually be a much lighter bullet and maybe a higher pressure to account for it. A .243 still gets excellent drag properties and with good bullet selection can make a big gooey wound channel with a lot of wasted rib and shoulder meat. From this, I’m certain that each chambering has a mathematically optimal bullet weight/profile, twist rate, powder charge, powder type and barrel length that will get the most energy out of the cartridge and keep the most energy down range. This could all be calculated to a gnats ass but calculations always make assumptions and the the devil is in the details. I’m also almost certain that the optimal package would be with the most expensive components, it would be with some finicky powder that didn’t like to play nice when the weather changed, and testing wouldn’t back up the theory because the real world likes to eat hopes and dreams. The game is all about compromises but Newton and Bernoulli always win.
200 meters (1000 vs 800).
And a bit heavier bullet (b-32 vs m61)
weight shouldn’t be an issue, as well (16.5 lb vs 27.6 lb)
I've had a 308 for a few years now. I skipped the 6.5cm because for my use and range there wasn't enough difference for me to jump on it. But, when I first heard about the 6.5 prc. Well I just had to have one. I finally picked one up a week ago. Will be trying it out later today.
Dear God, we’re still hung up on the PKM. Yes, 800 vs. 1000 m is the max EFFECTIVE range while the max range I quoted is 3725 vs. 3800. Crabbyballface or whatever his name was declared that our designated marksmen, machine gunners and snipers werent capable of engaging PKM and DSHK crews who were a mile away because they were using a puny .308 so in that case, I don’t think differences in EFFECTIVE range apply. Even if it did, the 240L has a MER of 1100 meters. My suspicion is that the PKMs range advantages are removed by the addition of a laser range finder and modern optics to the 240 compared to the 1897 Nagant sight that most of our middle eastern aggressors are using. The heavier bullet no doubt aids in extending range, but the weight of the weapon is a two edged sword, helps for the hike in but there’s no way it’s as stable under recoil unless you’re also bringing the tripod, which is more weight. Does it make sense that we are paying 20x more for a heavier weapons system made from laser welded titanium when our adversaries have a lighter, competitive product using stamped steel from recycled washing machines riveted together? Probably not, our government is wasteful with our taxes. Is the new .338 NM going to be a big improvement? Probably so, but it too will have it’s drawbacks, probably cost and maintenance. None of that changes shit for the topic at hand because most people don’t own a PKM or an M240L (poors, I know). Is the .308 a good round for current PRS matches where you compete against a $5500, 27 pound DBM fed 6 brX that doesn’t need any sight corrections out to 800 yards, no, horrible choice. Is it a good round for hunting pigs at night while suppressed or deer/bear/moose/buffalo/elk/ squirrel, at daylight or disintegrating concrete block walls, stopping car engines at check points, shooting out to 1200 yards for fun, buying lots of cheap Ammo to stockpile, service rifle matches, F/TR matches, and generally enjoying shooting? Yes.
You don't shoot against the 6mms. You are in Tac class with the .308 competing against other .308s and .223s. Also with the right bullet they hit to 1200 pretty easily. Of course with a good wind call that is
You don't shoot against the 6mms. You are in Tac class with the .308 competing against other .308s and .223s. Also with the right bullet they hit to 1200 pretty easily. Of course with a good wind call that is
Understood, but the same people who bitch about the .308 being outdated bitch about there even being a Tac class, somehow it offends their honor that “ancient” cartridges can still shoot well, they don’t want us around. I’m just making the point that you wouldn’t race a Ford raptor (.308) against a mclaren p1 ( 6 brx) on a road course and expect to win (though it may keep up pretty well). But you also wouldn’t run a mclaren for the Baja 1000, or put a trailer hitch on it and tow your boat, or carry four other people and a bed full of tools to a job site, or go 500 miles without something needing a $10,000 repair. What works for PRS or benchrest shouldn’t be the sole driving force behind every decision in riflery. Especially since we aren’t limited by the number of rifles we own, if you’re out of room in the safe, buy another safe, and then you can fill that one up too.
jphil108 - as a .308 kinda guy, I was jiggy with your caliber analogy until you mentioned the 4-letter F word
Haha yeah, I forgot that the second most divisive topic on here besides cartridge choice was truck brands, that’s another choice where it really seems to matter what you grew up with. I’ve liked every Ford and Chevy I’ve driven but I always felt that Cummins wasted good engines by wrapping them in Dodges.
Well we all know the Honda Ridgeline is the best truck so just fight over second. LOL
Well we all know the Honda Ridgeline is the best truck so just fight over second. LOL
Car/truck hybrids aren't that blasphemous. Cause you see at bible camp we created a flow chart or what ever that proves that the honda ridgeline is a gift from god Rob01.
Then again, god sent his only son to die on the cross to forgive our sins and the way we repay him is by developing shitty cars like the ridgeline.
Now on topic the 308 AKA the desolate one, the spoiler of virgins and the master of abortions, we should all abandon Lucifer's cartridge and bask in the holy glory of the 6.5 Jesusmore.
Which would you rather have. All in the same class.Well we all know the Honda Ridgeline is the best truck so just fight over second. LOL
Ford ranger
Chevy colorado
Dodge dakota
Honda ridgeline
Toyota tacoma
Tacomas are much better as trucks.
Now back to the topic at hand.
I'm pretty sure the ridgeline is the 9mm vs 40 vs 45 debate and saying 7.65 French long is the best for a factory loaded defensive round.
Ridgeline. Toyota might be better as a truck if needing a truck to ride in sand dunes but Ridgeline is much more comfortable to ride in, better mileage, tows the same and costs less.
the only sensible post in this whole thread.....
I think hunters would laugh at your question. If it's out of style for you, it won't be hard to get rid of
What's your barrel length? Since you have so much R17, you should try the really heavy stuff. It's been done before, a 208 pushed to 2650ish if I recall correctly.
I only say this because of reading how you decided to go 300WM after not liking the 308 so much, then 6.5, then 6, etc.
Pushing the 308 might be fun and you might just realize it's no slouch after all...
11 years over.. wow. You have been deployed longer then anyone I’ve ever heard of. Glad you are back safe.
Oh I see. My bad. I guess that makes sense. I have 3 years of deployments then. Take care. Thanks for your detailed posts. Good knowledge
Solid post, what then, would you say is your ideal round? after running the numbers like I spoke about above, I modified my spreadsheet so I could compare rifles and loads, its interesting to play around with bore diameters, case capacity, and barrel length to see how it effects muzzle velocity, recoil, and remaining pressure at the muzzle. All theoretical But the model is close to the data I have compared it against. it and ballistic AE are leading me to see a real happy spot with a .284 Win. Or a .280 AI running heavies out of a 26” barrel. I’ve also found a real happy spot with an 18” .308 running 125-130 gr. Bullets for local hunting, looks like I can keep it at good velocities for nice expansion under 350 yds and its draggy enough that it shouldn’t go very far beyond that, minimal sound and recoil, cheap, no appreciable barrel wear, and a nice compact package.
Well to follow the previous direction of this thread; “you’ll never kill anything with a .22 and a .17 hmr shoots flatter and has a better BC and that’s why the afghanis are better squirrel hunters than we are.” I’m kidding. What’s your furthest animal with the 6.5 Grendel and did it perform well?
