School me on case design..

[rscott5028]

I don't think progressive, or gain twist barrels are a good idea. A century ago, they worked well for lead bullets, but these days I dont' believe they'll consistantly do well. Comments from top match rifle barrel makers reflect their knowledge that the biggest single flaw in barrels that causes accuracy is an inconsistant twist rate. When the twist rate changes, it moves jacket material around on the bullet and can easily upset its balance. Unbalanced bullets just don't shoot accurate.

I also hear gain twist (since it's continualy morphing the jacket as you point out) may tend to increase copper fouling which can already be a problem with the longer bearing surface of very heavy bullets.

Bart - not to hijack the thread, but would you recommend IMR4064 vs Varget for 308 Win pushing a 175gr bullet?

thanks
richard

 

[Bart B]

Bart - not to hijack the thread, but would you recommend IMR4064 vs Varget for 308 Win pushing a 175gr bullet?

Thread hacking is authorized if its for a good cause.

So, 'cause your a good guy, here goes.

IMR4064 under 168's, 180's and 190's in a .308 Win. case probaby won more matches and set more records than all other powder-bullet weight cominations used when that case was "the" one to use in high power matches. Therefore, I predict that a 175 would shoot somewhere in the middle of the sub 1/2 MOA test groups at 600 yards the other three produced (perfect hardware assumed). I suggest 43 grains as 44 was the favorite for full powered 168, 43 for the 180's and 42 with the 190's. The 175 is close enough to 180 to work just fine.

 

[rscott5028]

Thread hacking is authorized if its for a good cause.

So, 'cause your a good guy, here goes.

IMR4064 under 168's, 180's and 190's in a .308 Win. case probaby won more matches and set more records than all other powder-bullet weight cominations used when that case was "the" one to use in high power matches. Therefore, I predict that a 175 would shoot somewhere in the middle of the sub 1/2 MOA test groups at 600 yards the other three produced (perfect hardware assumed). I suggest 43 grains as 44 was the favorite for full powered 168, 43 for the 180's and 42 with the 190's. The 175 is close enough to 180 to work just fine.

Thanks Bart B. Put me on the pre-order list if you ever write a book about this stuff.

 

[JimBoIHN]

Just keep the questions flowing - he's writing a book right now. So, Bart, the Lapua .3008 Palma switch to small primer is for slowing the powder burn? Might they also do it with other calibers? So, what other cases utilize small primer and is it for the same reason? You mentioned one you used.

 

[Bart B]

Just keep the questions flowing - he's writing a book right now. So, Bart, the Lapua .3008 Palma switch to small primer is for slowing the powder burn? Might they also do it with other calibers? So, what other cases utilize small primer and is it for the same reason? You mentioned one you used.

Jim, yes, I think small primers in a .308 case was done to slow the powder burn at the beginning. That would make the pressure rise slower and make the bullet more gently start into the rifling. The objective was probably to distort/deform/upset the bullet as little as possible.

I've only used the Remington .308 case with the small primer pocket. I'm not aware of any other 30 caliber case that does this except for the Lapua one. And magnum primers have to be used. I and others have tried standard primers and they cause hang fires most of the time. Which tells me cases larger than the .308 may have problems with magnum small rifle primers.

Some people on the US Palma Team use magnum large rifle primers in their .308 cases fired in Palma rifles. Wolf primers (made in Russia) are a favorite in high power competition 'cause they're quite uniform. And they don't deteriorate over time like virtually all others do; the chemicals change and old ones don't detonate as uniform causing greater velocity spreads and often lower speeds of muzzle velocity.

 

[edge]

Quote:
Originally Posted by Bart B
SNIP.

Another very important thing is, the most accurate cartridge cases typically hold one grain of powder for each square millimeter of the bore's cross sectional area.


SNIP.

Are you talking about water capacity or the actual powder weight?
If actual powder weight would you search for a powders bulk density ( within the appropriate burn rate ) to maximize this relationship?

Interesting concept that I never considered before.

edge.

 

[Bart B]

asks me about case capacity.

Are you talking about water capacity or the actual powder weight?
If actual powder weight would you search for a powders bulk density ( within the appropriate burn rate ) to maximize this relationship?

Looks like I forgot to answer this before. Sorry 'bout that.

It's the actual powder weight. And I'd use a medium speed powder for the case size. I've never searched for a powder's bulk density but they do vary. With the IMR extruded powders they're pretty much the same weight for a given volume as far as I could tell. Never did any "exact science" stuff to compare any of them.

It's possible and does happen that over bore capacity cases (those, to me, that hold more powder than bore capacity is) can deliver great accuracy. And come very close to what those cases at bore capacity will do. But the powder choice and exact charge weight may be more critical.

 

[edge]

Thanks for the reply

I shoot the equivalent of a straight case with 100% filling.
The reason I asked the question was that in a bottleneck case different powders can have drastic changes in density.

Here is 100 grains of N110 on the left and 100 grains of Lil'Gun on the right.
Both powders that I have used in the past.

Thanks again for your insight on this topic.

edge.

 

[JimBoIHN]

OK, Bart, let's get this book written. Here's a question about barrel length and velocity. As I understand it, longer barrels substantially increase muzzle velocity, so if a slower burn is generally desirable, why wouldn't target shooters utilize very long barrels with slower burning primers and powders? I had been under the impression that longer barrels led to more harmonics? and decreased performance. Are you familiar with the +1 BC bullets that are just waiting for a gun capable of shooting them, and have you considered a solution?

 

[Bart B]

Here's a question about barrel length and velocity. As I understand it, longer barrels substantially increase muzzle velocity, so if a slower burn is generally desirable, why wouldn't target shooters utilize very long barrels with slower burning primers and powders? I had been under the impression that longer barrels led to more harmonics? and decreased performance. Are you familiar with the +1 BC bullets that are just waiting for a gun capable of shooting them, and have you considered a solution?

First off, I'm convinced that the slower a powder burns, the more muzzle velocity spread there is. This applies to powders slower than IMR4350 which is about 2/3rds down the list going from fastest to slowest powders. And to cases holding no more than about 80 grains of it. Increased velocity spread causes more vertical shot stringing.

Second, "harmonics" is not the correct term and may be confusing some folks. A barrel has a fundamental, or resonant, frequency it whips (vibrates) at. A harmonic is just a multiple of that frequency. The greatest change in the bore angle at the muzzle happens at the fundamental frequency. The harmonic multiples are much lower in size and contribute virtually zero to any accuracy improvements or degradation.

Third, every barrel whips the same for each shot regardless of what load's used. It's like smacking a guitar string with a ball peen hammer; it makes the same note (very loud) as barely plucking it with your pinkie (very soft). Rifle barrels respond the same way. So how much a barrel whips has little to do with accuracy as long as it whips the same for each shot. But the timing of when the bullet leaves is important....so use the right one with the right amount of the right powder.

 

[Trickymissfit]

*6BR will push a 105 grain bullet to just about 3100 fps according to one guy in P.S. I suspect he's also hitting close to the 75K psi mark. My guess is that 2800fps is a very hot load

* a 6xc will do about 2850fps safely, but still a max load.

* my 6/250AI (similar to the 6xc but improved) will touch the 3100 fps mark, but the loads are also pretty hot. 3000fps is much better

* a .243 should be about the same as the 6/250AI as cases capacity is similar, but for some odd ball reason the 250 case seems to be more efficient. The 6XC, 6/250, 6/250AI, .243, .243AI all suffer from too short of a neck length unless the throat is pretty long. What happens is that the bullet base ends up being seated into the shoulder area, and you thus loose capacity

*6mm Remington has a much longer neck length, and the bullet will not be seated into the shoulder. The books list the 6mm remington at about 100fps faster, but that's crap. The round is much closer to 175fps. Ackley said that the 6x57 case was about all the 6mm bullet could handle without going into an overbore condition. A close look at loading manuals will tell us that he was very right. A 6mmAI is really only about 150fps faster than the generic 6mm round.

* then there is the 6BG. This little round is like a 6BR on steriods! It will push a 105 grain bullet to almost 3000fps and still be very safe. Has a .35" long neck with a 30 degree shoulder. Barrels should last a long time, and you won't use nearly as much powder to go just as fast. Plus it's a very tight grouping round like the 6BR.
glt

 

[Bart B]

*6BR will push a 105 grain bullet to just about 3100 fps according to one guy in P.S. I suspect he's also hitting close to the 75K psi mark. My guess is that 2800fps is a very hot load

That's about 61K CUP; about half way between most blue pills and normal loads.

 

[Trickymissfit]

That's about 61K CUP; about half way between most blue pills and normal loads.

I didn't read the articles about doing 3100fps in a 6BR, but most data I have is bits and pieces from here and there. The only serious pressure data I have for a bullet in that weight range is with AA2700 @ 2700fps. That was with 33.0 grains of powder in a compressed load with a 107 grain Sierra for 54.3K psi. Hogdons does publish pressure data as well, but they don't do the 105 grain bullet is my books, and their pressures are way down there. So your probably right about the 60K psi. Whatever it be the 60K number is way over the 52,000 cup of pressure considered max. 52K cup of pressure factors out to 62K psi, and could be dangerous. I can't say much for this data as my 6BR has a 1:14 barrel, and is throated for 70 thru 80 grain bullets. A different animal all the way.

By the way 61K cup's of pressures is about 75.6K psi, and a trip to the hospital just waiting to happen
gary