284 case design issues (relevant for all 284 derivatives and wild cats)

[TonyR]

The grip of the case body on the chamber walls is very real and makes an important contribution to containing the pressure. I learned this the hard way when I was shooting some light loads at 600 yards when it started to drizzle a little and the cases got wet. Little did I know so I just kept shooting. Hard bolt lift and they all came out looking like they had been "proof" loads. I'm glad they were on the light side to start with or I might have had more than beaten up case heads.

 

[camac]

The grip of the case body on the chamber walls is very real and makes an important contribution to containing the pressure. I learned this the hard way when I was shooting some light loads at 600 yards when it started to drizzle a little and the cases got wet. Little did I know so I just kept shooting. Hard bolt lift and they all came out looking like they had been "proof" loads. I'm glad they were on the light side to start with or I might have had more than beaten up case heads.

To add a little weight to that story is my experience once again with the original 6.5*284 chamber. When I rechambered the barrel but roughed up the chamber a little (the original was baby b__ smooth.) I did gain significant velocity with even shortening the barrel but could up the charge significantly with less case head pressure sgns. Also there are many stories about the difference between leaving sizing lube (or even oil) on cases and removing it.

 

[RonAKA]

I think one method of determining the effect of the cartridge shape is to imagine a cut right through the barrel and cartridge just ahead of the head. Stresses on each side of the cut line have to be equal and opposite, or the gun blows up. You have a 60,000 psi gas pressure on both sides of the cut, and acting over a large area. Next you have a very thin ring of brass, that really can't take 60,000 psi. The stress in the brass will be equal to the gas pressure, and the brass is saying "I give up, and I am Yielding". The barrel is next and in proportion is massive, and it is holding essentially all of the stress caused by the gas pressure. It is not yielding (I hope!). My thoughts are still that the brass is a bystander in the process. To the extent it can yield, it will yield, and resist essentially nothing.

I think where the differences in cartridge shape can come in are in how the brass flows ahead of this imaginary cut line. A steep shoulder and relatively straight body probably contains the brass flow, at least in the body area. It does not have any easy place to go. A rough chamber wall probably resists this forward flow some as well. On the other hand, I would expect a gently sloped cartridge would flow brass forward more easily (it really can't go back), and the case would grow more with each shot especially if the pressure is high.

 

[camac]

Ron, I think it can go back. It can go into the ejector port, head expansion, primer hole and squashed against the head. This is what we mean by head flow issues. It can only go "out" to chamber wall but can flow forward (case length) and back (head bass flow - out etc). You could theoretically under enough pressure force a lot of brass back into the extractor slot, and fill in all the cavities around the bolt head. For every action there is an equal and opposite reaction. for things to go forward there must be something going back..At least I think >>>??? In principal you are correct if the "chamber contacted all the shell everywhere and there was none of these gaps at the head. The case heads rarely match perfectly in diameter to the bolt either.

 

[RonAKA]

Perhaps to resolve your concerns about the head design of the .284, you should make a wildcat (at least I think it would be) by taking a 6.5 Remington Magnum and machine off the belt. Increase the shoulder to 40 deg.. Should have about the same or a little more powder capacity as the 284, but with a more sturdy head. Without the belt you could headspace it like a regular cartridge. 6.5 Belt-less Remington!

 

[15Tango]

Camac,

I think with your design idea here you may have to keep your shoulder pretty tight in the chamber for headspacing or you might separate case heads with the 'improved' design. All that pressure is normally taken up by the bolt face but in your design thrust would be lessened by the body grip on the chamber walls - all that pressure is still there inside the case so it would be pushing the rear of the brass backward thinning out the wed area on each firing. In theory it could end up separating like belted magnums do. If you found it to be a problem it would just be a die adjustment but it's something else to consider.

Maybe the guys with Shehans can chime in on if they've experienced this issue at all or if I'm just totally wrong.

Wayne

 

[TonyR]

The only issue I've had with the 284 Shehane has been loose primer pockets. They don't fall out, but they are much easier to seat after one firing. Another shooter I respect has suggested a few firings with reduced loads will harden the case heads enough to stand higher pressures later but I haven't bee in a position to test any of that.

 

[15Tango]

Tony,

Are you using Norma brass? I thought that stuff was supposed to be pretty hard? That'd be really frustrating after ONE firing with how much we spend on brass and how much labor goes into prepping the stuff with cutting flash holes, primer pockets, necks, trimming necks . . . I haven't owned a .284 derivative yet, I wasn't aware that they have this known issue with primer pockets loosening up.

Wayne

 

[TonyR]

Wayne.
I'm using Lapua exclusively. I bought one of the guns as a 6.5X284 and it came with a bunch of Lapua and Norma brass. I sold the Norma and bought more of the Lapua. I wouldn't make too much out of this primer pocket thing. I've got 7 or 8 firings on one set of 284 Shehane cases and they are still going strong; however, a friend that shoots a 30 BooBoo based on the RWS 8X68S case is still going strong after 40 firings. If I could figure out how to match or improve the overall performance with a significant improvement in the brass life, I would probably try something else when this big set of prepped cases is done. But brass life is just one more variable to consider when choosing a cartridge. I did shoot out about half of the first barrel figuring out how to load for it, but subsequent barrels have just required a tweeking of the powder charge back to the correct velocity. In addition, I shot most of my fireforming rounds in 600 yard competition, so even that knock on brass life is not as bad as some make it out to be. Overall, I am very pleased with it, and it has done very well for me in competition.

 

[holstil]

This thread has my interest. I was also thinking about the 7mm chamberings. I shoot the 7 shehane. I shot the best group ever from the gun with the lightest loads, I just can't get it to repeat. I'm not sure if my issues are weather or brass or both. I cut my str.taper barrels down to around 24" to make weight for LV. I am currently at 49 grs with 4350 and speed has not diminished much at 2860fps. Don't know why. I used two diff. chronys to check.
So now I am thinking if I can get that type of speed with that much chamber/powder, I'm thinking 7DAsher or 7-08 Ack imp. I would like to understand more about burning coefficient. I find myself thinking what would be better ballistically, a 7mm something or other shooting the 180 at 2800 to 3100fps or the 6 DAsher or 6br shooting the 105,7,8 at 2900-3100? Got to look at more numbers.
When ever I get to thinking about how to make something better it seems a toss up of barrel life, BC and cost of components not to mention recoil and accuracy. If there is a way to make the 7mm even better, I'm all ears.

 

[TonyR]

Jim.
You could give Norma MRP or RL-22 a try in those short barrels or IMR7828SSC in anything over 28 inches with the 180s. I think you will see higher velocities with less pressure and no loss of accuracy with the 284 Shehane.

 

[holstil]

One other thing in regards to case taper. Why is it that you can have very little if any clearance in the neck that has no taper and does not stick? How much spring-back effect goes through out the case? Maybe not as much on one end as the other? What happens if you have a case with say .002 taper over 1.75 and a die that allows .001 in resize dim?
These are probably basic questions, but I need to get up to speed.
As of now, I think the shape of the case means a lot when I think about what happens in micro seconds. Maybe how the design of shape charges (explosives) got their start?
Jim

 

[holstil]

Tony, thanks for that info. I can use all I can get. I'm trying to take advantage of the sick 7mm BC as far as speed goes for when it's windy. To me, accuracy trumps all. If I were to get a 7mm to shoot like my DAsher, I might be a one platform kinda guy.
Jim

 

[TonyR]

Jim.
I think that the need for case body taper versus neck taper has to do with the relative surface areas of the two parts of the case. All of the necks we deal with in target shooting are less than three eighths of and inch in diameter and length compared to case bodies that are a couple of inches long and often more than a half inch in diameter. The minimum tapers seem to be about 0.017 inches from the head to the beginning of the shoulder which doesn't seem like much but it is enough to break them loose.

 

[camac]

Back again guys - more on the body taper. I remembered I did an interesting chamber recently. I made a 6x47L with a 6BR reamer not long ago. More as an experiment than anything else. When this is done, the back 8mm of shell (roughly 1/3 inch) has a parrallel taper. I went back to look through the load development data and noticed we had very little pressure signs except for the "crack" of the bolt at the top of the lift. More interestingly we acheived some pretty awesome velocitities for a 26 inch barrel at this stage (was a 12 twist but 3750 fps for 70 gr and very accurate). My thoughts now are indeed the straight taper relieved some case head bolt pressure and chamber pressure may have been high without the usual signs. (Oh and we sized down large rifle Lapua 308 brass instead of normal 6.5x47L).

I can imagine the two bolt lift issues could be quite separate and affected differently at both ends of the taper range.
1) If you have "slippery shells" or significant taper this would expand shell more in length and cause a little more "headspace" jam. this makes bolt stiff from start. With less taper, the shell expands less in length (not pushed hard back on bolt) so only sees the tight "crack" at top of throw where the shell is being released from chamber.

 

[holstil]

Tony, agreed, .017 is not much and my reamers are less than that at .010 taper on my 284. I ran a tight neck for a while, I don't thnk it was touching the neck walls after fired. The shells came out like .0005 smaller.
I rough my chambers with 220 grit also. I don't get any stickness due to that. I do get some stick at the top after one or two hot loads though.
Jim

 

[RonAKA]

This concept of the cartridge shape affecting the bolt thrust is just not passing the smell test for me. I find it worthwhile when things just don't smell right, to take an extreme case and work it through to see what you get.

Here are extreme assumptions using some real world dimensions and pressures. Consider the cartridge a short distance in front of the head.

Diameter - 0.5"
Barrel OD - 1.5"
Brass Thickness - 0.025"
Pressure in case - 60,000 psi

The total force that has to be handled one way or another is easy to calculate. It is just the pressure times the area of the cartridge and works out to be 11,775 lbs or nearly 6 tons.

This force is resisted by the axial stress in the cartridge brass and the stress in the barrel steel. Lets assume the cartridge expands under the internal pressure to fill the chamber and welds itself to the cylinder wall. It doesn't of course but this would seem to be an extreme assumption.

Next the bolt has to deal with the pressure force. Through the bolt lugs it stresses the barrel, and the brass case now "welded" to the barrel. They both stretch an equal amount and stress the brass and steel in an axial direction, until this stress equals balances all the forces. Since the brass stress does not transfer to the bolt, the total force is equal to the barrel stress times the barrel area plus the brass stress times the brass area. The barrel stress or total minus brass stress is what the bolt has to deal with.

This can be calculated knowing the modulus of elasticity of steel and brass, the areas, and the assumption that the brass and steel are welded together so they must stretch the same amount.

I ran those calculations and found the force resisted by the brass given these assumptions was 160 lbs, with the rest resisted by the barrel and essentially the bolt, or about 11, 615 lbs. Or in percentage terms 1.4% brass and 98.6% bolt.

So again if you took the extreme in the other direction that the case totally slipped against the chamber, and held nothing, then force on the bolt would go up 1.4%. I'm thinking in either case I want the bolt lugs totally intact, if I'm going to pull the trigger.

I'm open to any thoughts that these assumptions are not correct, and if you really must I can show you the math (hurts my aging head though...).

 

[camac]

Ron, Thanks for the calculations. I see where you are coming from now and will put my head to it (as you said this will hurt) and come back later. A lot of gunsmiths recommend this practice and a lot of observations say it is true but as always sometimes things turn into chinese whispers and the effect can get grossly exagerated. My first thought and only thing I can suggest at this stage is the webbing of the brass is much thicker than your assumption and indeed the brass thickness tapers down to 25 thou further up the chamber. This aside you may well be in the right ball park anyway. Assuming your calculations are correct but the webbing offers 2-3 times the support, 4-5% is still a lot to add on top of these loads and can make the difference between brass deformation and not.
Perhaps a simple test would be to lube up a case or two (at the maximum load level) with some super slippery stuff and look for differences.
Or perhaps the other way to get a feel for the grip a shell can impose is to look at some other things. Have you ever had a shell stuck in a FL sizer because of not enough lube (I hate to admit it but I have) I can tell you I was swinging off that rock chucker press to get it out. Neither the rim nor the shell yielded before it came though. I certainly know a stuck shell in the chamber can also take some removing.

 

[TonyR]

While Camac is developing his headache, maybe we could turn to the other end of the case? I have been testing one of the 7mm short magnums and another in the 280 Remington class and I believe that the shape of the case also influences pressure and velocity for a given amount of powder. I am sure there is a technical term for this, but I would call it efficiency. The manufacturers have been in love with the short fat cases, of which the Shehane is not one, but I have also had very positive experiences with some of the longer thinner cases. My gunsmith is one of the best 1000 yard benchrest shooters ever and he belives that the 280AI is the best 7mm case he has ever worked with. I would like to hear comments on this aspect of case design.

 

[RonAKA]

I based my thickness estimate on the 6.5x47 Lapua cartridge diagram at Steve's Pages. It shows a nice detailed cross section of the case, and I just eyeballed thickness based on neck thickness and scaled it.

The thing to keep in mind is that brass develops stress at about 1/2 the rate as it stretches. Compared to steel the E value of brass is 16 million psi, while steel is 29 million. So if the brass is "welded" to the chamber there is just so much it can do. It is not thick, and it is not strong (stiff), compared to the steel barrel around it.