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

[camac]

Ron - Still thinkin - still hurtin. Looking for some nice pictures to include but do believe you may be in the right ball park as said before. I also have a fe 6.5*284 cases cut in half so will take some measurements. At this stage I generally agree. The front part of the shell back to where it gets to 25 thou (oir so) can only hold somewhere in the ball park of what you say before yielding. But the web gets thicker and thicker and the base and web are designed to take these pressures (The base and head apply the full force to the bolt after all without being crushed into a blob). But as we get to the base there is less and less grip offered.

Tony, Yes and let's not forget our friend German who swears by his 30-06. A lot of guys also like the 6.5*55. I also consider the humble old 222 (+223, 222mag, 204 etc) as a long cartridge and these were extremely accurate and efficient cartridges. They did however never achieve the ultimate accuracy level of the ppc, BR case designs.
Let's face it we all want PPC accuracy in our 7mm cartridges. A 0.1moa edge might not mean a lot on a windy day but it is still what we strive for. The long cartridges can be efficient, can be accurate but whether another world record will come from them I am not sure (maybe in prone??). A couple of guys over here are trying to talk me into the 280 AI instead of 284 Shehane. I personally believe the 284 is a very accurate case design, it is also efficient. It just doesn't seem to be able to be pushed to the same pressures (even with quality Lapua brass).
If someone brought out a 284 Shehane with .500 head it could be a world beater....(needleass to say you lose that HUGE advantage of .308/30-06 bolt head.)

 

[RDavies]

Camac, if you build up some dies/reamers for the short 6.5/284, I would buy some off you to ease the cost.

Holstil, I also get best accuraccy with the lighter loads of H4350. I try the higher node (2920 fps) with all barrels I get, but always end up back to the lighter H4350 load (2800fps). As we move to a different larger ICFRA target down under, I might start trying the higher node again, otherwise, I am looking to a 7mm/08 or improved version, to hopefully get something more like a big Dasher (my new favourite cartridge).


RDavies

 

[holstil]

Are the questions at hand about case efficiency through how the case/brass reacts to the combustion or, how the powder is burnt and expelled through case design? Or both and more? I am interested to hear about either/all, I just did not want to get confused.
I was hoping to gain info on burn coefficient and the effects of case design.

Mr. Davies, I have heard from a few people that I might be wasting my time with this case because I will not be able to get to the next node accurately and that I should just shoot the 284. Maybe so, I am just trying to get the speed required to get the ballistic advantage over a 6 Dasher for the windy days. Not really trying to just go fast, that would burn barrels faster than I care to. Think how much barrel life we'll get at 2850+/-. If I don't get where I want to, I'll have Dave regrind my reamer into a ........, 7-08 express and try again.
Jim

 

[camac]

Jim, I think all those questions have been asked but the original thread was the case head design and body taper (can decreased body taper take the pressure of the small head of the 284?) Needless to say it has turned into an interesting discussion.

Ron I found a photo from my library (sorry out of focus) of the bottom of a LApua 6.5*284 shell. The case wall thickness tapers up from 50 thou to 25 thou over the bottom half of the shell.

To complicate I am sure it doesn't get a "full grip" (or "weld") at this point though but may add enough to the grip from above to be reaching close to it's max. - I am not sure??. I do know that the front, "thinner parts spring back a little better and it is the rear of the shell that gets stuck under pressure so am thinking it can add close to max grip under the high pressure loads -But It is without doubt beyond my ability to calculate.

If we followed Rons assumptions and calculations there may be in the ballpark of up to a few hundred lbs grip (MAX). But the rearward force is actually only the inside dimensions (0.400" diameter or 0.125 square inches) by pressure which is less thrust force than we originally thought (7500 lbs at 60,000psi). - So this could indeed be up to 4 or 5%. Certainly not enough to make up for the 13% loss spoken about earlier but could help a little.

But after getting that far I started thinking. It is more complicated than this. Why do we get more recoil from a heavier weight bullet than a light one. Recoil after all is a rough measure of this backward force on bolt and chamber. Theoretically chamber pressure can be the same for both. We all know the "for every action there is an opposite and equal reaction". I believe recoil is based more on momentum than energy and there is a little "jet" recoil added by the burning gunpowder (hence higher recoil with more powder burn). However, chamber pressure seems to be more defined by energy output (ie we roughly load heavy bullets and light bullets in same chamber to a similar energy for the same given pressure). Most of the recoil effect needs to act through the chamber and bolt as well. Without going into pages and pages of my ramblings - I got lost. I got to the point of thinking it is pressure over time thing and started working out it takes more of the same pressure a little longer to get the big projectiles moving but then got lost and gave up.

Summary of what I am now thinking.
I still believe chamber taper and "grip" adds a small part to it. Is it significant? It probably is not as much as I first thought but I believe it could be enough to help negate a little of the head deformation at the top end loads.

Please any one else that knows or works on this area please chime in and help out...

 

[RDavies]

Have people played with different finishes in the chamber? maybe a rougher finish near the rear of the chambers, where you want the grip, while still keeping it smooth along the rest of the length to help extraction?
Has anyone played with anything like this? I,m not sure how it would be done, maybe you machinists out there might have ideas?

 

[Steve Blair]

Chamber pressure is internal to the firearm and does not contribute to recoil directly.

The factors that contribute to actual recoil are:

projectile mass and acceleration
propellant mass and acceleration
muzzle pressure at projectile exit
firearm mass

Some factors that contribute to perceived recoil are:

stock design
manner in which the firearm is held
shooter mass

Internal ballistics are very complex. So complex that, when I was a member of my employer's team on the ultralight howitzer program with Picatinny Arsenal, Benet Labs and Watervliet Arsenal, the howitzer recoil impulse was characterized using empirical data. All other calculations were fully modeled. The above factors ignore the presence/absence of a muzzle brake which has a series of dependent factors associated with it, too.

 

[RonAKA]

I updated my spreadsheet based on the 0.050" brass thickness, and used the ID of the case for the total force.

Axial Force in brass - 175 lbs (2.3%)
Axial Force in steel - 7361 lbs (97.3%)

Stress in Brass - 2475 psi
Stress in Steel - 4487 psi

What isn't being considered in my calculation is headspace. If the front of the case welds to the chamber and then the head moves back due to headspace, I suspect stress in the brass goes much higher possibly to yield. Have to think about that one longer. It may significantly increase the load resisted by the brass, to reduce load on the bolt potentially to half.

Agree with Steve that recoil is a totally different matter of dynamics, not statics. It is basically the weight and speed of the bullet going one way, and the weight and speed of the gun going the opposite way.

 

[camac]

SGator thanks -I understand that with recoil. All forward forces (action) have an equal and opposite reaction (recoil). How that is felt can be different from stock weight recoil pad etc etc.
BUT my thinking is -the only way the rearward motion can be forced onto the rifle is in the vertical surfaces (ie case to bolt head) or chamber grip. This is enacted on the rifle by chamber pressure vs time. Chamber pressure is constrained in the radial plane but is released in the forward direction with bullet weight and escaping gases out the muzzle and in the other direction by pushing rifle back via case head and grip of case on chamber. I think of it as the bullet and escaping gases pushing back on the rifle via chamber pressure. I may be quite wrong here though.

Thanks Ron for the updated calcs. I must be missing something though -I don't quite understand how jumping from 25 to 50 wall thickness only put force up 15 lbs (160-175)?? BUt personally I believe we are in the right ballpark anyway. I think somewhere between 1-5% reduction in case head force may be possible but very hard to calculate. This is based on the fact that no one is magically coming out with double the velocity and it is rare to hear people loading to same pressure signs for a 300+fps gain in same barrel length which would roughly be 10%.

 

[RonAKA]

BUT my thinking is -the only way the rearward motion can be forced onto the rifle is in the vertical surfaces (ie case to bolt head) or chamber grip. This is enacted on the rifle by chamber pressure vs time. Chamber pressure is constrained in the radial plane but is released in the forward direction with bullet weight and escaping gases out the muzzle and in the other direction by pushing rifle back via case head and grip of case on chamber. I think of it as the bullet and escaping gases pushing back on the rifle via chamber pressure. I may be quite wrong here though.

Thanks Ron for the updated calcs. I must be missing something though -I don't quite understand how jumping from 25 to 50 wall thickness only put force up 15 lbs (160-175)??

The recoil thing has nothing to do with internal dimensions. It is just gun weight, and bullet weight/velocity. It is a conservation of energy or momentum thing. Every action has to have an equal and opposite reaction.

On the calculations keep in mind with the reduced force (.400 diameter instead of .500) total thrust is reduced to 7500 from around 12000 lbs. So while the brass takes more in proportion, there is less to take.

Don't underestimate the effect of headspace. I've not thought of a quick and dirty way of calculating it(yet). You may need someone like Varmint Al to run a finite element computer analysis if you want a realistic and accurate answer. My guess is that headspace is a big factor.

 

[Steve Blair]

Recoil is the result of the total accelerated mass and is not dependent upon chamber dimensions. The projectile and propellant both have mass and are accelerated to some velocity. Their total kinetic energy is ½MV2. At the point the propellant gases are released into the atmosphere, they quickly accelerate to about twice projectile velocity. In many small arms cartridges with typical bullet weights, propellant accounts for about 40% of total recoil. This will vary considerably with projectile weight, projectile velocity, barrel length, propellant weight and propellant progressivity. The combined energy, as Ron noted, reacts and accelerates the firearm mass and produces recoil. The total recoil is affected by firearm mass but not the contributing factors.

 

[camac]

The recoil is hard for me to explain. I think we are on the same page but I am not making myself clear. We all agree that the mass (propellants, gas, bullet) cannot be forced forward without pushing something else back (the rifle and shooter etc). A bit like a guy throwing a ball in space - The ball goes one way and the guy the other. It is the contact force between the ball and hand that forces each his own way. It is this force by time that gives the momentum. Momentum of the forward ball is the same as the backward guy. It is not forward energy but forward momentum that is equal (mv not 1/2 mv squared) that creates recoil. This is why heavy projectiles with same energy as lighter ones kick much more.

So the propellants, bullet gas goes forward, The rifle, shooter etc go back. It is the interaction between the two that I am talking about (or force between hand and ball) not the result. I believe the key to some of what we are talking about lies between. I am not concerned about how felt recoil is affected by weight etc. but on the force back on the rifle to create it.

What I am saying is the only way I can see these propellant gases and bullet pushing back on the rifle is via the case (case head on bolt and case wall grip on chamber wall (maybe some friction of gas on wall of barrel??). Before I go on anymore can anyone else suggest other rearward forces acting on the rifle that I haven't considered?? There may be some but I cannot easily identify them.

 

[Steve Blair]

Perceived recoil is affected by many factors including the rate at which the mass is accelerated. You must be referring to perceived recoil because total measured recoil is exactly the same if the reacting force is the same. If the total force on the rifle is generated sharply, rather than more gradually, the perceived recoil will be greater.

I have done a little work with muzzle brake development using a recoil dynamometer and a recording strip. (it was in the eighties) The total recoil could be identical but the perceived recoil very different.

 

[camac]

Perceived recoil is affected by many factors including the rate at which the mass is accelerated. You must be referring to perceived recoil because total measured recoil is exactly the same if the reacting force is the same. If the total force on the rifle is generated sharply, rather than more gradually, the perceived recoil will be greater.

I have done a little work with muzzle brake development using a recoil dynamometer and a recording strip. (it was in the eighties) The total recoil could be identical but the perceived recoil very different.

I had a moment of clarity last night (rare as they are) and believe I came to a better understanding of my own ramblings. I was a bit backward and came out wrong in my earlier statements so will try and convey better now.

Firstly Steve you are right. Felt recoil is more akin to the energy departed by the rifle on the shooter. This indeed feels worse from a lighter rifle moving faster than a heavier one moving slower. Newtons Law that we are all quoting earlier "for every action there is an opposite and equal reaction" refers to momentum. For an unheld rifle not touching anything the momentum of the bullet and gases going forward would be the same as the momentum of the rifle going back. Momentum is mass by velocity. Energy is 1/2 the mass by velocity squared. So for the same momentum a stock weighing half as much imparts twice the velocity which results in loads more energy and felt recoil (as it is a square thing).

I believe TOTAL RECOIL (sounds like some movie) should really refer to the momentum but wouldn't reflect felt recoil (energy imparted). They are quite separate.

I was however way off track in thinking the momentum imparted backwards could be a good measure of case head force plus case grip force. The reason is this: Momentum of the recoil (mass* velocity) is the result of the force (case head and case body pushing back) * TIME. A heavy bullet takes longer to get out of the barrel. The pressure curve for a heavy bullet stays high for longer and imparts more momentum back on the rifle. This momentum is then "felt" by the energy which it imparts on the shoulder. Heavy bullets have a longer pressure curve, lighter bullets get out quickly. Peak force on bolt head may well be the same but it is the time exposed x force (area under curve) that imparts the momentum not the peak force.

As I said this seemed to come quite clearly to me all of a sudden so I hope I am right.

 

[camac]

Moving on from that train of thought because I think I was wrong anyway. Uncannily something came into my possession today that fits right up the line of this thread. A guy from the range brought over a stuck shell in a FL die today for me to try and remove for him. He had ripped off the rim of the shell trying to pull it out with the press and even more impressive was he had been hitting the expander stem with a hammer to drive out the shell. The expander stem was severely paned over. I am not sure how much force this would require but it took a flogging by the looks. Also the inside of the primer pocket has been pushed in severely. I will get some photos and post them. The amount of force he was putting on the shell was pretty immense and may bear some interest. Looking at the shell it appears the main grip is coming from right at the base.

 

[RonAKA]

The base would be the strongest part of the cartridge, and most able to grip the sizing die. Not much resistance up in the shoulder and neck area. The thermal coefficient of expansion of brass is double that of steel. So, the first step would be to put the thing in the deep freeze overnight before getting the BFH out.

Yes, OK, thinking out loud, you probably have to put the whole press, die, and cartridge in the deep freeze!

 

[camac]

Thanks Ron, Die and shell are already in the feezer. but I am thinking this is giving us an idea of how much grip a shell can really get. I entertained bushing up expander/depriming rod and putting it in the press to see what force it would take to remove but it is too far bent and gone.

 

[holstil]

Brass cools and heats very quickly as Ron pointed out, just drop in some ice shards wait 2-3 minutes and smack it. If anything, heat the die first with a hair dryer. Like putting on a bearing with a press fit. Worked like a champ for me.

 

[TonyR]

I have run dry cases into a full length resizing die that were hard to remove with a press-based stuck case removal tool. This thing with the freezer should be interesting. I hope the process is reversable.

 

[camac]

Stuck case is removed thanks guys. Even after being in the -30 degrees C freezer (-22F) it took sveral hundred pounds force to press out. Apart from the rim being torn off and a dint on the inside of the primer pocket where decapping/expander stem was pushing there was little other damage to shell. Expander stem is one big zig zagged bent mess though. My conclusion - cases can get a good grip. If I can find my camera I will try and post a few photos.

 

[Hengehold]

I have been thinking about putting together a 7mm Shehane for 1000yd competition but am now second guessing that decision. I am of the opinion that the desired end-state would be to push a 180gr 7mm bullet 2900-3000fps. I have not shot with a 284 rebated style case before and must say that I am a little discouraged from heading in that direction after hearing about issues with pressure signs. There must be other options that can bring us to the 2900-3000fps window with a 7mm bullet. Has anyone ever tried a 7mm/30-06AI, or a 280AI, or a 7mmWSSM? It would seem that a magnum size case head such as the WSSM would be a fine solution, am I missing something?