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

[camac]

Hi guys, After working with several 284 and 6.5*284 owners we had noticed a common theme. Case Head expansion and extractor mark issues when starting to push the loads at what seemed like (with all calculations) fairly reasonable pressures. I also noticed some other readers seeing same issues. After puting the thinking cap on (and believe me this hurts nowadays- This old head not made for thinking) I believe I may have struck on the reason. This may have been mentioned or discussed elsewhere but I cannot find reference to it.

THE ISSUE: a large diameter case pushing back on smaller diameter head. The 284 case is 0.5008 inches diameter (0.197 square inches) pushing back on 0.473 square inch bolt head. Compared to the standard 0.470 diameter cases (0.173 square inches) there is over a 13% increase in force back on the bolt for the same pressure.

Add to this the steep taper of the shell design (which gives chamber less resistive force to backwards force) and there could well be over 15% increase in force on bolt (and case head) for the same chamber pressure.

 

[TonyR]

Re: 284 case design issues (relevant for all 284 wild cats)

Sounds reasonable. I spoke to Bill Shehane before I did the 284 Shehane and he claims that he reduced to body taper to ease extraction of fired cases, not to increase capacity. That would appear consistent with your theory.

 

[camac]

Re: 284 case design issues (relevant for all 284 wild cats)

Thanks Tony, The Shehane taper is an interesting part to this discussion. In designing rifles that take pressure off the bolt we like to let the shell do some of the "gripping" work. The Shehane taper should be very beneficial for this and significantly lower the case head expansion. It would also lower pressure for same given velocity and thus extraction force required and force on case head.

 

[camac]

Imagine if you like, the case head and extractor groove as a brass rivet. Then press on them at 9850lbs, the other at 8650 lbs (equivalent force from same 50,000psi chamber pressure - not including any body taper isues). Remembering we are on the edge of the brass yield strength.

 

[TonyR]

I have been working up loads for a Short Magnum. The case is rated at 65,000 psi. I know I have fired strings loaded very close to max pressure with nothing other than a little cratering of the primer around the firing pin hole. Nothing at all on the case head. I have also worked up loads for another case with a 308 boltface and a lot of body taper, and it gives me resistence at the top of the bolt handle lift, nothing at the start of the lift and nothing on the primers or boltface. Any thoughts on either of these?

 

[RonAKA]

Interesting discussion. I certainly do agree that the larger the head of the cartridge the larger the force towards the bolt face, for the same pressure.

However, you have to keep in mind that pressure is defined as force per fixed unit of area (lbs per sq. inch). So if the area goes up and the pressure stays the same, the force goes up. But, the lbs per square inch does not change.

So, for the extractor pin issue, if the area of the pin bore is the same, and the pressure is the same, I don't think there is reason to to see more flow into the bore. The force on that hole is the same.

The shape of the cartridge is a bit different issue. Consider a CO2 cartridge or some other irregular shape vessel that is under internal pressure. Because the cartridge is not symmetrical does not mean the CO2 cartridge under pressure and laying on the table will go anywhere. Same if you imagined a cartridge laying on the table had 60,000 psi inside it. Assuming the bullet is welded to the case and the case would hold the pressure, it does not want to go anywhere. It does not matter what shape the cartridge is. It is not moving. Where I am getting to, is that in a gun the barrel and bolt obviously give the case the real strength, but the shape of the chamber really does not matter.

So the force on the bolt is increased due to the increased surface area, but at least in my thinking, not by the shape of the front part of the cartridge.

Just my thoughts,

 

[15Tango]

Richard Kayser wrote of P.O. Ackley's 'improving':

" . . . as least as much value to the reloader is the ease of extraction and vastly increased case life. Cutting hte case body taper also lessens the casehead thrust, the rearward movement of the case into the boltface . . .
. . . For reasons unknown to me, the sharp shoulder and minimum taper improved case seems to direct the pressure more evenly, and at a greatly reduced bolt thrust pressure. Ackley's tests on several Winchester 30-30s showed the same general velocity increases over factory ammunition and lessened bolt thrust pressure. In support of his claims, Ackley took a '94 Winchester, re-chambered it from a .30-30 to his improved version, and removed the two locking lugs from the rear of the action. Firing the improved 30-30s with just the lever to hold the bolt in place, showed the decrease in rearward thrust."


Based on that, I'd say Shehan's come up with the solution to the issue if there is any case head thrust issues in the .284 design.

Wayne

 

[camac]

I have been working up loads for a Short Magnum. The case is rated at 65,000 psi. I know I have fired strings loaded very close to max pressure with nothing other than a little cratering of the primer around the firing pin hole. Nothing at all on the case head. I have also worked up loads for another case with a 308 boltface and a lot of body taper, and it gives me resistence at the top of the bolt handle lift, nothing at the start of the lift and nothing on the primers or boltface. Any thoughts on either of these?

Tony, I do have some thoughts on this but would like to qualify that I am not a hugely experienced gunsmith or case designer. My background is a machinist with a lifetime of professional and semi-professinal shooting. My gunsmithing duties and indeed long range target interests are only relatively recent (last five years as I get near retirement) and I am still learning a lot. No doubt there are guys here with more knowledge on the subject than me but I will put forward my thoughts with anticipation of some corrections.

OK I will pass on my thoughts one "case" at a time. The magnum case and magnum bolt face is fairly easy to deal with in the above context. Firstly, although there is considerably more "force" put on the bolt by a magnum case head run at same chamber pressure due to larger contact surface area. The effect on the brass is reduced as it is spread over that same larger surface area. You won't see any increased brass signs. So a magnum case head will put more pressure on bolt and lugs (more force) but spreads this pressure out over a larger contact surface between case and bolt head so has lower effect on brass.

Similarly all pressure being equal - primers act in the same way. A primer should be considered independently of the case head size as it will act by itself (can move easily quite independantly of brass). The primer surface area is the same for all these shells as they use the same Large Rifle Primer size. Irrespective of flashole size given the same chamber pressure you will push primer back on bolt face with same force no matter what size case head. The different pressure signs you see on primers are due to cup strength and firing pin to bolt hole tolerance (cratering). Not case head size.

Now - Body taper. This one is a little more difficult for me. All things being equal (same pressure and same shell capacity), a tapered body should extract easier than a straighter one - especially at the top of the throw where it is "breaking away from chamber." I can think of a couple of reasons why this would not be the case. Firstly With increased body taper the case capacity is reduced and therefore increased pressure. Secondly it could come down to the chamber cut itself. I like to build into my chambers a little "roughness" by applying a 220 grit paper lightly to chamber wall. This allows the shell to get a little bite onchamber wall and take the pressure off the bolt. The chambers cut like this do indeed require more bolt lift to break this grip. Personally I have a preference for wearing out the "extraction" part of bolt (lift) than the pressure lugs (closing and firing) and don't mind a bit of stiff bolt lift for the right reasons.

To add a bit of my experiences to yours. My first 6.5*284 confused me to no end. 30+ years of reloading and there was very little of my traditional pressure signs showing in running up the first load. Primers looked great and bolt opened easily. There was some extractor marks though but I remember shrugging it off thinking "must be soft brass - so much for Lapua". I went back to reloading room to deprime and size and low and behold the two hot loads wouldn't fit in the shell holder. I miked them and they had grown considerably. I was totally confused but backed loads down anyway. Now it makes more sense.

 

[camac]

Richard Kayser wrote of P.O. Ackley's 'improving':

" . . . as least as much value to the reloader is the ease of extraction and vastly increased case life. Cutting hte case body taper also lessens the casehead thrust, the rearward movement of the case into the boltface . . .
. . . For reasons unknown to me, the sharp shoulder and minimum taper improved case seems to direct the pressure more evenly, and at a greatly reduced bolt thrust pressure. Ackley's tests on several Winchester 30-30s showed the same general velocity increases over factory ammunition and lessened bolt thrust pressure. In support of his claims, Ackley took a '94 Winchester, re-chambered it from a .30-30 to his improved version, and removed the two locking lugs from the rear of the action. Firing the improved 30-30s with just the lever to hold the bolt in place, showed the decrease in rearward thrust."


Based on that, I'd say Shehan's come up with the solution to the issue if there is any case head thrust issues in the .284 design.

Wayne

This makes sense to me on bolt thrust. Bolt opening still remains a mystery to me (and Richard Kayser too by the looks... but I will have a few more coffees and consider this and Rons comments above...... Good to get the old brain ticking again.....

 

[TonyR]

I think camac is correct. The pressure inside the 284 case acts on the area of the 0.500 diameter (less the case walls) while the case head metal resisting this force is the area of the 0.473 case head diameter. I think the point is that ratio of these two forces is higher if the case head diameter is smaller the the case body maximum diameter, as it is with the rebated case head, than if the two diameters are the same.

 

[15Tango]

This makes sense to me on bolt thrust. Bolt opening still remains a mystery to me (and Richard Kayser too by the looks... but I will have a few more coffees and consider this and Rons comments above...... Good to get the old brain ticking again.....

I believe the taper under pressure doesn't get the same amount of friction lock on the chamber whereas the nearly straight cartridges do. As said in the writing, we might not know the exact reasoning but the straight cases have been proven to lower the thrust against the bolt face.

I wonder if bolt opening is more an issue of brass growth/migration than anything. We all know that if we're neck sizing only that eventually we have to kick the shoulder back or the bolt's tight on closing and opening. The straighter body and sharper shouldered cartridges are less likely to need trimming as well, .223REM. Vs .223AI is a good juxtaposition. I imagine a rough chamber that allows brass to flow into the chamber wall surface under pressure and heat will have an effect not unlike bedding a stock and accidentally getting putty into a locking point.

Who knows.

Wayne

 

[camac]

Wayne, I can understand that.

If all this is correct - The case head is "the" place to look for pressure signs in the 284 based cases. It seems to show up with extractor marks and case head growth well before max pressures are reached. ie the case head cops a flogging before anything else. Probably not a bad thing really.

As an aside I am keen on two 284 case derivative designs and the above theory makes them even more appealing. One is the SHehane and I am moving towards that path but I am also quite keen on a new 6.5*284 design. (Oh and I know just mentioning this will get a few negatives but we can't help wanting to improve things if possible - or at least try) One with less body taper but also squeeze the shoulder back a bit and allow for more neck length and less case capacity. I think (and may be wrong) the same velocities can be achieved but with greater barrel life but maintain (hopefully improve) the accuracy properties of the cartridge. I saw Preacher made a reference on the same thoughts recently.

 

[JHardy]

Re: 284 case design issues (relevant for all 284 wild cats)

Sounds reasonable. I spoke to Bill Shehane before I did the 284 Shehane and he claims that he reduced to body taper to ease extraction of fired cases, not to increase capacity. That would appear consistent with your theory.

Tony:

Exactly what you said.

Jim

 

[15Tango]

Camac,

That's interesting actually. At first I was like oh geez at what you were basically describing as a 6.5-284AI but then knocking the shoulder back to keep the same rough case capacity of the straight 6.5-284 seems like a great idea. I'm kind of picturing a 6.5PPC on a much larger scale.

In theory it'd have the same ballistics, though I wouldn't be surprised if it were a little better with the more efficient dimensions. It'd have less case head thrust. It may be a little more accurate and maybe even more forgiving with reloading. Brass growth and trimming needs would be nil. If you kicked the shoulder back far enough and trimmed the neck back a little it may suit medium length actions very well. It's an interesting thought to be sure!

Wayne

 

[TonyR]

Another way to go is a 9.3X62 necked down to 7mm. Lapua or RWS is available and no rebated case head. I have seen this mentioned a number of times on the "net" and have thought about going that way. 280AI is another alternative, but inventing a wildcat sounds like a lot more fun! Too many projects already and my Shehane shoots really well despite primers that seat easier than I would like. Camac is moving in the right direction, in my opinion. My 31 inch barrels have all tuned at 2900 fps but the case is good for 3000 fps so what to do with the "wasted" capacity? Hopefully, something to improve accuracy and there are lots of possibilities for someone to explore with the motivation and time.

 

[camac]

Yeah Wayne and Tony, The 6.5*284 Shehane or AI or your suggestions Tony are all great rifles but a couple of goals for me would be 1) shorter OAL , 2) Longer neck.
Individual preference would probably determine as to what length to go to but if you could maintain a fair bit more case capacity than a creedmore or 260 but keep that sort of length it would be good. If the goal was to acheive 2950 fps with 140 class bullets then I think this can be acheived with a significantly shorter case design capable (with less taper) of running higher pressure. If it was to squeeze everything you can out then the AI or shehane etc versions of 6.5 would be the go........

The less taper also works well in magazines so a 6.5 short fat 284 with less taper but worked in 308 length repeaters could also be nice (and accurate)

 

[TonyR]

Keep us informed!

 

[camac]

Will do Tony
- the 284 Shehane still excites me and I am going down that track first. Maybe the body taper theories above explain a little as to why it sems to punch above its weight and the modest case capacity increase seems to give a better than expected velocity gain.
I will still have all these 6.5mm barrels here though so may as well try.....

 

[RonAKA]

I think camac is correct. The pressure inside the 284 case acts on the area of the 0.500 diameter (less the case walls) while the case head metal resisting this force is the area of the 0.473 case head diameter. I think the point is that ratio of these two forces is higher if the case head diameter is smaller the the case body maximum diameter, as it is with the rebated case head, than if the two diameters are the same.

Agree, and I mis-spoke about the pressure on the bolt face and ejection hole. I did not look at a cartridge drawing and assumed from the post that the face of the head of the .284 was .500. I see it is still .473 while the case is .500. So yes, pressure is increased from the 270, 30-06 configuration of .473 head and .470 case. Ignoring the case thickness which is reasonable, pressure on the face of the .284 configuration would increase a nominal 60,000 psi pressure in the case to 67,000 on the face of head. The same pressure in a 30-06 would reduce the pressure from the .470 case to 59,200 at the head face. There is about a 13% difference. That may very well explain differences in indications of pressure on both the primer and extractor hole imprints.

The mag case reduces face pressure even more with a case of .513 and head face of .532. So the mag should take the most pressure before showing it at the primer and extractor hole.

Still thinking the shape of the cartridge ahead of the head is not material though.

 

[camac]

Ron - on the shape of cartridge, you may or may not be right. It would be a hard one to test and I am a bit uncertain myself - I don't really fancy repeating Mr Ackleys 30-30 demonstration and filing off the locking lugs and holding onto it. but I can "imagine" it making a difference. Think of a case expanding and pushing on the walls of the chamber. Now if the walls of that chamber are parrallel. There would be little push backwards. If however it was a cone (extreme taper) then the expanding shell would indeed push backwards. How much I am not sure but as stated I can "imagine" it happening. I can also certainly imagin a parrallel side getting a much better grip and resisting the backwards force. That is why we have the taper so the shells can be extracted.