reloading 243 ackley improved

[harrygrey382]

I got news for you: There is very little to be gained with an Ackley chambering. The case capacity difference between a 243 and a 243 IMP is 6.3%. The rule I have gone by, and it's an old one, is that velocity increases at 1/4 the increase in case capacity. This means that you will utilize a 1.5% increase in muzzle velocity...when all other variables are the same. These being barrel length and type of powder and pressures.

Look at it this way. If you increase the usable case capacity by 6% and leave in the same amount of powder, you would expect (generally) lesser velocity, would you not? For a given bullet and powder it takes a certain amount of pressure to push a bullet out of a barrel at a given speed. By increasing the case capacity you have just lowered the pressure (say max @ 60K psi) and most likely, the velocity. So. What to do? You increase the powder until you reach the same velocity. You have achieved this velocity by generating the same pressures, with that same powder, as you had with the standard load. So now you have added more powder and gotten the same velocity and pressure. Now you add more powder because the Ackley Improved cartridges are stronger, right?? Wrong. You're done: 60K max pressure. Why would you get even that 1.5% increase in MV? I'm not sure but most likely it is due to the increase in powder weight. The added weight of the powder itself must be accounted for in internal ballistics and this weight will change the burning rate of many powders. (Check Lee's book on compressed loads) In any event, the velocity difference would be about what you would expect in standard deviation on a chronograph and most likely be within the accepted norm of pressure readings. If AI shooters measured pressures they would get a hairy surprise. Here it is the custom for gunsmiths to suggest ridiculously long barrels for their Ackley chamberings. One friend of mine had a 30 " barrel installed. His velocity increase is about what you'd expect for such a long barrel and he said his powder charges are about what he used in his standard 243. One fellow on this board boasted velocities of his 243AI and a 105 A-Max... at pretty much exactly what the standard 243 obtained.

Interesting post Muir. And I hate to be nit picky here , and I'm not meaning a total hi-jacking but... You say a 1/4 velocity increase for 1 volume increase with regard to different cases. Surely the reason for this would also apply to the AI volume increase - it may be a very small one as you've said, but it would surely account for an increase none the less. I'm not arguing for AI btw.

What do you think causes the increase in velocity when you increase the volume of a case? By that I mean say a 30 Carbine vs a 30-378 both firing the same bullet loaded to the same pressure. A computer model says when using these cases with the same powder, bullet, barrel length and pressure there is an 800 fps increase (that's a 1/5 velocity increase to volume increase by the way). Surely there is more at play here than powder weight? OK sure this is a computer model but I don't see these figures as wildly wrong.

My theory is - consider the cartridge/chamber and barrel as one pressure vessel, with the bullet a moving seal. When the powder is ignited a pressure is created. As the bullet travels down the barrel this pressure is reduced. Once it has reached the end of the barrel the volume has increased to a certain percentage of the original. So therefore the pressure has decreased by this proportion. If the original volume was higher (bigger case - or longer barrel), the percentage increase in volume (and therefore decrease in pressure) would be less. So that is why a bigger case volume means higher velocity.

Of course, this is a perfect world, and powder burn rates differ and make things more complex. And it may not have finieshed burning once the bullet'\s moving and all that but still I believe the basic volume/pressure theory holds. Or someone tell me if I'm way out...

 

[Muir]

Harry,
This is only a guide and not cast iron and actually, represents a best case scenario. There are some applications where this doesn't apply at all and in any event, pertains to when "all other things being equal" this means, the identical weapon, cases, etal, in the exact same barrel with the exact same components. It doesn't take into account any changes in the above. It's as it your 243 woke up one morning Improved and you could mark the differences bases on pressure and velocity.

As to the vessel theory, it's close to the mark from where I stand except that the peak operating pressure is obtained very close to the chamber in most instances and length of barrel doesn't effect this. It does effect the amount of space the expanding gasses, not burning powder if your powder choice was anywhere close to reasonable. Since we as shooters deal with pressure as our common denominator in all loads (as to what is safe and what is not) we must abide by what takes place in the chamber or close to it. Pressures begin to fall shortly after the bullet moves but by then, peak pressures have occurred.

It's too early in the morning to wrap my head around your 30 carbine VS 30-378 example! It is extreme, to say the least. Does your computer say that velocity increases 1/5 the rate of case capacity in this instance? If so, I can imagine that there would be fellows here who would have bet it would have a had a grater proportional velocity increase!

Burning rate is a misunderstood item. I was trying to explain to a person why a 22LR and a 22WMR have the same maximum allowable operating pressure as listed by SAAMI. He was adamant that they couldn't have the same operating pressure because of the difference in bullet velocity. I used the example of loading some of my cast bullets over 14 grains of shotgun powder to 30K pressure and 1800 fps VS the came cartridge and bullet using 35 grains of medium-slow burning rifle powder to get 2500 fps velocity at the same pressures. In this case it is a matter of time that determines the velocity. Intuitively, many people think that same pressure, same velocity. But they discount time. The 14 grains of quick burning shotgun powder ignite instantly, with the pressures spiking peak with an abrupt falling off, leaving a small volume of gasses to push the bullet out of the barrel. The slower powder reaches the same peak, in a longer amount of time (in a ballistic sense) and leaving a larger volume of expanding gas to push the bullet to a higher velocity. The fellow could not and would not accept this. Rimfire loading is different from centerfire loading in that you can vary the priming considerably from load to load, and adjust the powder to match, to achieve (within reason and pressure limits) different results but as you are aware, the same adjustments apply to centerfires.

This kind of internal ballistic juggling is why I always add that with this "rule" -which, by the way isn't one I came up with myself, but have used- states that all things must be equal. A change in any theoretical component changes everything.~Muir

 

[harrygrey382]

Muir,
I'm glad I'm kind of on the right track with my understanding then, and I get your arguement about the 243AI case. I'm thinking that change in volume is so small as to make other factors more dominent than my expanding vessel theory. I'm also interested in your powder burning rate tale - I guess I hadn't thought that far into that area but what you say makes a lot of sense.

I know my example was extreme, but I wanted two cases with the same calibre. They had to be very different capacities because I was worried if it were comparing say 308 vs 30-06 (or 243 vs 243AI for that matter) the differences would be so negligable as to make my example obsolete. Of course for a fair test the bullet, barrel length and of course powder had to be the same, which they were. And yes it did say there was a 1/5 increase in velocity, or there abouts. For 72gr (30 Carbine 19gr, 30-378 91gr) water difference in case capacity there was a 800 fps volicty increase at 40000 psi using I think a 110gr bullet.

By the way, I would have been one of those people that would have bet on a higher than 20% velocity increase...

 

[Muir]

Always a pleasure to chat with you, Harry. I am surprised that your program could even digest the data to come up with that figure as there is such a monstrous disparity in case sizes and suitable powders. In any event, the subject of burning rate is fascinating stuff...for Dweebs like me, at least! I once took a pristine Model 1884 Trapdoor Springfield infantry rifle in 45-70 to the range with some cast bullet loads. (these are the weakest 45-70) An onlooker was admiring the rifle and asked what my load was. I told him it was 53 grains of H-4831 behind a 400 grain bullet. The guy's eyebrows rocketed over the far side of his brow and he yelled, "You &^%$#%^@ IDIOT!!! That's the same &$^@#% load I use in my 30 Aught Six!! You're gonna KILL yourself!!!"

So I slipped a round in the chamber, closed the breech, and fired the round before he could scuttle to safety. Of course, the round went off with a thud, and the case extracted normally with a hint of soot around the mouth. Of course, H 4831 in a straight walled case just doesn't develop the pressure it would in a 30-06 due to the change in burning rate: in a 45-70 it rather mimics black powder. If this "expert" had given it a bit of thought he might not have jumped to ill placed conclusions. ~Muir

(And yes, I told him so.)

 

[Brithunter]

An intersting point about the straighter walls of the Ackley Improved case is that I have heard it can cause feedign problems. Something I checked for by making up three dummy rounds in my .280 and didn't find a problem. Last night I loaded up three more dummies but this time used a Hornady #2822 Flat Point bullet of 139 grains. In my 7x57 they proved to be nicely precise and grouped well but they would not feed from one side of the magazine as the flat stip hung up on the chamber mouth. The BSA Long actions have twin polished feed ramps rather than the single flat plain feed ramp of the ones meant for medium length cartridges like the 7x57. I wondered if these twin ramps woul guide the bullets better.

On trying them trhough the action thse Flat points fed without a hitch partly due to the feed ramps but partly due to the less taper of the case keeping the edge of the flat point away from the chamber mouth. I tried the bullets seated to the cannulure and seated out further with the same pleasing results. As I have about 250 of these bullets still it was a welcome outcome and gives me another choice for loading the .280. Now we just have to get it cited for expanding ammunition .

 

[Muir]

Any time you switch to a straighter case yuou run the risk of needing to alter the feed rails... especially in Mausers. I rebarreled many 98 Mausers with surplus Israeli (FN) Model 98 barrels in 308 Winchester and at least half of them needed feed rail alterations. I'm glad your rifle didn't need such attention! It seems like you project is coming together smoothly.~Muir

 

[Brithunter]

Well it's going slow as I have other things that need oing before I can get back onto the rifles. Here a is a photo of the feed ramps of the Monarch:-

You can see the twin guide ramps. This is not the .280 though it's a std Monarch .270. I have noticed one thing though as the shoulder is now further forward on the case that of course the shoulder no longer catches the little retaining bevels in the magazine side wall. So the bullet tips can now hit the front face of the magazine under recoil. We will ahve to wait and see how it goes. I have to finish the inlettign for the new barrel profile and get the bedding in this stock before I can really begin testing.