Ladies and Gentlemen:

Where can I find experiments and data on the reduction, if any, in bolt thrust based upon cases with little body taper like the Ackley Improved designs?

I have read in Parker Ackley's book about his 30-30 AI experiments, and would like to know some more.

Presently, I am building a 7x57 AI and a 416 Taylor in 1908 Brazilian Mauser actions, and would like to reduce the bolt thrust as much as possible.

Yalie

I've never seen any. But if you find it, let us know. I'm sure I'm not the only one that would read it.
Internal ballistics is all just theorizing since we can't really measure what's going on in there. Just the effects of it. That said, it makes easy sense to me that gripping the chamber walls more decreases the back thrust.
But I'm not sure what you're looking to do here. If you've decided on a chambering, you can't really affect the bolt thrust. It is what it is. In a sound rifle though, those chambers will not be a problem. In an unsound rifle, AI-ing is not the fix.

Dear Bwana-be:

The 1908 Brazilian actions appear to be quite sound, and have shot hundreds of 6.5-06 rounds in one of them with no apparent set back. I lapped it approximately .002-.003 for even lug engagement prior to barreling.

I chose the 7x57 AI for two reasons. One, more room in the military magazine for longer bullets like the Barnes TSX, and two as per Mr. Ackley's suggestion, less bolt thrust.

My goal is: if I can focus the expanding gas energy vector on the chamber wall of the barrel with a cartidge that "sticks" to the wall at 55,000-62,000 psi and thereby mitigates the energy vector to the bolt face, since the brass is not physically sliding backwards in the chamber, I've a winning combination. I will then have little or no receiver lug recess setback.

That's why I'm trying to ascertain if Ackley's opinion on reduced bolt thrust is accurate.

I haven't seen any data other than Ackley's. I don't buy his explanation. He believed that taper would allow the case being stretched rearward to pull off the walls, reducing its ability to cling to the chamber. I can see that the friction load is parallel to the wall, and if you change the angle of the wall, you get less friction component pointing towards the breech. However, you at the same time get a compensating force. As the case angles more towards the direction of the breech face, the internal gases give a greater shove forward -- directly from the inside. It seems to me this increase is affected by the change in shoulder angle just as much as is the friction force. Now, with the increasing angle, the case head might be thought to "peel" the case sides from the chamber, perhaps reducing the friction some in this way, but given that the brass is "plastic" at this point under the gas pressure, I can't see this being important.

Ackley explains it in his Guide for Shooters and Reloaders, Vol 1.

I buy it. A 7 x 57 AI convinced me. Hot loaded casess last forever.

I shoot several AI's, from the .243 to the .30-06 AI. I would think that case life is the direct result of the case not stretching quite as much as could be in a more tapered environment. However, I also believe that the custom barreled rifle has had far more attention to it's assembly than a Walmart special in whatever caliber. Consiquently, the custom AI chambering is far more likely to give satisfaction to the careful reloader. What is not mentioned here nor have I seen it elsewhere is the fact that the overall surface area of the AI'd cartridge case is greater than the standard chambering of said round. So of course it makes more contact with a larger chamber surface thus reducing the proportional amount of pressure exposed boltface/case head area. Whew!


good shooting

I'd say it depends on if you load at pressures high enough to cause the brass to yield. At pressures high enough for the brass to yield, IMHO, bolt thrust will be identical between the std and ackley chambers.

If you are using an action of moderate strength, my thoughts are the ackley chamber is an excellent choice for obtaining the same velocity as the std chamber at slightly lower pressure and for reducing the frequency of trimming brass.

If you're using the ackley chamber to achieve higher velocities, IMHO, the majority of the velocity gain cited by ackley loaders is due to opperating at HIGHER PRESSURES and not due to increases in case capacity or "efficiency".

quote:

I'd say it depends on if you load at pressures high enough to cause the brass to yield. At pressures high enough for the brass to yield, IMHO, bolt thrust will be identical between the std and ackley chambers.

If you are using an action of moderate strength, my thoughts are the ackley chamber is an excellent choice for obtaining the same velocity as the std chamber at slightly lower pressure and for reducing the frequency of trimming brass.

If you're using the ackley chamber to achieve higher velocities, IMHO, the majority of the velocity gain cited by ackley loaders is due to opperating at HIGHER PRESSURES and not due to increases in case capacity or "efficiency".

The majority of my rifles are my own 280 based wildcat. Kind of an improved Gibbs. Or AI. Based on my testing and findings I would say Paul H hit it square on the head. All my testing in the 06 sized case gave me 1% velocity for 4% powder. Only way I could reach many of the POSTED velocties was 70-80,000 psi(measured). In a tight chamber and fireformed brass you often saw no pressure signs until 75,000psi.

Yale, what I was driving at is, there isn't much to "do" about it. If you're building a 7x57, you have one and it does what it does.
If you had said, "I'm building a wildcat from scratch, and trying to design it with the least possible bolt thrust," that would've made more sense.
If your goal is to "focus the expanding gas energy vector on the chamber wall of the barrel," then again, what are you looking to "do?" i.e., change about your chamber?
Maybe you're just trying to get confirmation that you can load this thing hotter than a 7x57 without worrying about bolt thrust.
If so, I'd say yeah. You can load a 7x57 hotter than a 7x57, too, without worrying about bolt thrust. In a sound rifle, that casehead size at 60k psi just isn't a major issue. But there are other ways for a rifle to fail besides bolt setback.

quote:

Originally posted by Paul H:
I'd say it depends on if you load at pressures high enough to cause the brass to yield. At pressures high enough for the brass to yield, IMHO, bolt thrust will be identical between the std and ackley chambers.

If you are using an action of moderate strength, my thoughts are the ackley chamber is an excellent choice for obtaining the same velocity as the std chamber at slightly lower pressure and for reducing the frequency of trimming brass.

If you're using the ackley chamber to achieve higher velocities, IMHO, the majority of the velocity gain cited by ackley loaders is due to opperating at HIGHER PRESSURES and not due to increases in case capacity or "efficiency".

Right you are, Paul. I've told several people
I've known that seek higher and higher velocities to just buy a magnum rather than
do unsafe things or push their rifles to over
maximum! I.e., a 7x57 is not a 280, a 280 is
not a 7 mag, and a 50 caliber necked down to
7mm is just plain silly!

Your Dakota engine will never be a hemi!

There's an article on the Lilja barrel web-site about bolt-thrust you might like to read.

Being a handgun hunter who uses Encores and Contenders to hunt with, I have a slightly different take on the back thrust issue than the rest of you. There is probably a lot more information available on this subject on fourms which deal with break action firearms, since they don't have the bombproof lock ups that bolt guns have.

It has been well demonstrated time and again that break action guns can run higher pressures with no ill effects by using or improving to AI type specifications over using rounds with long gradual shoulders and cases with a lot of body taper. JD Jones currently has a series of propriatary cartridges with 60 degree shoulders that are based on the 06 case and they beat the velocities that can be achieved in 300 magnum based cases in 15 inch pistol barrels.

Straight sided sharp shouldered cases have definite advantages and do substantially reduce backthrust. They can and do bring with them some feeding issues in magazine guns that don't have to be dealt with in break action arms. Which isn't to say that those feeding problems can't be mitigated to a large extent with some extra care and work being put into magazines, feeding rails and followers. But fourty degree shoulders and 10 degrees worth of body taper are probably approaching the practical limits for reliable feeding in repeating firearms...Rusty.

Yale: I did some hunting around and came up with some information for you. David White, a custom barrel maker, did some interesting expirements comparing improved cartridges to unimproved ones with the barrels mounted in a vice with the breach left open. The improved cartridges were retained in the chambers in every case while the standard cases were ejected from the barrels by the fources generated in firing the rounds.

You will have to log on to Mike Bellem's sight to read the post. Click on the link below and the sight will come up. Log on and then go back to the link I will provide you with below by clicking on the back icon, then click on the link again and the information will come up if you want to read it:

http://www.bellmtcs.com/forum/index.php?act=ST&f=24&t=1...aa10400c3db7c23ba23b

While his expiriments provide no hard measurements, they do provide graphic information on how improved cases controll backthrust in any firearm...Rusty.

I have this friend...who is the wheel at Picatinny Arsenal in NJ. He runs the R&D dept. They developed the idea that a bullet actually has three BC's, based on velocity ranges. The algorithms he developed are the ones licensed to Sierra that they offer. Anyway, they have reams of empirical data to prove that in improving cases you get between 24 and 25% of the capacity increase in increased volume translated into increased velocity.

ie: if you improve a 257 Roberts and increase case capacity by 20% you will see a 5% increase (at the same pressure level) in velocity. The lessened casehead trust is the primary gain according to their research.

Several years ago I attended Oehlers' chronograph school with this gentleman and we learned all about setting up to pressure test cartridges. Basically, you use a calibrated strain gauge (several strands of verrrrrrrrrry thin copperwire glued to a strip of thin plastic that is then epoxied to the midpoint of the chamber on the barrel. When you fire a round the barrel swells several millionths of an inch and those thin copper wires stretch a millionth or two. The transducer gauge measures the stretch as it occurs and the software translates that into a pressure equivalent. It's pretty neat, as it also measures the total pressure curve. It is really informative to shoot a round thru a rifle with and without the muzzlebrake attached and watch the pressure flatten out as the bullet hits the brake.

Hope I didn't bore you

Rich

Dear Rusty Hook:

I just finished reading Mr. White's Ackley Improved experiments vis a vis bolt thrust on the link that you suggested. Excellent answer to my question, thank you.

I have also sent you a private message.

Yalie

Dear Idaho Sharpshooter:

Thank you for your insight from the "wheel" at Picatinny Arsenal. His 25% gain in velocity per 100% increase in case volume comports with my chronographed experiments, although I get slightly more velocity. This appears to be the case because of a fast Douglas barrel, and no more.

Yalie