Move shoulder by leaving headspace long?

[mattri]

Just a hypothetical question, not suggesting that anyone do this.

In an AI chamber you seat bullets long, a slight jam to keep everything stable. Then when the round is fired the shoulder is blown forward and the angle changed.

Any reason you couldn't do something similar with a standard round by intentionally setting the headspace on the long side?

For example, say you had a "go" and "no go" guage for caliber x. Instead of using the "go" gauge to headspace the barrel, if you used the "no go" gauge, and then used scotch tape as if using a "go" gauge, wouldn't you esentially create a semi-wildcat with slightly more case capacity, the same shoulder angle and a slightly shorter neck?

[KRP]

A true AI chamber shouldn't require jamming the bullets. What you propose will also increase the amount of unsupported cartridge case. Not much to be gained by those few thousandths available, except maybe to make some dies/brass work together better. With a chamber cut to fit it's a different story.

[J.Baker]

Theoretically yes...but some dumb@ss would eventually come along and think "well if it's ok to stretch it a few thousandths then surely it's ok to stretch it a few tenths and it would be even more better" without having the smarts to realize the more they move the shoulder forward the more area on the case they leave unsupported as the gap between the bolt head and breech face increases. Then they'll wonder why they have case separation issues... ::)

Naturally that raises the question: What's the point of doing it in the first place? If you want more case capacity just rechamber to the AI version or to a larger cartridge and stop half-@ssing things. Just because theoretically you CAN do something doesn't necessarily make it a good idea.

[mattri]

Excellent replies- thanks.

Again, I'm not suggesting anyone try this, or that there is a huge performance gain to be had- just confirming the actual mechanics.

[nsaqam]

The Gibbs cartridges blow the case out, sharpen the shoulder angle and move the entire shoulder forward a considerable amount.
You don't do this by jamming a bullet hard into the lands but instead by forming a false shoulder precisely where the new shoulder will be in the FF'd case.
Easy to do a false shoulder too. Just neck up the case to the next larger size (or two sizes which is my preference) and then run the case into your FL die just until the brass chambers with some resistance in the Gibbs chamber with the false shoulder firmly against the neck/shoulder junction.
You make a good false shoulder in the correct location and you'll have no potential long headspace issues.

[mattri]

A valid point, and well made.

To be honest I'm pleasantly surprised that an abstract question has been answered fairly without any flames on "responsible reloading" or "saami specs" etc. It was a thought I wanted to bounce around a little, nothing more.

Well done to those who have replied.

[Ray Gunter]

The Gibbs cartridges blow the case out, sharpen the shoulder angle and move the entire shoulder forward a considerable amount.
You don't do this by jamming a bullet hard into the lands but instead by forming a false shoulder precisely where the new shoulder will be in the FF'd case....

Your description of cartridges that were introduced by Rocky Gibbs is very good.

In fireforming a case to a Gibbs chamber the requirement is that you maintain a tight headspace for the initial fireform. This can be accomplished, as you stated by creating a false shoulder. It can however be accomplished by using a bullet seated very long and pushed into the lands to hold the case tight against the bolt. In order to get a good fireformed case the case head needs to be held tightly to the bolt face.

I have a 240 Gibbs that I have fireformed cases for using both methods. The only drawback that I have with using a bullet into the lands is it wastes a good bullet and needs a medium load of powder, Where using a false shoulder I can use a small charge of a fast powder and a cotton ball.

Here is a pic of a Gibbs before and after fireforming using 30-06 brass



To the orignial post ...... mechanically, yes you can do as you described. NOT a good idea but it would do as you described.

[mattri]

Thanks for the good description and pic. Again, not intending to do this.

[nsaqam]

The Gibbs cartridges blow the case out, sharpen the shoulder angle and move the entire shoulder forward a considerable amount.
You don't do this by jamming a bullet hard into the lands but instead by forming a false shoulder precisely where the new shoulder will be in the FF'd case....

Your description of cartridges that were introduced by Rocky Gibbs is very good.

In fireforming a case to a Gibbs chamber the requirement is that you maintain a tight headspace for the initial fireform. This can be accomplished, as you stated by creating a false shoulder. It can however be accomplished by using a bullet seated very long and pushed into the lands to hold the case tight against the bolt. In order to get a good fireformed case the case head needs to be held tightly to the bolt face.

I have a 240 Gibbs that I have fireformed cases for using both methods. The only drawback that I have with using a bullet into the lands is it wastes a good bullet and needs a medium load of powder, Where using a false shoulder I can use a small charge of a fast powder and a cotton ball.

Here is a pic of a Gibbs before and after fireforming using 30-06 brass



To the orignial post ...... mechanically, yes you can do as you described. NOT a good idea but it would do as you described.

That right there is a freaking nice cartridge!

My thought for my next Savage LA build was to do a super lightweight with a 24" barrel chambered for the 6mm-06 but seeing that sexy .240 Gibbs has me thinking a little more powder and a little more exclusivity may very well be a good thing.
I'd love to hear a report on that .240 Gibbs. If you don't wish to hijack Mattri's thread maybe a new thread on the .240 Gibbs is in order.

[mattri]

No worries on the hi-jack, I want to know too!

[gotcha]

Me too!................Me too!

[Blue Avenger]

left my phone / camera at work or I would show you 300 Win mag fired in a 300 weatherby. seen it enough times to know some people should not be allowed to own guns

[mattri]

Post a pic when you can for sure- I've seen similar cases but I don't think I've seen that paricular example.

[Ray Gunter]

The Gibbs line of cartridges runs from a 6mm up to a .358 all with the same case body, only the neck diameter changes.
That feature sure would make a neat switch barrel set, same bolt head and every round would feed.

Wolfe Publishing produced a small book on Rocky and his cartridges. Its a enlightening read. Rocky was a wildcatter in the true sense of the word and I get the impression from reading about him that he was real character. He left instructions to his wife that on his death she was to burn all his notes. She did too.

You can learn more about Rocky here http://www.riflemagazine.com/catalog...?productid=820

I would describe the Gibbs line as not "Improved" but "super Improved". The body (an 06 case) is blown out to a minimum taper. The shoulder is increased to 35*. Rocky believed that Ackley had it wrong and his shoulder was too steep.

Rocky's line up
.240
.25
6.5 mm
.270
7 mm
.30
8 mm
.338
.358

My 240 Gibbs will push a 100 gr in to the 3400 fps range. Reloading this round is touchy, its so over bore that there is a very fine line between "thats just about the right amount of powder" and "WHOA thats way too much". Definitely not a round for the novice reloader.

Be very carefull of any loads that you may find that were produced by Rocky himself, they are very suspect. He measured his barrel from the bullet tip not from the bolt face. So when he said he got this velocity from a 26 inch barrel, it was really a longer barrel. I told you he was a real character.

And to be honest, with todays powders and case choices I dont think the Gibbs has much advantage as it did 40 years ago.
I'm putting together a 6mm Rem AI that I believe will scare the hell outa that 240 Gibbs and be alot easier to load for.

[Fat Albert]

Gibbs had so much trouble fireforming cases that he went to a oil impact forming die as a step before firerforming the cases. Notheing like forming cases with a raincoat, facemask and a 5lb. hammer.

[Blue Avenger]

[img width=432 height=768]http://img.photobucket.com/albums/v675/gcidso123/photobucket-3773-1334535657790.jpg[/img]

300WM - fired in a 300 weatherby