Why does reusing fireformed cases make for more accurate ammo?

[raindog]

I'm rereading Lee's "Modern Reloading". He mentions that reusing fireformed cases in the same (bolt-action or single shot) rifle they were shot in leads to greater accuracy, all other things being equal. He contrasts this with full-length resizing.

Why is this? Does it make a better gas seal? If so, then I assume that means higher velocity. So is this just a case (no pun intended) of higher velocity leading to better accuracy, or is there something I'm missing?

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[bballer182]

The way I understand it, and Im sure there will be more to chime in on this who have benchrest experience, is that all chambers are different. 99% of all the F/L dies out there size to the minimum SAAMI specs for that cartridge. If your rifle was one of the first to be chambered with a fresh reamer you will have a pretty loose chamber. If your rifle was chambered at the end of the life of the chamber reamer you will have a tight chamber. either way the chamber is supposed to be bigger than the maximum SAAMI specs for that round. So you end up with some (couple .001") of what I will call slop between the outside of the case and the inside of the chamber. When fired the case expands and fireforms it self to the inside dimensions of the chamber. During that time the bullet has the opportunity to hit the lands at a non-optimum angle. So neck sizing is sizing a case at the mouth and neck only leaving the body of the case to dimensions of the chamber it was fired from. Other advantages are that you can gain extra case capacity. How much? Not a lot, as in a tenth, or half of a tenth of a grain of water. At any rate more case capacity equals more powder, which, equals more velocity (typically). This is not typical practice for a semi auto rifle nor will it probably even work as being a rapid action semi-auto requires a lot of slop to get the cartridge in the chamber quickly. Im sure there will be more specifics to come by others but thats the general gist.

 

[mjbskwim]

Well if firing the round off in a given chamber will expand the case to the chamber size,then doing so will take some of the energy created by the explosion,no?
Now if you have the cases that were "fire formed" to your gun,then this has already been done and now no energy will be robbed by the expansion process.
Then when tuning a new load,this one variable will be eliminated and you will have consistent burn and pressure,sending the bullet down the barrel the same every time.
Now you can concentrate on the rest of the variables such as powder,bullet weight,and the rest.

That's my story

 

[bballer182]

Well if firing the round off in a given chamber will expand the case to the chamber size,then doing so will take some of the energy created by the explosion,no?
Now if you have the cases that were "fire formed" to your gun,then this has already been done and now no energy will be robbed by the expansion process.
Then when tuning a new load,this one variable will be eliminated and you will have consistent burn and pressure,sending the bullet down the barrel the same every time.
Now you can concentrate on the rest of the variables such as powder,bullet weight,and the rest.

That's my story

Feasible point.

 

[deadshot2]

To illustrate one of the reasons, take a full length sized case and reinsert a spent primer in it. Then weigh each empty case, then weigh again with the case filled with water. If you're using a digital scale just put the empty case on the scale, hit the zero button, then fill and weigh. The indicated weight will be the capacity of the case in grains of water.

The fired case will hold more water and in some cases considerably more. This translates to the possibility of using more powder without having to resort to compressing the load.

Another factor is that a fire formed case will more likely locate the bullet more accurately so it can enter the rifling in a straight line rather than starting it's flight slightly cocked with the possibility of being deformed.

One more benefit is that by using a fire formed case, only sizing the neck enough to hold the bullet, means less working of the case. this leads to longer case life which for some expensive cases can mean less trips to the wallet for large bills.

For most shooters it's not really a big deal to just shoot full length sized ammo. When hunting it's often preferred so you don't run the risk of having a round fail to chamber easily while your trophy animal is getting ready to bolt. Also makes it possible to interchange ammo among several rifles of the same caliber.

For most target shooters that are satisfied with groups they can measure with a ruler, not needed. For those that use calipers or target measuring software, fire forming is just another step in the process of eliminating all the variables possible.

As for mjbskwim's feeling that "energy might be robbed" by case expansion, just how much do you think will be "missed" from a charge that generates 60,000 psi, or so? The brass of a case moves out of the way pretty darn quick with that kind of kick

 

[Greenbug]

This^^^^^^^^^^^^^^^ +

Fireformed or "custom fit" cases fit your chamber better, and usually perform better.

 

[raindog]

Thanks for all the comments. I certainly learned a lot.

It sounds like this all comes down to greater velocity, for various reasons, presumably leading to a flatter trajectory.

 

[mjbskwim]

Hey deadshot let me sound somewhat smart will ya?
Smarty pants

And we like smarty pants in this section

I would figure ,as you said,when measuring with calipers,all these little thing will add up.

It sounds like this all comes down to greater velocity, for various reasons, presumably leading to a flatter trajectory.

One thing I have learned on here is that velocity does not always equate to accuracy.

 

[skywag]

They fit your chamber better.

No need to get all complicated.

 

[deadshot2]

One thing I have learned on here is that velocity does not always equate to accuracy.

That doesn't mean that one shouldn't go looking for an accuracy node at a higher velocity. The higher velocity will then allow one to hit a target at greater distances. I can load rounds that will stack in a single hole at 100 yards traveling 2200 fps but won't even reach a target over 1k yards. The 2800 fps rounds do so nicely.

 

[Otter]

Usually my most accurate load is in the middle of the suggested range for a given powder, not at the highest, so I don't agree with the more capacity idea produces more accurate ammo. Finding the accuracy nodes for your barrel is far more important to accuracy than reaching the highest possible velocity.

I tend to agree neck sizing produces the most accurate ammo, and better yet neck sizing with a bushing die so that you don't size the entire neck. You only want to size down slightly farther than you seat the bullets. This leaves an expanded section of the neck near the shoulder that perfectly centers the brass in the chamber and barrel. On the other hand, a custom FL die cut to the specs of your chamber reamer will produce the most accurate ammo, at least that is what the benchrest guys do now.