Case Length

[golfr]

what effect does the case length have on pressure, accuracy and overall performance of a cartridge? I am reloading 243 win and have been trimming to 2.033 inches. Max length is 2.045... If I go to 2.040" what can I expect to happen?
thanx in advance for your expertise.

 

[rcmodel]

Case length has NO effect on pressure if you seat the bullet to the same OAL.

Case capacity is determined by how deep the bullet base protrudes into the case.
And seated to the same OAL it makes no difference how long the case neck is.

It may have some unmeasurable effect on accuracy, but I doubt you could prove it without a $10,000 benchrest rifle.

But.
Trim Too Length for the .243 Win is 2.035".
That's what you should trim to.

rc

 

[savanahsdad]

in every book I have seen it says to trim .010 under max case length, however case max length is set but SAAMI and so is chamber depth , you would need a chamber under min and a case over max before you would have an issue , and that issue would be that you are pinching the bullet tighter in the case when you chamber the round , but you would/should feel the bolt close harder (assuming you have a bolt gun) and by pinching/squeezing the bullet you could have pressure spikes , same as if you jam the bullet into the lands ,

I made button gauges for all my guns by trimming back fired cases from each gun and put aluminum buttons in them, turning the rod down to fit snug in the case and turned the button .003 larger than the neck , and found my guns run .005 to .008 over min. so my cases only get trimmed .005 below max , yes it was a lot of work to get out of some trimming , but I found I almost never have to trim anymore just check length ,

but like RC said trim to 2.035 if you don't know your chamber depth , best to play it safe



.

 

[gamestalker]

The only length I maintain, or concern myself with is SAAMI max.. I do however keep my brass trimmed to the same lengths when possible, but some times we come across brass that is shorter than SAAMI trim too, actually it's not all that unusual, especially with new or once fired brass. I just load it, and it will usually stretch enough on the first loading to accept a trim that will allow me to get them all to the same lengths.

As for pressures, it's not even worth the consideration, as it has no discernible effect. Case volume, as it pertains to wall thickness in the body or the neck can effect pressures, sometimes quite significantly. But as long as a load is worked up using that thicker / thinner brass / heavier / lighter brass, it will have no noticed effect, as we are developing the load using that brass.

Case volume is also effected by how much bullet is seated into the case. This isn't really something we notice much with bottle neck cartridges, but certainly noticed on a much large scale with handgun cartridges, especially AL-ing rimless cartridges. This is where relatively small variation in seating depth can have a significant impact on pressures. And example of this is demonstrated in the Speer# 10 in which technicians seated a bullet .030" deeper in a 9mm cartridge with a known pressure. This caused pressures to more than double, by doing nothing more than decreasing the oal by .030".

GS