Dangerous case trim length?

[walkabout]

I cant remember but I thought I remembered reading that you can trim a case too short?

I get that if cases are to long it can be a problem in the chamber, but too short?

Not enough neck to grip the bullet?

Can you trim too short?

 

Thoughts gentlemen?

 

Mark

 

[MrCetrizine]

I don't know for sure but I always thought this only applied to handgun cartridges that headspace off the case mouth. Not bottleneck rifle cartridges that use the shoulders.

[gbal]

Mark,every firing of a rifle bottleneck case stretches it just a little-you don't want them getting too long .For standard SAAMI chambers as in all 'factory' rifles you need to check every fewfirings-stretch is load dependentas well as brass etc so there is no one number for all-but there are recommended max/min cartridge case overall lengths-SAAMi too-to fit with tolerances to the SAAMI chambers.It means at most a small trim every 5 firinds (as a guide)-or at least check them-it is best to be uniform-have all your cases in the Max-min range,and close to the same....the other issue is bumping shoulder back a though or two-when chambering gets stiff-similarly as needed-many FL dies will do this-stiff bolt closure indicates it's time...headspacing. Is on a "datum place" on the shoulder,and it gradually stretches away...needs bumped back...

This ensures smooth operation,and minimal unknown pressure increases,but it's not something to get uptight(!) about-just monitor them as above...you can buy trimmers,etc.

Custom fitted chambers stretch less,but do stretch too soe cartidges are hard on brass-the hot loads usually expand primer pockets too loose before brass is no longer useable.

Annealing helps all this a bit.Longer overall brass life.Clearly more brass extends the 'need to' time too 500 cases might mean 5000 firings if spaced evenly-but of course a fair trimming session eventually...or newer brass...good idea to measure any unknown fired brass before you buy if possible...threee firings/need trim isn't too bad-but they have had a hard life so far...anneal soon?

 

 

gbal

[snakeman]

walkabout

 

You do not state the cartridge you are trimming but, the below link is for SAMMI spec on .223cal

 

My understanding of the drawing is:

 

1.760 = maximun COAL

 

minus the '.030' would be the minimum COAL

 

Therefore on my calculations minimum 'safe' trim case-length would be 1.730..(wouldn't trim below this)

 

edit..see link to SAAMI in below post and scroll to find drawing of .223 cartridge

[snakeman]

Hmmm....got to be a member of SAMMI to view the drawings these days?

[snakeman]

click link and download SAAMI drawings..it should open on the .223

 

 

http://saami.org/specifications_and_information/publications/download/Z299-4_ANSI-SAAMI_CFR.pdf#page=13

[walkabout]

Hi snakeman

its a 6 x 47 Lapua, a necked down 6.5x47 Lapua.

[snakeman]

6x47 is a wildcat and so is unsupported by SAAMI...However, all of the 6mm cartridges are either .020 or .030 trimmed off the OAL for safe minimum OAL

 

Therefore I'd 'err on the side of caution and trim no more than .020 off your case OAL..(whatever that may be)

[Andrew]

I cant remember but I thought I remembered reading that you can trim a case too short?

I get that if cases are to long it can be a problem in the chamber, but too short?

Not enough neck to grip the bullet?

Can you trim too short?

 

Thoughts gentlemen?

 

Mark

 

Dangerous? No. Even straight walled pistol cases that headspace on the case mouth will not be 'dangerous' if trimmed a little short. They may not fire or give top accuracy, but dangerous? No, provided you seat to OAL and are not calculating 'seating depth' off of the case mouth to head measurement. I routinely make 6.5 Creedmoor from 22-250 and 7-08 from 308. The cases are short but function fine.~Andrew

[Chris-NZ]

+1 on that.

I initially used .308 cases to make 7-08 with no problems.

 

You read about a carbon ring building up ahead of a short neck that would then pinch std-length cases and up pressure, but I never saw it in mine. Not saying it can't happen though.

[walkabout]

Thanks guys

I will get on with the load development now

Mark

[Gandy]

Mark,

 

I have no idea how to answer your question but, its had me thinking, as the case shoulder and neck are used to form a 'seal' (I think the posh word is obturation) to create pressure behind the bullet to force it down the barrel it must be important.

 

Thinking this through logically I would imagine so long as the majority of the neck is left to form a seal this should be fine? as others have said accuracy may suffer. The acid test will be if you get a sticky bolt as this is taking the pressure rather than the case seal?

 

May have this wrong what do you think?

[gbal]

Gandy-you are correct in that obturation is indeed needed (sooty necks are one sign of slightly leaky gases).

And,as you say,the seal is more a function of neck diameter than length-a tight neck custom chamber may have only 2 thou clearance,a factory could be double that to seal on firing.And cartridges vary in spec neck length with no consistent effects,given other variables.

Firing also lengthens the case a very little each time-depending on the load pressure,and the brass- and it is this length stretching that needs trimming back to spec after some firings.Otherwisee there can be some pressure increases/bolt closure issues when extreme. Some cases havea reputation for more stretching,and shoulder angle may be involved.Ackley Improved eg, reduce the angle.

Some fire form wildcat cases by creating a 'false shoulder'-seating the bullet in only a short expanded bit of the neck ,rather than it's whole length-suggesting again that length is not the crucial measure.But that's for experts... :-)

 

gbal

[Chris-NZ]

..,the seal is more a function of neck diameter than length-...

 

The seal is more a function of pressure. As long as you're not operating in the dangerous pressure zone where web/primer integrity may come into play.. ;-0

[gbal]

Chris,of course -enough pressure to close the neck on the "chamber' wall- I meant that the whole length of the neck isn't critical (as in fireformig with false shoulder) and one calibre depth is usually enough-issue is some of the 'beyond the shoulder' has to obdurate-and obturation is achieved by a pressure process.Sooty necks mean sub optimum pressure,and incomplete obturation.

 

Enough pressue to obturate with enough of the case (neck)!

Can it -or much worthwhile in a complex world- be comprehensive and correct in one line..? :-)

 

(No- I read Turing back i the early sixties and was impressed-didn't understand his proof then,but he also had a maths equation for zebra stripes and leopard spots - uniqueness,but not random---his work of course underlies all computors-and I get the impression ballistivc calculators are no exceptions,though pretty good to mediun=m distances when well inputed...whatec=ver way Coreolis etc are going...

How's the 'hut' doing...my dad came close,but didn't have a rifle or a hut-well,not at the same time as he had me.... :-)