I use a Stoney Point Chamber Length Gauge (the straight one) to help set bullet jump to the lands when I'm reloading. I then use the SP bullet comparator so I'm measuring off the ogive and not the bullet tip.

With the SP Length Gauge, you order a threaded cartridge for every caliber you want to measure. While this is fairly accurate, it occurred to me that brass fire-formed to my specific gun should be better, so I bought the correct tap and threaded one of my fire-formed cartridges.

Using yet another SP tool, I measured from the shoulder to the base of the threaded .243 brass I bought from SP and also the piece of .243 fireformed in my gun. The fireformed cartidge was about .005 or .006 longer, which didn't surprise me.

What I'm trying to figure out mentally (before doing the physical tests) is whether measuring seating depth with my fireformed cartridge is going to be more more precise in putting my bullets the distance off the lands that I want them.

When I run either the shorter SP case or my fireformed case into the chamber with the SP tool, both enter the chamber until stopped by the shoulder, right? So I'm thinking the shoulder-to-bullet-ogive distance should come out the same using either piece of brass.

There's no question that the fireformed case is a smidge longer and when chambered fills the chamber more tightly than the piece of unfired SP brass, but if it's the shoulder-to-bullet-ogive measurement that really sets the jump, then I'm thinking it won't matter one whit which piece of brass I use on the SP tool.

If anyone remotely understands what I'm getting at, could you comment. Thanks.

I know exactly what you are getting at and I opine that the fireformed case will produce the more accurate measurement, as it is the distance between the contact of the shoulder on the camber, and the lands, that is being measured.

But that said I do not suppose it will make a heap of difference, as you would probably shoot test loads at various different COLs based around your original measurement.

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Regards

Richard

Kentucky, the distance that matters is the distance, as measured by, the S.P. comparator that gives you the accuracy that you want! Either case will do. It does not matter how far off the lands it precisely is!
However, the fire formed case gives the best measurement of bolt face to lands, and is an interesting number to have on hand.
Regards, Curley

Whatever the difference is between your fireformed case and the SP case, add that value to the measurement derived using the SP case. For example, lets say using the SP case, your measurement to land contact using their comparator insert is 2.994". Lets further assume that there is a .006" difference between the case head-to-datum on the shoulder between the two cases, with your fireformed case being the longer of the two. Just add the .006" to the 2.994" measurement to get a 3.000" casehead-to-land contact for that bullet make/model along with your fireformed brass. From that dimension you can start there and then work backwards during your load development to tune the offset from the lands.

If you change bullets, you have to establish a new length to the land contact point since the ogive on the new bullet will likely be different.

Hope this helps.

Martindog

[This message has been edited by Martindog (edited 04-06-2002).]

Martindog, the distance from the boltface to the lands is built into the rifle and does not change (hopefully)! Bullets will be seated more or less depending on ogive shape, but the measurement is still the same.
Regards, Curley

Curley,

I agree that the distance is fixed for each rifle, but when using the SP modified case, we're not indexing off of the bolt face of the rifle in question, but rather off of the case head of a modified SP case. The SP cases are formed from brand new, unfired brass (i.e., short by a couple thousandths of an inch cases). What I'm saying is that when you use fireformed cases, for all practical purposes the bolt face position (our desired index point) is equivalent to the casehead since the rear of the case (casehead) already expanded to come to rest up against the boltface and the front of the case (shoulder/datum) has come to rest against the chamber's datum.

Just try to picture the SP tool in use with your calipers. Assume that we are trying to calculate the proper seating depth for a standard .308 Win case. Also try to picture using it on a case where it is extremely overly short from the casehead to the datum -- picture the same 308 case, only in a really exaggerated PPC form (short and fat). Assume for a moment that this is the basis for the modified SP case that screws into the tool and you use with your calipers. It's obvious that when we measure from the casehead of this case to the datum line, we will end up with a measurement that is way out of whack (way short). If you substituted a fireformed 308 case by tapping its head for the SP tool, you would end up with a much longer dimension. Since either case indexes off of the datum and the lands when you insert a bullet, the difference between the two cases' would be found in the caseheads-to-datum measurements. This needs to be taken into account when using the SP tool with fireformed cases to establish a casehead-to-land contact point baseline measurement.

Martindog

[This message has been edited by Martindog (edited 04-06-2002).]

im thinking about getting a stoney point tool may i ask what tap it takes to make cases. thanks jason

Martindog,you are correct, but why bother since no one (almost) seats a bullet into the lands. On another forum, a poster wrote that using the same dummy round, measuring with both the S.P. comparator and the "hex nut" type from Sinclair, there was a considerable difference in length from case head to ogive.
He then measured the inside diameter of the two tools and found several thousants difference! In all probability, neither tool matched his bore diameter exactly. I think S.P. knows what they are doing, and are coppering their bets by giving a slightly short measurement to please their lawyers!
Personally, I feel that if you want the exact (as measured by S.P.) distance from b/f to ogive, chamber a dummy round with a snug, but slip fit on the bullet so as to jam the bullet in the rifleing, then measure.
Regards, Curley

Jason,

I read on these boards a while back that it is a 5/16 X 36. These are hard to find in the local HW store and you'll probably have to order it. I haven't gotten one yet, but I will one of these days.

Take Care,

-Steve

Steve ive got a msc industrial supply here in town they sould have one. thanks jason

Thanks for all the input. The whole question is rather splitting hairs, but isn't that what accurate reloading often comes down to?

Martindog, I think your analysis and suggestion are the best I've seen: measure the shoulder-to-casehead difference between the (shorter) SP brass and (longer) fireformed brass, and add that difference to my seating measurement when using fireformed brass.

I emailed Stoney Point about a week ago, asking them to have a tech email or call me. I haven't heard a word, but then they may not monitor their website on a daily basis.

KF,

Glad to help. One last suggestion -- whichever bullet you use to make your initial measurement, save it. Every 500 rounds or so use it again to remeasure the casehead-to-lands dimension. That way you'll be able to track throat erosion and adjust your OALs accordingly.

Martindog