Case length.
- Thread starter DGY
- Start date
Leatherstocking
CGN Regular
- Location
- BC Rockies
It’s not difficult to measure your chamber.
Size a case
Make sure it chambers
Trim it shorter than usual
Cut a ring off the end, I used a fine triangular file
Use your fingers to start the ring on the base of the bullet, trimmed end towards the base
Seat the bullet backwards a bit at a time until it chambers
Measure your max case length
Then deduct your.01 or .05.
HTH. I’d post a pic, but I’m away from home for a few days.
Size a case
Make sure it chambers
Trim it shorter than usual
Cut a ring off the end, I used a fine triangular file
Use your fingers to start the ring on the base of the bullet, trimmed end towards the base
Seat the bullet backwards a bit at a time until it chambers
Measure your max case length
Then deduct your.01 or .05.
HTH. I’d post a pic, but I’m away from home for a few days.
Yes, quite a big surprise! I have the Sinclair chamber length gauge for 6.5 (.264) which I used to measure the chamber of my Tikka T3X in .260 Rem (.264).
The SAAMI max case length is 2.035". To my surprise the chamber measures 2.068", which is 0.033" longer than SAAMI spec.
- Location
- Western Manitoba
SAAMI 2015 - is two drawings on one page - top is cartridge, bottom is chamber. You are correct - for cartridge case, SAAMI calls out 2.035-.020 to cartridge case mouth. For chamber, SAAMI calls out 2.0450", so your 2.068" is well past (.023" longer than) SAAMI call out for the chamber length to mouth. I wonder if your throat was moved forward accordingly, or if that extra neck length encroached into the throat area? Is your chamber headspace to SAAMI spec? 1.630" = Min 1.640" = Max.
Good clarification on the SAAMI chamber vs the cartridge. Yes, I just checked the SAAMI drawing and you are correct for the chamber length. (That extra 0.010" between max case and max chamber makes sense, thinking about tolerances of ammo manufacturing, different brass metallurgy, and tolerances in measuring equipment, plus maybe an idiot factor. I am guessing they decided on that dimension of buffer to reduce likelihood of bad stuff from happening).
My Tikka's chamber is bang-on SAAMI spec for headspace. I use the Whidden Case Gauge (mic) which measures headspace to the datum line. My fireformed brass consistently measures at 0.000" to -0.001", so the Tikka headspace is on the slightly tight side.
That rifle has about 3000 firings and I wondered about throat erosion around the chamber end. (The actual throat for bullet seating depth is very much eroded from when the rifle was new, as expected and normal). But I checked my notes from 2 years ago when it had less than 1000 firings and the chamber measurement was exactly the same.
So I don't know if that is normal for Tikka to bore the chamber a little longer, or it is factory variability in machining? The throat length for the new rifle was nice and tight. I could easily seat 123gr to 140gr VLD shaped target bullets to touching the lands with plenty of case neck grip on the bullets. As the throat erodes past 3000 firings I can still reach the lands no problem if I want (but I seat off the lands anyway).
- Location
- Western Manitoba
Biologist - bear with me - not sure how headspace can be "bang on" - I think all SAAMI drawings call out headspace as a range - so in the case of 260 Rem is 1.630" to 1.640" - any dimension between those numbers is compliant with SAAMI headspace spec. I understand for new barrel installation, most makers will try to be Minimum to plus .004" over Minimum (between GO and NOGO gauges) Maybe you mean that you are "bang on" at Minimum - 1.630"? I do not do precision shooting, so I am still learning about the details of this stuff.
Hi Potashminer: You are teaching me stuff I was not aware of! I had no idea the SAAMI spec chamber drawings had that 10 thou variance.
I always thought that the head space number shown on reloading manual cartridge drawings, was "THE" headspace number. For example my Lee reloading manual labels the headspace for the .260 Rem (and all the other .308 Win family cases) as 1.634". No variance is cited. That is where I got the idea that there was one specific "bang-on" number for headspace, having never looked at the SAAMI drawing details until now. So you learned me some stuff!
That 1.634" is conveniently almost the midpoint between 1.630 and 1.640. I wonder why they picked 1.6340 instead of 1.6350?
By "bang-on" for my head space measurement, I meant that the Whidden Case Gauge, which is a micrometer headspace gauge that measures case head to the datum line, measures my fireformed cases typically at 0.000 or -0.001", where 0.000 on the micrometer cap means "THE" SAAMI spec exactly (as per reloading manual), so to speak. My assumption (I may be wrong), is that 0.000" on this gauge equals a base to datum line length of exactly 1.6340". I use Lapua brass. When my fireformed cases grow to +0.001 on this gauge, I know its time to full length resize for my rifle, since it has a tight chamber.
I have a set of Forster go and no-go gauges for .260 Rem. In the Whidden headspace gauge (which Whidden calls their "case gauge"), the go gauge measures -0.002". The no-go gauge measures +0.002. This is the typical 4 thou range of go and no-go gauges. My assumption is that the go and no-go gauges are made 2 thou shorter and longer than 1.6340 as the midpoint.
Unfortunately I do not have a tool to attach to calipers to measure the fireformed case head distance to the true datum line. I have the Hornady headspace comparators which are only relative to itself, not to the actual SAAMI headspace spec. The .260 Rem (and .308 Win family) uses Hornady headspace comparator D400 according to Hornady's website. This measures my go gauge as 1.622" (and my fireformed brass), which is obviously not the true measurement to the datum line.
So, when I see the reloading manuals specifying the precise headspace number, I assumed for practicality that this is the precise spec, and one's actual rifle headspace, as measured by the surrogate of the fireformed case, is compared to that number. My assumption is technically wrong as you have pointed out with the variance on the spec.
The other true headspace micrometer I am aware of (and own for 30-06) is the RCBS Precision Mic, which does the same thing as the Whidden, with 0.000 mark on the mic as "THE" headspace bang-on spec, (i.e. midpoint in the variance) so to speak.
But now I have more questions than answers.
