Meuasuring bolt face to ogive (jam) distance (E. Cortina's method)

[bsbxl]

I am trying to get a reference number for seating depth variation in load development (.308 Winchester in a Tikka T-3 Tactical). In another thread I read the instructions from Eric Cortina to measure the bolt face to ogive (jam) distance. If I understood his method correctly, he says to seat a bullet long in the (unprimed) case, put some sizing wax on the ogive, chamber te round and let the chamber seat the bullet to ogive jam distance, lift the bolt quickly and then retract the bolt and measure the cartridge with a comparator. I followed this procedure (loaded single shot with a single shot follower in the magazine) for four different bullets, all with Lapua cases:

• Hornady 168gn. AMAX: 2.274"
• Hornady 208gn. AMAX: 2.281"
• Hornady 178gn. HPBT: 2.273"
• Lapua 155gn. SCENAR: 2.302"

There is a 0,029" difference between the longest and shortest measurements. This worries me a bit. I realise that the distance to jam depends on the ogive shape and the 155gn. SCENAR has quite a different ogive. Do you think these measurements are plausible or should I do the procedure over again? Thanks.

bsbxl

 

[savageshooter86]

:-[

Each bullet is different. So if you have those 4 different bullets(weight/style/brand) you need to measure each one and log the numbers of the "JAM" for each bullet in your chamber. Who cares if the there is 29 thous difference between the 155 scenar and the 208 gr Hornady. Two different bullets and will be two completely different loads and seating depths

 

[bsbxl]

@ savageshooter86

Thank you for your reply. I expected to see different measurements given the different bullet shapes. However, I was a bit surprised by the 0.029" difference... I logged the numbers of the "jam" for each bullet and also kept the cartridges. I will use them for setting up my Forster micrometer seating die. The 155gn. SCENAR will be the first candidate for load development.

bsbxl

 

[jlow]

What is much more important than differences between different types of bullets is variation within the same bullet.

Would suggest first measuring either bearing surface length (two comparator in a pair of calipers) or bases to ogive (one comparation in a pair of calipers) to see how the bullets lot you are most interested in (or all four if you are not sure) varies. Then do what you have already done but do it with at least 4-5 bullets of the same type and lot and see what the range of distance is. This will give you good information for choosing bullet type and where you will seat them to.