Thread: CBTO new projectile

Originally Posted by gimp

There's a lot going on here,

1 - I'm not familiar with the Sinclair tool, but the Hornady comparator (by necessity) measures to a point along the ogive, not the point of maximum ogive diameter. Because bullets have different ogive profiles, using this measurement for 2 different bullets will not necessarily result in the same distance off the lands. You may risk touching the lands depending on the specifics (e.g. how far off your reference load is)

2 - ELDMs usually have a much more aggressive ogive profile than SMKs, so using the OAL from an SMK of the same weight to seat an ELDM will usually result in the ELDM being seated further off the lands. This may be a problem depending on the specific components - e.g. ELDMs are often very long, and the bullet shank may intrude excessively into the powder capacity. Using the same die setting doesn't mean that you'll have the same OAL, depending on the die.


If you are worried about being seated too deep using the same OAL as the SMK, I'd recommend taking the necessary measurements, and seating to either -
1. Maximum OAL that the magazine allows, OR
2. An arbitrary OAL 0.020" - 0.050" off the lands

Whichever is shorter.




I'm not sure I've demonstrated that, exactly. What I haven't been able to do is demonstrate any difference in precision between seating depths with the combinations that I've tried. It is possible that there is a real difference in precision between loads at different seating depths (within the realistic functional window for the combination) for some bullets, cartridges, etc - after all a straight start into the rifling to minimise principal axis tilt is a key to reducing dispersion. I just haven't been able to find a difference.

I have not seen anyone else demonstrate this either.

Typical results of seating depth tests with small sample size groups simply show that all depths produce shots that fall into the wider true cone of fire of any of the loads.


It's also really clear that -
1. "actual" precision (cone of fire) is a lot larger than what most people think they have when you measure it properly
2. achieving a result that meets functional requirements doesn't require "tuning", it is a lot more efficient to define requirements (based on more realistic expectations) and test if a combination of components meets those that it is to try optimise to find the "best" load with that combination of components with small changes


This brings me to


@Kelton, can you talk us through what we're looking at here?

In short a seating depth test with 180grain vld from my 7mm saum I will endeavour to shoot 10 of what I consider the worst and 10 of the clear best (6shots) measuring .355 center to center and see what happens it won’t be immediately but il post results open and honestly