Thread: CBTO new projectile

I despise load development.

I plan to try ELD-Ms on two rifles and use the same CBTO measurement from the previous SMK loads, measured using the Sinclair tool.

Would you measure the new projectile touching the lands. I can’t see that it’s necessary other than to measure possible throat erosion rather than as a safety precaution. I can’t see that the shape of the projectile changes the distance from the case base to where the ojive engages the rifling.

Any contrary thoughts?

That's what I do for my 2 223s, you just have to watch if you'll exceed mag length. As Gimp has demonstrated the seating depth has no effect of hunting accuracy, if at all.

Ta.
Yeah, on one of them I’m trying to get down to mag length.

Originally Posted by Tentman

That's what I do for my 2 223s, you just have to watch if you'll exceed mag length. As Gimp has demonstrated the seating depth has no effect of hunting accuracy, if at all.

Literally the only difference in all of these is seating depth

Originally Posted by Kelton

Literally the only difference in all of these is seating depth

I like statistics and science. 3 shot groups are neither.

Originally Posted by Moutere

I despise load development.

I plan to try ELD-Ms on two rifles and use the same CBTO measurement from the previous SMK loads, measured using the Sinclair tool.....I can’t see that the shape of the projectile changes the distance from the case base to where the ojive engages the rifling.

Any contrary thoughts?

Well that would be a fair assumption if the "gauge" had exactly the diameter of your bore and that may not be true. If there's a mismatch, the different nose profiles of the two projectiles will make the outcome less certain.

In the end, it's what happens on paper that matters. Avoiding a hard jam is always wise in a hunting gun.

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.

Originally Posted by Tentman

As Gimp has demonstrated the seating depth has no effect of hunting accuracy, if at all.

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

Originally Posted by Kelton

Literally the only difference in all of these is seating depth

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

For example this gentleman has done a tremendous amount of testing different loads using the usual methods, powder charge and seating depths - and doing a heap of faff like weight sorting brass and bullets - and every single one of the groups shot during the "development" process just falls within the cone of fire he gets when he actually shoots 20rd of his "optimised" load - which he ultimately isn't happy with


https://youtu.be/BSjTbf26EpA?si=0hhmMNaGCP4ilN2z

The seating depth tool measures the distance from the base of the case to the lands. This datum's then used to measure OAL/CBTO using a comparator. This distance is specific to the rifle and surely not how the projectile interacts with this point.

The only variable is whether the comparator is the same size as the bore.

The Sinclair tool is more or lees the same as the Hornady tool, but is not calibre specific.

I'm just being lazy cause the gun cabinet is buried behind piles of crap at the moment and I didn't want to repeat the measurements.

Originally Posted by Moutere

The only variable is whether the comparator is the same size as the bore.

Thats the query right there, I've alway assumed the comparator ID and bore ID were the same.

Looks like I'm having a big tidy up and repeating measuring to the lands.

Originally Posted by Moutere

The seating depth tool measures the distance from the base of the case to the lands. This datum's then used to measure OAL/CBTO using a comparator. This distance is specific to the rifle and surely not how the projectile interacts with this point.

The only variable is whether the comparator is the same size as the bore.

The Sinclair tool is more or lees the same as the Hornady tool, but is not calibre specific.

I'm just being lazy cause the gun cabinet is buried behind piles of crap at the moment and I didn't want to repeat the measurements.

Glad you made that point, different projo's are just that, different. To assume they be the same is not in the shooters best interest. Good luck with your tidy up, let us know what the difference was between the bullets.
I load ELDM's and Siera's (both 155 grain) they go to different specs on the comparator.

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

Please do, I'm interested. VLDs specifically are reputed to have precision issues with seating depth - the single load I've tried with them was 10rd into 0.6MOA at a totally random arbitrary depth & charge