6.5 Creedmoor Question

[kjmdrumz3]

So, measuring my OAL I used the Hornady tool and got a consistent measurement of 2.280” BTO. Using the Eric Cortina method of finding “jam”, I’m getting a consistent measurement of 2.313”. The American Gunner round I’ve been shooting (which shot pretty great btw, out to about 800 where the ES of 52 and SD of 19 really started showing itself) measured between 2.208”-2.218” with most being in the 2.213”-2.215” range BTO. Where should I begin? I like the EC method. Seems to make sense. But starting at 2.293” is still about 13 thousandths longer than what the Hornady gauge is saying. I’m working up a load for the 130gr Sierra SMKs right now. Both the Hornady gauge and EC methods were measured with the SMKs I plan to use. I just want to be safe as I work up my OCW.

 

[918v]

I like .050” off to begin with.

 

[kjmdrumz3]

Yes, but .050” off of which measurement?

 

[Wisconsinner]

If it were me, I would seat .030 off of "jam". Using Eric's method. That .013 you are seeing with the hornady tool is barley in the lands assuming you pushed the rod lightly.

 

[Jefe's Dope]

Does the Eric Cortena method use a bullet in an empty case IIRC?

I'd go w/ the Cortena measurement. It's pretty accurate. The Hornady tool requires a special 'touch' to feel the resistance.

 

[918v]

Yes, but .050” off of which measurement?

The Hornady comparator one. The hard jam measurement is unrealistic and unworkable. No one loads ammo that way. It is stupid.

I like .050” off where the bullet is touching the lands. That much jump prevents pressure spikes and weird shit when you’re trying to find max pressure.

 

[sheepdog697]

Following this thread.

 

[kjmdrumz3]

So, it’s reasonable to have 33 thousandths of space between just barely touching the lands (Hornady) and being at “jam” (EC)? If that’s the case, start working my OCW at 2.283”? That’s still just in the lands.

 

[kjmdrumz3]

The Hornady comparator one. The hard jam measurement is unrealistic and unworkable. No one loads ammo that way. It is stupid.

I like .050” off where the bullet is touching the lands. That much jump prevents pressure spikes and weird shit when you’re trying to find max pressure.

The purpose, according to EC, of finding the “jam” is to find your true do not exceed seating depth. In my case, I don’t think that would fit in my mag anyway. Oh, Tikka TAC A1, btw. I forgot to mention that.

 

[kjmdrumz3]

Does the Eric Cortena method use a bullet in an empty case IIRC?

I'd go w/ the Cortena measurement. It's pretty accurate. The Hornady tool requires a special 'touch' to feel the resistance.

Yes, resized, decapped, empty case.

 

[jakelly]

IIRC they shouldn’t be that far apart. I tested this a couple different times with different bullets, but I don’t think you can jam over ~.020”.

 

[kjmdrumz3]

IIRC they shouldn’t be that far apart. I tested this a couple different times with different bullets, but I don’t think you can jam over ~.020”.

I’m not disagreeing with you, because this is the first time trying to do a super accurate work up. But I measured both ways 3 times or more and got the same numbers each time. On the Hornady gauge, I’m stopping as soon as I feel contact, however light. So maybe I could have lightly tapped it in another .013 or so But I do know that seating the bullet long and letting the rifle seat it into the case is accurate. My bolt will only exert so much pressure and seat the bullet so deep.

EDIT- just watched a Hornady video where the gentleman specifically says to seat the bullet firmly. Everyone else I’ve seen says to seat it lightly. Like just kissing the lands. Well, that looks like the reason the 2 measurements are so far apart. Thanks @jakelly

 

[jakelly]

Yeah man, there’s a bit of a feel to that Hornady method, but I like to make sure the bullet is touching on all points of the rifling when I do it. That requires a bit of push without flexing the little plastic or engraving the rifling. Try to seat the bullet firmly and see what measurements you get.

 

[kjmdrumz3]

Yeah man, there’s a bit of a feel to that Hornady method, but I like to make sure the bullet is touching on all points of the rifling when I do it. That requires a bit of push without flexing the little plastic or engraving the rifling. Try to seat the bullet firmly and see what measurements you get.

Yup. See my above edit.

 

[jakelly]

Good deal. Out of curiosity what numbers are you getting now? 2.295?

 

[kjmdrumz3]

Good deal. Out of curiosity what numbers are you getting now? 2.295?

2.298, 2.298, 2.294. And the last one I know I was a touch lighter on the plunger.

 

[kjmdrumz3]

So now that the numbers make more sense, where would you start? I feel like starting my OCW testing at the 2.295” is best because backing off of that by .010 or .020 later when I do my seating depth will reduce pressure, not raise it (to a point).

 

[Graye2]

I personally do not do my testing that close to the lands...bullet variations/seating depth variations can put you right back in and cause pressure spikes as noted above.

I would start .020 off but in fairness I don’t to OCW. I find my velocity node 1st and then do a seating depth test in which I largely go shorter in .003 increments.

 

[rady]

Not fond of the Hornady plunger method of sliding the bullet in. I found E.C's method to be much better for my 6.5 C MRAD barrel. It is very consistent. Another method that works is to lightly drop the bullet into the chamber by tipping the barrel and allowing the bullet to fall and self center. Of the two methods, I prefer the E.C. way.

For my barrel, the cbto measured 2.273, 2.258, 2.281" for 3 different bullet manuf. that I was using.

 

[SicVic]

I've meticulously tested both with the 6.5.
EC method is designed to start you @ 100% one end of the jump/jam measurement. This being NO jump/ max jam.
How much Jam you achieve will be directly related to the tension applied to the bullet from the case neck among several other factors.
Without going into specific details when using the EC method I encountered the pressure spike issues.
Again, skipping all details; in the end of testing 100 rounds I ended up with .0415" jump. Surprisingly, this was the EXACT same outcome I had already achieved using the Hornady.
In my case it just confirmed that the way I currently use the Hornady provided relatable results to EC.
My end take was just two ways to skin a cat.
Using the Hornady tool I do put some effort into seating the bullet with the rod.
I'm also of the mind set of backing off that max while developing charge weight then fine tune after.
You will be chasing your tail if you're inducing pressure spikes into your ladder testing unknowingly.
I can be more detailed if necessary.

 

[918v]

The purpose, according to EC, of finding the “jam” is to find your true do not exceed seating depth. In my case, I don’t think that would fit in my mag anyway. Oh, Tikka TAC A1, btw. I forgot to mention that.

Jamming a bullet .030” into the lands to find some number that you will never use? You can’t see the retardation in that?

 

[Bradv86]

If your limitation is in fact mag length, I would start there and perform seating depth tests at and below that number. Anything longer is irrelevant unless you intend to single feed it. No need to over think it IMO. Tikkas are known to have some long-ish throats.

 

[CK1.0]

Jamming a bullet .030” into the lands to find some number that you will never use? You can’t see the retardation in that?

I use the number I get. It's great for those of us that don't chase the lands because determining the jam number takes no time, and requires far less dicking around than using one of those Hornady tools or really any of the other methods to supposedly "find the lands".

Every single method I've ever seen to measure the lands is flawed, so really they all require a certain amount of "faith" in the number one chooses to work off... some methods are just shittier than others.

The jam number is used only as a guide and is supposed to only really be measured once, as it's relative, so you could measure it 3 times in a row and get slightly different numbers every time and it doesn't even matter: you just pick one to work off... it's just to get a decent "do not exceed/end of the road" number on one's chamber, that's it. It's used kind of like a "zero" or the tare on a scale for measuring other stuff.

The number you get can actually be really useful for some of us, as it's used as a guide to base one's relative bullet-jump measurements off of... it's just a coarse measurement, just gives an idea of where the "brick-wall" is as far as max worst-case cartridge length.

For example, these days I don't even bother starting at less than a .060" jump, so if I find jam with EC's trick and get a CBTO of say 2.313", then I know I can start with my rounds measuring 2.253" CBTO to have a .060" jump-to-jam.

To me, "jump-to-jam" actually means something, whereas the ubiquitous ".020 off the lands" doesn't really mean shit as where exactly the lands actually are is a moving target, and 10 different guys will measure it 10 different ways and get 10 different numbers. So in a weird way, while not exact, jam is probably a more precise measurement to work off.

I honestly have know idea where the lands are on my rifle and really don't even care.

 

[Praeger]

Here's the thing about using any method to find your starting point for bullet seating depth - where the bullet begins to make contact with the throat is not a hard stop. The angle the lands begin to rise from the freebore is a very shallow angle, around 1.5 degrees. The bullets ogive is not as shallow, but it's a ramp also which is why it is hard to get consistent measurements.

The good news is that it doesn't matter. Where ever you get as a contact point, just subtract 0.020" (or whatever jump you want to start) and start your load development. From that point out you'll be using the CBTO measurement which is much more easily repeatable. Tune to what works best and don't look back.

 

[kjmdrumz3]

I use the number I get. It's great for those of us that don't chase the lands because determining the jam number takes no time, and requires far less dicking around than using one of those Hornady tools or really any of the other methods to supposedly "find the lands".

Every single method I've ever seen to measure the lands is flawed, so really they all require a certain amount of "faith" in the number one chooses to work off... some methods are just shittier than others.

The jam number is used only as a guide and is supposed to only really be measured once, as it's relative, so you could measure it 3 times in a row and get slightly different numbers every time and it doesn't even matter: you just pick one to work off... it's just to get a decent "do not exceed/end of the road" number on one's chamber, that's it. It's used kind of like a "zero" or the tare on a scale for measuring other stuff.

The number you get can actually be really useful for some of us, as it's used as a guide to base one's relative bullet-jump measurements off of... it's just a coarse measurement, just gives an idea of where the "brick-wall" is as far as max worst-case cartridge length.

For example, these days I don't even bother starting at less than a .060" jump, so if I find jam with EC's trick and get a CBTO of say 2.313", then I know I can start with my rounds measuring 2.253" CBTO to have a .060" jump-to-jam.

To me, "jump-to-jam" actually means something, whereas the ubiquitous ".020 off the lands" doesn't really mean shit as where exactly the lands actually are is a moving target, and 10 different guys will measure it 10 different ways and get 10 different numbers. So in a weird way, while not exact, jam is probably a more precise measurement to work off.

I honestly have know idea where the lands are on my rifle and really don't even care.

Thank you. I couldn't have hoped to explain it better.