Comparator -vs- COL

[Akula69]

Now this may be a question that others have had (and I did do a cursory search but did not find a direct answer to the question) so here goes:

Is there any correlation between cartridge overall length (as shown in reloading recipes) and comparator length?

I recently reloaded some 300 AAC with a 147 grain tracer projectile (its a bit longer than normal 147 grain projectiles due to the phosphorus pocket at the end) using the comparator. The projectile seats with the cannelure just north of the case mouth.

The powder I'm using (CFE-BLK) does not create a compressed load and requests a COL of 2.235. Using the comparator on the projectile ogive, (after slugging the barrel) I have a caliper reading of 1.600 (+- .002). This comparator length results in an average COL of 2.18. Both cartridge lengths will fit in the magazine and feed reliably (although I suspected the 2.235 cartridge is touching the lands and confirmed it by 'coloring' a projectile with a sharpie to see the marks)

Now, I realize using the comparator is to get as close to a 'no bullet jump' situation as possible, but I have a concern about the .055 overall length difference. Should I consider a different powder (AA 1680) which allows for a shorter (edited, I originally posted "longer") COL? or just use the 2.235 regardless what the comparator indicates?

 

[Walkalong]

Base to ogive is important if you’re concerned about distance to the lands (and of course chambering without being into the lands).

OAL is important for fitting the mag.

I log both for some applications.

OAL is set by your bullet/gun, not the powder. If the data with a powder uses a different OAL than you settle on, adjust charge as/if needed.

 

[243winxb]

I suspected the 2.235 cartridge is touching the lands and confirmed it by 'coloring' a projectile with a sharpie to see the marks)

I would make the col shorter by .020" or a col of 2.215" when measured from case head to bullet tip. Must feed & fit magazine.

The bullet ogive should always be above the case mouth.

 

[jmorris]

The powder I'm using (CFE-BLK) does not create a compressed load and requests a COL of 2.235.

Powder doesn’t dictate COL, that data would have come with some other data like a charge weight range and a bullet choice.

Now, I realize using the comparator is to get as close to a 'no bullet jump' situation as possible,

It’s really just a way to get a more consistent measurement than using the tip of a bullet. Some designs differ in length quite a bit. Can you imagine pulling out your hair trying to get consistent OAL using say SMK’s? Where base to tip of just the bullet can differ by .005.



But are identical base to a datum along the ogive.



Having the same OAL/COL using them would leave you with different amounts of jump to the lands between the rounds.

 

[Akula69]

I may have misspoke when I indicated the powder 'requested' a particular COL.

What I was trying to express was my belief that when a certain load using a certain powder requests a COL of a given length it may (in part) indicate pressure range as well. I've always believed seating a projectile lower than the COL could cause pressure spikes (hence our aversion to bullet setbacks) but I don't know if the variance I have could cause an issue.

I suggested a switch to AA1680 as an alternative because the load for that cartridge has it requesting a COL closer to the comparators' COL. Trouble is, I like the accuracy I get in my platform using CFE-BLK.

But then again, referencing Walkalongs' post, I also believe if the projectile starts is journey already touching the lands it could cause a pressure spike as well.

 

[CMB]

Seating depth will influence case volume.

Maybe I shouldn't say this but to be safe- if you are reducing the case volume by seating deeper for any reason (projectile is larger than listed projectile) you may want to reduce the charge by the delta of the volumes- if near max charge. Develope the load from there.

Cheers,

 

[GeoDudeFlorida]

when a certain load using a certain powder requests a COL of a given length it may (in part) indicate pressure range as well

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Seating depth will influence case volume.

This exactly. What it seems to me you're really asking is NOT how much bullet should be sticking out of the case but how much bullet needs to be seated in the case. Yes, you can (and maybe should?) use a comparator, or some other reliable measuring device, to determine where the listed bullet will seat and where your substitute bullet will seat so you have no change in volume. I use depth micrometers directly on the bullets and down the cases, and pencil/paper, to calculate the relative change in seating position between similar bullets when I make a substitution. A good fit comparator is probably a lot more accurate. A bad fit comparator is probably about as accurate.

 

[jmorris]

What I was trying to express was my belief that when a certain load using a certain powder requests a COL of a given length it may (in part) indicate pressure range as well.

That too will accompany a given bullet. The pressure curve with a given bullet, load and barrel. Won’t be linear or even in one direction. With the seating depth out to touch the lands, despite increasing the volume in the case, it will be higher than say .020 off them, with less case volume and shoved into the case so they are .250” off the lands might make it higher than .020” as well due to the greatly reduced case capacity, despite a huge running start.

In any case the projectile is going to be a key variable in this equation.