Bullet (Ogive) Comparator - Do you use it and how

[Haus]

After hastily shopping on Midway and accidentally purchasing a bullet comparator set instead of a headspace comparator, it got me to research this tool. On its face, it seems like a useful tool, and the reasons for using it pass the smell test: bullets aren't 100% uniform, so measure bullet depth from the Ogive. So, my question to y'all is, does anyone on here use a bullet comparator when loading for a 308 gas rifle? Most videos and articles I've seen were for bolt guns, which makes sense as they have more freedom in OAL length, whereas gas guns are limited by the magazine. When you guys reload, do you base your bullet depth on OAL or Ogive measurement? and if you use a comparator, how do you determine your depth as getting "close to the lands" isn't really what you chase with a gas gun given its constraints? 

Also, (and I'm sure I can figure this out by putting a few rounds through the press) does the bullet seating die set the bullet depth bases relative to the ogive? Basically, if i get an ogive measurement I like, and the bullets aren't all the same length, will I have to adjust the bullet seating die for every round to get consistent results? or vice versa if I choose the stand OAL of 2.8" does the seating die consistently pumped out OAL no matter discrepancies in the bullets themselves?

[98Z5V]

I'll add what I can, in parts.  Easier for me to keep up with it that way.

4 hours ago, Haus said:

how do you determine your depth as getting "close to the lands" isn't really what you chase with a gas gun given its constraints? 

Don't chase the lands.  In a gas gun, you're constrained by internal mag length anyway, so you'll lever get to "load xxx off the lands."  Here's information on that, at any rate:

 

 

I have a Hornady comparator, and use it on my precision loads.  On bulk loads, I just set them to OAL and be done with it.

When you get a couple boxes of the same projectiles, and measure them, you'll find projectile OAL all over the place.  So, using OAL to seat those projectiles has some of them further into the case than others, which changes case fill, which changes pressures.  Which changes muzzle velocity, and with that, potentially the accuracy. 

Using the comparator to check seating depth will be a better way of seating the projectiles to the same depth in the case, and either eliminate or reduce those inconsistencies, in turn making your hand loads more consistent and accurate.  Once you use the comparator on a batch, check the loaded cartridge OAL, and you'll find some longer or shorter than others.

Start by finding several projectiles that are the same OAL.  Use the cartridge OAL that you want, seat them there.  Then take the comparator readings of all those loaded carrtidges.  That's the comparator number that you want to seat all of them to.  Sometimes you have to give some of them a bump to get them in there further.  Sometimes you need to knock a few of them out a little longer with a kinetic bullet puller to hit the right comparator number.  It'll definitely help your ES and SD numbers, and even out your muzzle velocities.

[Haus]

Great thank you, that makes a lot of sense. I've seen those videos as well too, they're very informative. I appreciate the share of knowledge on how you determine your seating depth with the comparator. I'm sure just like everything else with reloading, you can adjust the bullet seating depth on different loads and see how it impacts groups. Thanks!

[98Z5V]

12 hours ago, Haus said:

you can adjust the bullet seating depth on different loads and see how it impacts groups.

You can definitely do that.  I do seating depth tests 0.003" apart.  Take the 5.56 for example.  Max OAL is 2.260".  Mag internal depth might buy you a few more thou, depends on the mags.  Once I find a good powder load with good ES and SD, and decent accuracy, I'll load those things at 2.250", 2.253", 2.256, 2.259", etc.  Go with the best one, and believe it or not, the shorter one is better, if it's a good load.  As the barrel goes through throat erosion, and your accuracy starts to drop off - you bump that load out another 0.003", and the accuracy comes back...   That's your new load...  until the throat wears more with more use.  Repeat as necessary...  

[NF1E]

Makes a huge difference when all the stars align. With P-Mags you can file some material off the rear inside to give you a few thou of extra clearance. I prefer a jump of .004 over a jamb. Whatta Hobby!

 

 

 

Edited September 14, 2023 by NF1E