In this youtube video, Erik Cortina (US Team Lapua F-Open) calls out the Sinclair Hex nut. He measures the Sinclair nut 7mm orifice saying its .272" when it should be .276" so its not even measuring where it should be. In the video I think he mixes up some of the comparators, but his point remains the same, being that the Sinclair hex nut does not measure at the correct point on the bullets ogive.
I use the Hornady bullet comparators and measured my 7mm bullet comparator to be .272" also. It must follow that my Hornady bullet comparator is equally offensive as the Sinclair, compared to his "sticker shock" expensive Bob Green comparators. For whats its worth, I measured my 30 cal to be .298", the same as his Bob Green version and the Sinclair version.
That being said, I don't think the exact diameter of a bullet comparator is critical to most of us as long as we can get some reference point measurement firstly to the lands, and secondly to some repeatable point on or close to the ogive of the bullet. You will be able to take accurate CBTO (or close enough to ogive to call it that) measurements whether its exactly on the ogive or not. Once we have a starting point, use the same tools to measure and adjust seating depth as needed.
Lightly start a bullet into a resized deprimed case about as deep as the bullets diameter, close your bolt on this case, then carefully extract and measure CBTO with your tools and your bullet comparator. Repeat 2 more times, more if necessary, and take the average. In your rifle, with your tools, with this bullet, that measurement is kissing the lands and is your reference maximum CBTO for that bullet without seating into the lands. Does it really matter that the measurement was taken at the bullets ogive? No, you can duplicate this CBTO anytime using your calipers and comparator, and more importantly you can use this length to start your seating depth adjustments. Using the Hornady OAL gauge should get you the same results, except you paid for an extra tool to get the same information.
I start my seating depths at kissing - 3 thou and move deeper by 10-30 thou depending on calibre, using finer +- adjustments once I've found something worth investigating further. As mentioned above, whether your comparator measures this length from the ogive, or slightly off of, you will still be able to adjust bullet seating depth by the same amount provided you use the same tools and comparator.
Knowing the maximum CBTO measurement will hopefully prevent you from creating an over-pressure situation by seating into the lands when you had not intended to. As another precaution, I test fire the bullets that are seated deepest first, working up to the ones that are seated closest to the lands, being under the impression that bullets seated closer to the lands would develop a higher pressure, although I have no test data that verifies this relationship.
If closing the bolt did not cause the bullet to further enter the case, the barrels throat or leade may be excessive, likely due to the number of rounds down the tube, and probably will not fit in your rifles magazine anyway so you've no other choice than to seat deeper, unless you want to single feed your rifle.
All bullets with an identical bullet ogive profile will have the same CBTO when touching the lands, irregardless of the mass or composition of those bullets. You can use this to your advantage if say you're working with bullets from the same manufacturer and type. For example 50 and 55 gr .224 Nosler Ballistic Tips both share the exact same bullet ogive profile. No need to have to re-measure maximum CBTO if you had already measured one of them. So if you've already found a good shooting CBTO for the 50 gr projectile, that same CBTO might work equally well with the 55 gr'ers. Worth a shot anyway.
I reloaded for many years happily measuring case base to bullet tip as taken from the manuals. Comparators as we now know them were not generally available way back then. Trying to return to the same seating depth after using the seating die for another bullet or rifle was probably a +- 5 thou proposition. Certainly close enough for a hunting round, but over the years I've become more demanding, ok OCD'ish and using a comparator will enable you to scratch that itch and get within +- 1 thou every time. The critters won't know the whether you did or not though.
