Sinclair Comparator Body with .26 Bullet Insert

Sinclair Comparator/Bump Gage Body and Inserts

The Sinclair Comparator/Bump Gage Body, along with various inserts, allow accurate measurements of bullet seating depth and sizing die adjustment.

One of the most important skills a handloader can learn is how to make accurate and consistent measurements. When I first started reloading, I knew very little about it. I can remember seating 210 grain Nosler Partitions for my 338 Winchester Magnum to the cannelure. I did it that way because I had no way to measure the overall length of the loaded round or the knowledge to think it mattered. The cannelure looked like a good line to aim for.

When the time came to size the brass, I followed the directions that came in the RCBS die box. I was completely ignorant about brass stretch or work-hardening. As long as the loaded round fed into the chamber from the magazine and went boom, I was happy.

A lot has changed in twenty years. I can’t even imagine loading a round now without knowing precisely where the ogive of the bullet is in relation to the rifling in the barrel. Likewise, the volume of fire I now send downrange dictates making the brass last as long as possible. At the same time, I try to make that brass fit the chamber as snugly as possible while still functioning under field conditions.

Sinclair Comparators and Inserts

To measure for bullet seating depth and case shoulder bump, I use Sinclair Comparator/Bump Gage bodies and matching inserts.

The bullet inserts allow me to measure at the ogive of the bullet, rather than the tip of the bullet. With one comparator/insert combo, the Cartridge Base to Ogive Length(CBOL) can be measured. With two, bullet bearing surfaces can be accurately compared. Because of machining tolerances, I mark the comparator bodies and inserts, noting which ones I used for a particular measurement.

The bump gage inserts are used for consistent measurements from the case head to the shoulder. They’re machined to match the case’s shoulder angle. I use these to set up my sizing die.

The comparators are made of anodized aluminum. One end is slotted to fit over the jaw of a dial indicator, while the other is bored to accept an insert. Both ends have set screws to hold it all together.

The inserts are machined from stainless steel, and marked with either the bullet caliber diameter or case shoulder angle.

To use the Sinclair Comparator Body with a bullet insert, attach the appropriate insert to the comparator, then clamp the comparator to the dial indicator.

Once the appropriate insert is in the comparator body and the body is clamped to the dial caliper, I zero the calipers with the comparator/insert in place. Now I can measure from the case’s head to the bullet’s ogive for a consistent CBOL value.

The procedure is the same for comparing bullet bearing surface length variations—zero the caliper with two comparator/insert combos clamped in place. For this measurement, I usually zero the caliper with a bullet in place then note how much each bullet varies from the initial measurement.

Sinclair Comparator Checking CBOL 338 Edge

Checking CBOL on a loaded 338 Edge +P round with a bullet insert.

The Sinclair Comparator Body with a bump gage insert is used to set up a full length sizing die.

The bump gage inserts are machined to fit the shoulder angle of a cartridge case. The setup is the same as the bullet insert—once the insert is in place, zero the calipers. Measuring the case like this allows me to set up my full length sizing die for a precise amount of “shoulder bump”, usually .001”-.002”.

260 Terminator Comparator Bump Gage Insert

Checking shoulder bump on a 260 Terminator case with a bump gage insert.

The Sinclair Comparator Body and inserts, along with a good dial caliper, are tools that I consider indispensable. I recommend them early and often to any new handloader. Here are some links: Sinclair Comparator Body and bullet inserts, bump gage inserts.




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As an Idaho native, avid hunter, and long range shooter, Sam has written numerous articles and gear reviews for various online publications. Specializing in long range hunting in the mountains of northern Idaho, Sam founded Panhandle Precision as a way to continue sharing his passions.

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  • Paul Fakenbridge
    June 29, 2016 at 21:08


    Nice review on the bullet comparator. If I remember correctly a comparator needs to have the same leade angle as your chamber to go from one bullet to another. I make my comparitors by running the reamer into a barrel stub. Works great and is exactly the same as my chamber. Just food for thought.


  • John Haus
    August 23, 2018 at 11:55

    Regarding the Sinclair comparator body inserts, I have been using the RCBS Precision Mic and it seems to work well. But as you know, it registers off the shoulder datum line and not the entire shoulder like the Sinclair insert does. Do you like the insert that reads off the entire shoulder better? Do you get a more accurate reading?

    John Haus
    Spirit Lake, ID

    • Sam
      Sam Millard
      August 27, 2018 at 14:48

      Yes, I’ve used the RCBS Precision Mic in the past, but I prefer the Sinclair comparator and inserts. I don’t know about them being more accurate, but they are definitely more consistent.

      • John Haus
        August 29, 2018 at 13:42

        Thanks for your response, Sam. I have used the Hornady comparator and it is a toss up as to which one I like the best.

  • ralph oppeneer
    February 8, 2022 at 07:55

    i’m wondering if those sinclair bump gauges will fit in the hornady body. thanks,

  • Vince F
    July 8, 2023 at 01:20

    If brass takes 2 to 3 firings to fully form in the chamber:
    A) does shoulder only get bumped after brass is fully formed to chamber?
    B) is there a way to determine brass will stretch more and has not fully formed?


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