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.

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.

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”.

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.