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If you handload Ackley Improved brass and want tighter control over how much you're sizing your cases, the Sinclair #40A Shoulder Bump Gage Insert is the piece that makes it happen. This insert is machined specifically for cartridges with a 40-degree shoulder in the 25 to 338 caliber range, so it seats against the case shoulder cleanly and gives you a measurement you can trust pull after pull.
The 40A is cut for the longer Ackley family, including rounds like 257 Roberts Ackley Improved, 280 Ackley Improved, 338-06 Ackley Improved, 260 Ackley Improved, and 30-30 Ackley. The insert is engraved with its shoulder angle so you can confirm you grabbed the right one at the bench. Just match the 40-degree marking to your cartridge's shoulder and you're set.
Instead of measuring to the datum line, the gage indicates off the shoulder of the case, which gives you a cleaner read on how much you're moving that shoulder back. The insert threads into a Sinclair comparator body or a Hornady comparator body and locks down with a 5/64 hex wrench. From there you clamp it on a caliper blade, let it center itself, zero out on a fired case, then check your sized brass. A common target is around .001 to .002 inches of bump for a bolt gun and a bit more for an AR, though you'll fine-tune that to your chamber.
Because the insert matches your case's shoulder angle, it helps square the case head to the caliper blade, which is where a lot of the repeatability comes from. Fire-formed Ackley brass lives longest when you only push the shoulder back the minimum needed to chamber smoothly, and this gage takes the guesswork out of setting your die. It's also handy for keeping brass sorted and checking loaded rounds if you run the longer comparator body.
The #40A is machined for 40-degree shoulder cartridges in the 25 to 338 caliber range. That covers Ackley Improved rounds like 257 Roberts Ackley Improved, 280 Ackley Improved, 260 Ackley Improved, 338-06 Ackley Improved, and 30-30 Ackley. If your cartridge has a 40-degree shoulder in that caliber window, this is the insert for it.
It installs in either a Sinclair comparator body or a Hornady comparator body, so if you already own one you only need the insert. Keep in mind the standard Hornady body may not clear a loaded round because of how short the Sinclair insert sits. If you want to gage loaded ammo, the longer Sinclair comparator body handles that job.
A common starting point is about .001 to .002 inches of bump for bolt action rifles and roughly .003 to .005 for AR-type rifles. You measure a fired case to establish your baseline, then check your sized brass and adjust your die until you hit that target. The idea is to push the shoulder back only enough for reliable chambering, which helps your brass last longer.
Reading off the shoulder gives you a clear, repeatable measurement of how much you're moving the shoulder during sizing. This is a comparator, so it's showing you the change between fired and sized brass rather than an absolute headspace number. Because the insert is machined to your case's shoulder angle, it also helps square the case head to the caliper blade for consistent readings.
| UPC | 050806111301 |
|---|---|
| Manufacturer |
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