Estimate the accuracy of your firearm.

Before you try to improve gas check quality, you should establish a baseline for your firearm's performance. Experiment with a variety of commercial gas checks until you achieve best accuracy. Set a reasonable standard such as one minute of angle (MOA) accuracy.

Select appropriate gas check tools.

The die should fit the outside diameter (OD) of the bullet. The punch mandrel should be appropriate for the inside diameter (ID) of the check. FreeChex II is appropriate for low volume production. FreeChex III makes checks quickly but requires hand-operated press such as an arbor press.

Select metals of appropriate thickness.

Estimate the distance between the bullet shank OD and the gun barrel groove ID. Measure the bullet shank ID with a micrometer. Slug the bore to determine the groove diameter OD. Choose sheet metal thickness between 0.006 and 0.022 of an inch (6-22 thousandths). Excess thickness may deform the bullet shank and cause gas check metals to extrude and weaken. Gas checks that are too thin may be too elastic to stay on the bullet. Ideally, a gas check should fill the region between the bullet shank and the gun barrel groove precisely. Use a micrometer when purchasing gas check metal. Sheet metal is not always labeled. Roofing materials can vary between 0.008 and 0.014 inch thick. Material thicknesses often vary on the same roll.

Know the composition of your bullets.

The gas check should be harder than the lead bullet. Bullets made from pure lead deform easily when the gas check is swaged on the bullet. Antimony and tin are added to lead to improve hardness and strength. Brinell Hardness Number (BHN) is a scale for defining hardness. For example: Pure lead is BHN 5. Pure aluminum is BHN 15. Mild steel is BHN 120; tool steel is BHN 1500-1900. Wheel weights were commonly 3-4% antimony, 0.5% tin, BHN 10-12. But EPA is encouraing lead-free alternatives such as zinc, iron or aluminum. Well-formed bullets require “eutectic” alloys that melt and solidify at constant temperatures. Eutectic 4/12/84 "linotype" is an alloy gives a flat cooling curve. (Linotype is a broad name for five categories of lead alloys.) A common bullet material is Lyman #2 eutectic alloy (BHN 15).

Aluminum alloy gas checks

Roof flashing is inexpensive and available in many different thicknesses. It may be sold according to thickness but the alloy composition and hardness (BHN) is rarely designated. The International Alloy Designation System is a four-digit naming system for aluminum wrought alloys. The 1000 series is essentially pure aluminum. Roof flashing 1060 has 99.6% aluminum (BHN 19-35); 1100 is alloyed with 0.12% copper (BHN 23-44); 3003 is alloyed with 0.12% copper and 1.2% manganese (BHN 28-55); and 5052 is alloyed with 2.5% magnesium and 0.25% chrome (BHN 47-77).

Copper alloy (brass) gas checks

Copper is desirable because it is malleable and hard and becomes stronger with work hardening. Unfortunately, the price of copper has quintupled since 1999 and future supply is uncertain. Brass is an alloy of copper with zinc and/or tin and sometimes nickel, iron, lead, arsenic. Gliding metal for jacketed bullets and commercial gas checks is 95% copper, 5% zinc. Copper/brass alloys are sold in different tempers designated by ASTM B370 as: 060 (soft), H00 (cold rolled), H01 (cold rolled, high yield), H02 (half hard), H03 (three quarter hard), and H04 (hard). Brass for gas checks should be soft. H02 (half hard) brass needs to be softened before it can be sized and seated.

Work hardening

Work hardening increases the gas check’s tensile strength, which makes it less likely to shear in the barrel and more likely to stay with the riflings. Work hardening also causes metal to become less springy by plastic deformation. A gas check is work hardened when it is created and when it is sized. Finding the best material and thickness usually requires experimentation. A change as little as 0.002 inches can make a substantial difference. If the gas check is too springy (lacks malleability) it cannot adhere to the bullet. But slightly over-size gas checks can be crushed and these stay on the bullet.

Avoid deforming the bullet.

The bullet's reduced shank diameter (rebate) allows the gas check to be fitted. Reducing the shank diameter further by sizing is undesireable. Lead bullets are soft and inelastic. Gas checks are hard and somewhat elastic. Sizing the bullet always causes a minor degree of deformation. But if the shank is squeezed too hard the deformation can be so extreme the gas check slips off. A highly elastic gas check cannot stay on a highly deformed bullet shank. Some users (not Darnall) have heated overly-stiff gas checks to make them softer to stay on the bullet. Instead of heating, the user should experiment with different alloys and thicknesses. Sizing the bullet without the check often solves the looseness problem. A gas check requires only a minor amount of annealing (if any at all) to stay on the bullet.

Develop a system for testing your gas checks.

Try a variety of commercial gas checks until you achieve best accuracy. Hornady sells a gas check designed to dig into the shank and “crimp” the bullet. Gator Gas Check has no web site but they can be ordered through group buys at James Sage makes aluminum gas checks that can be purchased from Experiment with only one variable at a time. Determine the accuracy of each prototype material with the same or similar thickness. Compare different thicknesses for each prototype material. Compare each prototype with the accuracy of commercial gas checks.

FreeChex II for 44 caliber (movie)