The Lyman Gas Checks are gilding metal cups that
fit the cast bullet base and protect it from hot powder gases. Easily seated
during bullet sizing. Calibers: 30, 32, 35,
.375, 44 and 45.
I only buy Lyman gas checks for one reason, they are made to fall off. Not to mention the consistant quality necessary for this to happen. The trick is to have them fall off the slug at the moment it reaches still air in front of the muzzle. The then step base is exposed which acts like a "boat tail" bullet. This requires care in mounting the cup to the base of the bullet so it is flat against the base before sizing and lubing. Plus the bullet base should never be into the bottle neck case beyond the flair at the shoulder of the case thus holding the the cup to the base of the bullet by metal to metal friction. Straight wall cases aren't a problem for bullet depth. The gas check has no where to go but out. After 20 rounds you should see the Lyman gas checks on the ground a few yards out. Don't bother counting them because some will fly like a frisby when they come off. Good shooting!
I use gas checks on most of my cast bullets and I have never had to spend much time scrubbing lead out of the bore. I have a 44 Mag that I have probably fired 25000 rounds through and have never had a problem while using gas checks on the Keith cast bullets.
I've been using these gas checks for my 5" Ruger Redhawk .45 Colt. Using a rest at 25 yards, I get a 3/4" group. I've had good accuracy with both Lyman and Hornady. Actually, I can't tell the difference between them.
I've heard that these gas checks will get stuck and come off in a Lubrisizer, but I have not had that problem. I have sized and lubed 2000 bullets in .45 caliber and not had one come off during the process.
I took a doe this year with my .45 Colt using an LBT 265 gr. Keith style bullet and a Lyman gas check. I clock my load at 1350 fps. Hit her in the throat. Obviously didn't recover the bullet, but it killed her dead.
Leading is no problem at all in pistol velocities. I've used these too in my .45-70 up to 2000 fps with no leading issues. I lose accuracy at higher velocities, but do not get leading.