If gas check fit is right, there is no real installation. Just put it on and run it through the sizer. If the check shank is to large to let the gas checks go on till the bullet base hits the bottom inside of the check, they must be forced on, which is what the Lyman gc seater is for. Don't use it with LBT bullets but return the mold and I'll fix If I don't get it right the first time. With other molds, I guess a seater might be required, but it causes shaving of the check shank which puts the check off center with the bullets center of gravity, and hurts accuracy, even though they installed straight and look pretty.
Lyman dies have a stop plug that is cupped out, and rarely is the outside sharp edge dead square with the body, therefore, if a bullet is bumped against the stop plug at all the base will be knocked out of square, whether gc or pb. So, square all of them, leaving about a 1/32 flat ring to imprint the gc or bullet base when the bullet is bumped firmly against the stop plug. This will insure dead square bases every time, and is very benificial accuracywise, with the benifit being most apparent on short handgun bullets.
Hornady makes RCBS and Lyman gcs to the same specs as they make their own, and Gator checks are made to the same specs AND QUALITY, so the lowest price and availability determines which are my favorite check, which is GATOR, because their prices are lowest and they offer the largest selection. Call them evenings LA time. Larry Blackmon is the owner. Phone number is 1 318 387 3266
