1. Gas checks can only be used on moulds that are designed for a gas check.
2. In most cases, shooting a gas check designed boolit without a gas check does not produce very good results regarding accuracy.
I use a gas check on all my rifle boolits designed to take one regardless of how fast I intend to shoot the boolit - I only have 3 pistol moulds that are gas check designed -- 1) a 150gr (.358) SWC; 2) a 440gr 500 S&W; 3) and a 240gr .44 mould. Always use a gas check on the .44 and 500 but have shoot some of the .358s SWC without a gas check ... Out pass 25yrds, the .358s wanted to start to keyhole. I have about 20 rifle moulds and they are all gas check designed with the exception of 2.
If you are purchasing a mould, the question is usually - do I buy one designed for a gas check or one without? Rifles - most are gas check designed; however, non-checked are available. Also, I have read about several casters actually modifying gas check designed moulds into non-gas checked versions. If you plan on pushing a boolit at or above 1200fps, I would suggest a gas check designed mould. Of course, this can vary based on the type metal mix you plan to cast with and whether you water quench or heat treat the drooped boolits - either of which can make your boolits harder; thus reducing the need for a gas check.
In rifles, I have found that gas checked boolits produce better accuracy that non-gas checked boolits; however, I have read of others using non-gas checked boolits with great success.
Of course there is always a downside to almost everything -- gas checks are expensive!! ... about $30 a thousand for .22 and .30 up to $50 a thousand for .500.
Will post a couple pictures later of a gas checked designed mould and a non-gas checked mould.
Here is a picture of a 440gr .501 gas checked boolit -- the one on the left is straight from the mould. As you can see, it has a indent on the bottom of the boolit for the gas check to fit on. The one on the left has been run through a sizer and the gas check seated.
'neck -- the Lee push through sizers WILL seat a gas check on a gas checked designed boolit.
I do believe that accuracy improves with the use of a gas checked boolit. I am not an expert; however, the check helps prevent gas leakage from going around the cast boolit as it travels down the barrel - also, the check is usually made of copper and comes in contact with the barrel's rifling .....