What is it? And is important?
What is it? - Simply stated it is the distance from the Case head to Breech face measured in thousands.
How do I know what mine is? Good Question, there is a couple of things you must know first.
First thing you need to know is, what is the gap, in thousands, between your barrel and any given frame breech face, in the TC family it is the firing pin bushing. The only reason this needs to be known is to calculate your headspace. It serves no other function. It will not make it a poor shooter or a good shooter. It is only a gap, and has nothing to do with your headspace.
There is one exception, and that is when the barrel hits the firing pin bushing, and you have to correct this before you attempt to set your headspace with your ammunition. This is rare but does happen from time to time. If you do not have some gap, other problems show up.
The Second thing you need to know is, where is my case head in relation to the barrel surface? Is it flush with end of my barrel? Is it below the end of my barrel? Or is it sticking above the end of my barrel? This is the second number we need to adjust for proper headspace.
There are many ways to get this number, with a Depth Micrometer, With a straight edge and feeler gauges. With some of the high dollar gismos that will tell you this. Or you can just give it the good old eyeball. But whatever you use, it needs to be as accurate as possible. I have found by using a Dial indicator mounted in a Bellm Indicator Base to work fast and easy for me. And gives me the accuracy I need to get a good measurement.
Now it is time for an example, this might help you understand it a little better.
My barrel mounted on my pet frame measures a barrel to frame gap of .004
Remove your barrel from the frame. Now I take a brand new piece of brass in my favorite brand and the proper caliber and drop it into the chamber. Now measure it. Lets say it is below the rim of the end of the barrel a minus .006 This added to your gap of .004 Gives you a headspace of .010 SAAMI Max is .006 so you have an excessive headspace. This will drive you nuts, miss fires, poor accuracy etc.
Now lets do another example.
Same old favorite frame with a different barrel on it. The barrel to frame gap is now .002 Does this make it a shooter? No...we don't know the headspace yet. Remember we needed to know where the case head is in relation to the end of the barrel. So after removing the barrel from the frame I again grab some good old brass that I just cleaned and did a full length resize on, and stick a round into the chamber. I make sure it is in all the way in, with a modest push with my thumb. Now I take my gauge and measure it. It sticks up .001 above the end of the barrel. This is a plus and must be subtracted from the barrel to frame gap. And now I have that much desired headspace of .001 I should expect no miss fires and if the load is a good one I should start to see some good accuracy.
Is it important?
Only if you desire to have no miss fires. Only if you want good accuracy.
Ok you have convinced me, now how do I get my brass where it needs to be?
That is another topic we will go into later.....Jim