To measure headspace, you must first know what the distance is from the end of the barrel to the firing pin bushing. Take this measurement with a feeler gauge set. Your sized cases and ammo should stick out of the end of the barrel .001" less than the barrel-to-frame gap measurement. 

This quality set starts with the thinnest blade at .0015" and goes to .025." Normally, this covers the full range of barrel-to-frame gaps you will encounter. 

Start by closing the barrel on successively thicker blades until one is gripped firmly between the firing pin bushing and the end of the barrel, then go back to the next thinner blade that just will slide out from between the barrel and firing pin bushing. 

Call this your barrel-to-frame gap measurement. Make a note of it. 

I like to mark it on the bottom of the barrel lug for future reference. If you have more than one frame the barrel will be used on, you might also mark the last two or three digits of the frame serial numbers and the barrel-to-frame gap measurement of that barrel for each frame next to each serial number. 

This also lets you monitor the frame for any stretching that may occur with use. 

If the barrel firmly grips the .0015" thick blade in the set, call the barrel-to-frame gap .001." You can double check this with one thickness of standard household aluminum foil which is usually somewhere between .0008" and about .0012." 

Your sized cases should not protrude from the end of the barrel more than .001" nor should they fall below the end of the barrel with the barrel off the frame of course. 

Includes sizes 0.0015, 0.002, 0.0025, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.010, 0.011, 0.012, 0.013, 0.014, 0.015, 0.016, 0.017, 0.018, 0.019, 0.020, 0.021, 0.022, 0.023, 0.024 and 0.025.