I just finished building a new upper. The last thing I need to do is check the headspace. I have a DPMS A2 upper, and Bushy 20" heavy barrel. A friend of a friend is letting me use their headspace gauges and I have no idea in hell on how to use them or what i'm looking for. Any input would be a big help. The gauges are made by Forster's and i have the go, no-go, and field.

I'm certainly not an expert here but I think you merely close the bolt on the gage, it should close completely on the go but not the no go, field....ya got me there.

Well, I seriously doubt you need to headspace it in the first place.  The bolt/extension would have to be SERIOUSLY worn, or defective from the factory.  If it didn't headspace, and the bolt was good, you are virtually screwed as to adjusting headspace as that is set at the factory.

However, in general you are going to have to remove the bolt ejector and extractor and throw the gauge in the chamber.  The bolt should close on the go, but not the no-go and field.  However, I believe most will close on either/or the no-go and the field with the Forster.  I have the Forster, Bushmaster, and Colt field gauges, and they are ALL different dimensions.  Some have reported that the bolt would close on the field and possibly the no-go with certain brands, even with a good bolt and barrel extension, but I can't remember which.  I have yet to come across a bolt that would not headspace, and I have done quite a few just for fun.  Perhaps someone else more experienced can chime in here...  

Tweak gave a great explanation of headspace and a how-to here:

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