I see alot of uppers for sale with no bolt my question is can I use any bolt or what? I have a Bushmaster with a 20" upper and I want to get a 16" upper to use on certian hunts and was wondering if I can use my bolt from my other upper. Or do I need to get the guages to check it.Thanks Denns

Yes, if you slap a bolt in a upper, you should headspace it.
It is just a fact of life.
Spend the 45 bucks and get a go and no-go gauge and learn how to use them.
If you wish to spend another 15 or 20 bucks get the field gauge. And learn to use that.
Do a search, if it is working and research the subject.
It is really quite simple.
One fits, one doesn't and one will show you if it might.