I am getting a new upper for my AR. (not sure what brand but most likely a DPMS or ASA. My question is that I have a bolt and carrier from a bushmaster that has had only 300 round put through it. I sold the bushy upper and kept the bolt and carrier to use on a new upper. Will this work? What problems am I going to run into?

Before you all go and tell me to get a gauge I have access to one. What I need to know is what to do if the thing has too much or too little headspace.

Quoted: I am getting a new upper for my AR. (not sure what brand but most likely a DPMS or ASA. My question is that I have a bolt and carrier from a bushmaster that has had only 300 round put through it. I sold the bushy upper and kept the bolt and carrier to use on a new upper. Will this work? What problems am I going to run into? Before you all go and tell me to get a gauge I have access to one. What I need to know is what to do if the thing has too much or too little headspace.

Chances are good that the bolt will work.  If there is too much headspace, you're kinda screwed: requires that the barrel be set back and the chamber then reamed to the proper headspace; of course, this is a problem if your barrel is chrome lined.  Too little headspace (won't close on GO gauge) is a little less of a problem; chamber can be deepened or possibly bolt face trued back.  Again, deepening a chamber on a chrome barrel is not possible to my knowledge.  You may need to get to know a got AR smith real good, but again I think your chances are slim of having a problem.

shooter

You propably won't have trouble with a new upper assembly from a reputable manufacturer.  Stay away from ASA and DPMS.  Bushmaster is the better choice with fewer potential conflicts.  Check the headspace using a military field gauge at the minimum.  A .223 Remington field gauge is the equivalent of a military NO-GO gauge.  

If the barrel is chrome-lined, you can only try another bolt.  If it is stainless steel or chrome-moly, and the headspace is short, you can have a competent gunsmith cut the chamber and throat a bit deeper.  If headspace is long, you can try a new bolt and/or barrel.  AR15 barrels cannot be set back and a new chamber cut to correct long headspace.

Quoted: I am getting a new upper for my AR. (not sure what brand but most likely a DPMS or ASA. My question is that I have a bolt and carrier from a bushmaster that has had only 300 round put through it. I sold the bushy upper and kept the bolt and carrier to use on a new upper. Will this work? What problems am I going to run into? Before you all go and tell me to get a gauge I have access to one. What I need to know is what to do if the thing has too much or too little headspace.

You shouldn't run into problems.  I used a DPMS bolt & carrier with a BM upper with no problems.  Still, it can't hurt to check headspace.