I bought a gun show parts kit for an AR I'm building (first time build) and I may have to do some minor tweaking to get it right.
I've heard that some gun parts come specially treated/harded and coated from the factory to keep them from wearing out quickly and rusting, is this the case with AR parts?  Will doing a little custom fitting(grinding)hurt the parts and/or shorten their lifespan?  

Thanks for any help

P.S.  Is it common to have to tweak some parts when building an AR or should they just fall into place?

Do yourself a favor and get a lower parts kit from DPMS,RockRiver or bushmaster.You shouldnt have to tweak AR parts to fit.If they dont fit they are out of spec and should not be used especialy just to save a few bucks.And depending on what you are grinding I suposed you could get a part hot enough to damage the heat treatment in that area.Get good quality parts that dont need fitting.I know match triggers and hammers some times need to be fit as well as oversized parts I use in my 1911s.If they are just standard parts get a good set of parts from a reputable company.

You shouldn't have to play with it, to give you an example, I had a Oly lower that would not lock in a mag when the upper was on (oly said it was not out of spec just made for one of their own uppers) my choice? either file my upper to match in that it would only fit this lower or pay oly to replace it with another, A long story short, my new lower will be delivered this week.

RUSTING?

****RUSTING?****

Yea, what was I thinking, Aluminum doesn't rust, should have said corroding.  Whatever the case, it's a problem solved, I order BM parts last night and will try to return the "bum" ones.

Thanks for responding and bringing me back to my senses!

No its not common - and it should not be needed if you're using quality parts.


However if you buy crap and have to modify them to work - I wouldn't be expecting a long and/or reliable life from them.

Yes, Rusting is correct depending on the parts being talked about. Not all parts of an AR are aluminum or plastic.

My only build that require major tweaking for it to work properly was when I was using a Hesse lower.

that pretty much says it all [puke]