I just got a new RRA A3 upper receiver installed on my AR. I have my prevoius A2 upper receiver now which is stripped. I'm gonna build a 20" AR out of it but I have a question on two Bushmaster barrels, the 20" Goverment post-ban barrel which is listed as being light and is more money than the 20" National Match heavy barrel. Out of these two which is the way to go, my current DPMS AR has a heavy barrel.

I would go with the Govt. profile barrel.  

It really depends on what you want.  The Gov't profile is light under the handguards which saves you some weight plus it will be chrome-lined.  You might even be able to get it with a 1/7 twist if you call Bushmaster and they have it in stock.  The NM is heavier and not chrome lined.  For a closer to mil spec go with the gov't profile, more serious target shooting go with the NM.

gov't profile all da way!

I believe that Bushmaster's 20inch NM barrels are chrome lined.  It is their DCM barrels that are not chrome lined.

The 20" BM Nat'l Match barrel is chrome lined, 1/9" twist. I used to think that the 1/9" was the way to go since it would stabalize the 55gr. M-193 and the 62gr M-855 well, and is very well suited for using the 69gr. BTHP Match bullets. But the future of the 5.56mm seems to indicate the use of heavier bullets like the 77gr OTM's. Some 1/9" barrels will stabalize the heavier bullets, some won't. And in any case, accuracy is generally less than outstanding with heavier bullets in the 1/9" barrels that will stabilize them.  

Let's assume for a moment that you decided that you wanted to shoot in High Power competition and you wanted a heavy match barrel to do so. There are a lot better choices out there for heavy competition barrels than the chrome lined Bushmaster Heavy Nat'l Match. You'd be better off choosing a dedicated match barrel from one of the top barrel makers if competition is your goal.

As one who owns one of the BM Nat'l Match barrels on an A2 AR, I'd reccommend a Gov't Profile in the 1/7" twist instead. You'll be able to shoot anything from the 55gr. bullet through the 77gr and all with acceptable accuracy.

The BM Nat'l Match barrel is sort of a "tweener". It's between a Gov't. profile A2 and a real competition heavy match barrel. Slightly more accurate than the Gov't. barrel, but short of a competition barrel's accuracy. Why carry the extra weight if it's really not doing you that much good?

As always, just MHO, YMMV.  

Thanks everyone. Looks like I'll be getting the Govt 20" for my new build!

Search around the EE and see if you can find a Sp1 or A1 barrel.  I got one and I love it.