I was thinking of buying a M1A for competition shooting. DCM, NRA, combat, whatever. But someone said the ARs are winning more. And easier to shoot.
Would a stock Bushmaster be ok? Does it have to be a heavy barrel? Can an AR have a good trigger job?
Give me some tips.

I use my stock Bushmaster 20" to shoot the High Power matches.  We have some guys that use an M1A and do ok.

Quoted: I was thinking of buying a M1A for competition shooting. DCM, NRA, combat, whatever. But someone said the ARs are winning more. And easier to shoot. Would a stock Bushmaster be ok? Does it have to be a heavy barrel? Can an AR have a good trigger job? Give me some tips.

1) Yes, AR15s are the rifle of choice for NRA and CMP Highpower, as well as all the other alphabet soup tactical run-and-gun competitions out there.  You can use the M1A, but it will be an expensive ordeal in terms of rifle and ammo and the learning curve is going to be very steep.  I am a highpower shooter, and while I don't have an M1A, I have an M1, and I can tell you that the recoil of such rifles, combined with their old schoot stocks make them a bitch to shoot well, particularly in rapid fire.

It takes about $3000 (including rifle) to give an M1A the level of accuracy and consistency than can be had in an AR15 for less than $1200.  Expect to pay about double for match-grade 308 Win ammo, compared to 223.

2) A stock Bushmaster is fine, but life would be easier if you bought an upper built specifically for the type of competition you would like to do.  Which brings me to another point: "competition shooting" is pretty broad and you will need to better define what that means to you  if you expect to get meaningful answers.  What works in highpower does not work in USPSA or IDPA.

3) An AR can have a good trigger job.  But most people simply replace their stock, crappy single stage trigger with an aftermarket unit.  Geissele, Jewell, Chip McCormick, JP Rifles, Jard, and Rock River all make good triggers for competition.

I guess I would need two ARs then.  For now I want a DCM style. So I need a replacement trigger group.
 But what about the HB? I'd like to get an A3 with government profile barrel.  Disadavantages?

Try here: Competition Shooting & the Accurate AR

See if you can look up some of the posts by bigbore- he's had a lot to say about the topic.

Quoted: For now I want a DCM style.  I'd like to get an A3 with government profile barrel.  Disadavantages?

Do you want to be competitive, or do you just want to play around?

If you want to just go out and shoot in matches, anything will do.

If you want to get serious your upper will need the following:

1) A quality, heavy under the handguards barrel.  Why?  Because it is stiffer, so it will be more accurate.  Because it makes the rifle heavier, and a heavy rifle is steadier offhand an easier to control during rapid fire (where precision is more important).

2) A 1/8 or faster twist rate.  Why?  So you can shoot all the popular bullets available for highpower, from 68 to 80 grains.

3) A service rifle legal free float tube and handguards.  Why?  Because sling tension will make for shifting point of impact, regardless of how heavy the barrel is.  Using a CMP legal float tube moves the sling swivel and front delta ring off the front sight and off the barrel, isolating the barrel from sling tension.  Plus the barrel will be free along its entire length so that it will not bind as it expands from the heat of shooting, again keeping POI constant.

4) A windage adjustable front sight base.  Why?  So that you can zero the rifle for windage while keeping the rear sight aperture centered.  You will see why this is good the first time you shoot at 600 yards with a 15 to 20 mph cross wind.  You will need all the sideways travel you can get from the rear sight.

5) A rear sight with enough elevation travel to reach 600 yards.  All A2s do this, but the only A4 rear sight that does is the Rock River National Match detahcable carry handle.  You also need at least 1/2 MOA clicks for windage and elevation.  1/4 MOA clicks are available in A2 National Match rear sights, but the RRA NM carry handle only comes in 1/2 MOA clicks.

Hope that helps.