I have DPMS AP4 and I like every thing except the barrel length. Shooting that 16" barrel is LOUD!!!!

Is there any gain in going to a 18" or 20" barrel?

Will 2" really make that big of a difference?

Will 4 additonal inches add alot of weight?

Yes. I cut the barrel on a Savage Scout to 17. It looked like a flamethrower at dusk. My 19in barreled Steyr doesn't do that and is much better mannered. FNFAL Para models have an 18in barrel. They stopped whacking at 18in for a reason.

http://www.tacticaloperations.com/SWATbarrel/

Having owned a 16" AR-10 and a 15" Encore, I understand. My 308 now is a 20" bolt gun. Even with it's heavy barrel, it still weighs less than an AR-10 with like barrel.

There's good reason the original Stoner design was scaled down to fit 5.56x45 shells.

I have a 16" AR10 - when it had its muzzle brake, it could efficiently clear the range. Unfailingly, folks would pack up & move rather than shoot next to me

Any AR-10 type rifle is going to be heavy, it's just the nature of the beast. A 20" barrel will add some weight of course but the gain in velocities will be worth it if you ask me. I think you loose too much with a 16" in a 7.62x51 but that's just my opinion there seem to be alot of guys that like them just fine.

Is there a light 308 semi auto?

Even an M14 weighs about 10lbs.

I have given it alot of consideration, and I think I just may stick with it pretty much as-is, as it seems to be working for me. I found a brake called a Claymore that will direct blast forward, I may try that

I lusted in my heart for an AR-10 type until I got my hands on one. They weigh more than my Model 70 .375 H&H.

I bought a 6.8spc instead. Still heavy at about 8lbs. scoped, but handles much better.

I have the barrel and some components in route now for a gun in 6.8 Will take a few months to build, but I intend to go the 'other' direction and build this gun as light as possible. I.E; carbon Fiber forend, Ace light buttstock, etc.

I popped Mrs. dosey-doe with one shot @ roughly 250 yds yesterday with my 'lowly' 16" barrel AP4. Broke both shoulders and there she dropped. My thoughts about a new/longer barrel are now gone!

No, I don't think so...if one exists I'm not aware of it. I recall a few years back DS Arms was messing about making 7075 T-6 FAL uppers in the attempt to reduce weight. If memory serves me correctly they all blew up at 90-100 rounds and the project was scrapped.

You can get a BAR under 7lbs

You read my mind my friend. After playing with a Steyr Scout, I was left with the impression Steyr hit the nail right on the head at 19�. I�ve compared a Steyr Scout to an 18� FAL and the Steyr was considerably less obnoxious, but it should be noted that the FAL had the FAL muzzle brake/grenade launcher which tends to enhance muzzle blast a bit. So 19-20� is my personal limit on full power rifle cartridges. If I need shorter but still need a bit of punch, I pick up my AK in 7.62x39. I will say that my Arsenal SGL-20 has the �74 style muzzle brake which I keep on just because it looks oh-so-right, but if I were ever to use that rifle in a fight, I�d either chit-can the �74 brake or swap it out for the Bulgarian 24mm flash hider which is much quieter and actually a decent flash hider.

We shooters need to concern ourselves with noise, we only get one set of ears. In Russia, there are a lot of former soldiers walking around with serious hearing loss due to that �74 muzzle brake. And short barrel 7.62 NATO rifles are just as loud.

A shooting range friend shoots an Encore in 308 with a 15" barrel IIRC. When he lights off 51+ gr. W748 under a 125 Bal Tip you know it!