I’m working up some loads from hornadys manual using 178 A-Max’s and Varget.

What are some details I need to keep in mind when loading for a gas gun?
Do I need to crimp the cases, would that work with the amax?

Make sure you are sizing the cases at least .003" when resizing.

I don't crimp my 556 rounds in my AR. Some do, some don't. Neck tension is very important.

OAL is going to be your biggest downside to the bigger bullets in 308 in an AR. You are limited by magazine length.

Start low and work up looking for pressure signs.

As long as you have good neck tension, and aren't using a compressed load, you shouldn't need to crimp.  I've only had one load that was a problem from crimping on a different caliber. That was a bullet that had rings in the base to reduce friction, but also reduced the amount of surface area the neck was holding the bullet.

Third recommendation for neck tension.  Am not expanding my case necks, and using comp seater.  This balanced with s die for differing brass.   Bullets seated to function with mag, consider using hard primer and aware of how some loads balance with the recoil spring/weight/gas port size.   Alert to case head swipe.

I had to get a small base sizer die for my 308 brass to chamber all the way.

Measure the COAL of a round.. put it in the mag with another round on top... shoot the first round then eject and measure the COAL again.. if it's slipped more than a few thou then you need more neck tension or a crimp.  May need to do it a few times to get an average.

Not using the neck expander will create too much neck tension and usually poor accuracy. Spin polish your expander ball using 600 grit wet/dry sandpaper, follow up with 1000 grit for a mirror like finish. I look at .306" diameter as being the maximum allowed for a semi-auto, .305" if .306" doesn't hold the bullet firmly enough to cycle. I would not go smaller than .305".

The brass will harden with use and you may have to go smaller with brass that has been reloaded several times. The expander balls are cheap and easily modified. There is no reason not to have two or three different sizes in your die box.

I use Lake City and IMI brass exclusively in my semi-auto rifles. It's thicker than commercial brass and can handle rough treatment better because of it. A 2.0 full grain reduction in powder charges are required when loading ammo in it.

My favorite bullet is 168 Sierra Match Kings. It's old school, but works in virtually any rifle out to 800+ yards. More tournaments have been won using that bullet out to 600 yards than any other .30 caliber bullet in history.

I just posted suggested loads using Lake City brass in the reloading page of the "Precision Rifle" section. Rather than repost the entire list here, please check it out under the thread titled "308 load advice".

Sorry, through direct experience am going to politely disagree about the neck tension.  Yes you can definitely create too much case neck tension if you try hard enough.  Holding moa out to 600 yds is good for me, don't have a 1000 yd range.  Worse yet, it is with SA mil surplus brass and berdan primers :).   Definitely works for 223/308 with a good comp seater.

Should be able to load them without a crimp op..just get your neck tension right..If you need a short bullet that is extremely accurate to fit mag length you might try the swift sciroccos..they have been very very accurate for us, you can get them in 150, 165, or 180 grain for 30 cal....