I am looking for suggestions on what caliber for AR-10 can provide the most precision.
I have a .308 bolt action that I am very happy with but I am looking for a new caliber to try for my AR-10 Build. Budget is not my primary concern, precision is.
I know that .308 and 6.5 Creedmoor are very proven calibers but any other options come to mind? Most likely I will do a 24 " barrel.

Short back story when I built my second AR-15 I told my dear dad (the one responsible for my love of precision firearms) I was going to do a 6.5 Grendel and what does he do? build a 6mm Arc lol
So before I commit to a caliber I want to make sure I chose wisely.  


Thanks for the Advice!


My precision .308

6.5 CM all day long if I could only do one.
Most of my parts are AP
The BBL is 22 inch BA premium fluted.
3/4 MOA
Or in true Arfcom fashion you could build both a .308 and a 6.5 upper with dedicated accessories and optics and throw inside a pelican case.

I’d go with 6mm creedmoor.

Link to GA Precision  7mm08 might be a nice choice but if you are already running a .308 why not stay same caliber.  6.5 seems to be getting pushed hard but heavy bullets carry energy a long way out there.

I have two AR-15 Uppers with optics that I swap with my one lower.

I tend to agree on staying with .308 since you already have that. The fewer calibers the better, in my opinion. If you didn’t, I would go 6.5 CM. That’s what I did.

See I reduced my calibers down to a few, then ammo shortages hit.  I have 6.5s since its the better round but there was no ammo, but .308 was plentiful and still is.  So I ended up building a .308 upper so I could actually practice and shoot.

Having fewer calibers is a good thing....to a point.  You still need multiple options.  Kind of like all of my pistols are 9mm, but I do have a .357SIG Glock and some ammo/mags stashed away

Caliber/cartridge has nothing to do with precision.  Quality of barrel and quality of loads are the biggest factors.  Cartridges aren't inherently more accurate than other cartridges.  The shooter counts as much or more than the rifle too.  Your skill/experience and expected range are bigger factors in choosing a cartridge as is the expected down range effects.  All else being equal (based on 308 parent cartridge) the larger bore will give greater energy at closer range and the smaller bore (to a point) will allow for a shorter time of flight and hence lessor effects from external forces like wind and gravity, but will not deliver as much energy since it's starting with less energy.  Barrel life is a consideration as well.  A 358 Winchester will last practically indefinitely.  A 6mm Creedmoor or 22 Creedmoor won't last long in a gas gun for most shooters.  That said, 6.5 Creedmoor is a great cartridge that's a nice middle ground, although I believe the 260 is a little better, particularly in a gas gun.  The 308 is a very capable round too though(more capable than most shooters), and it's more economical due to it popularity and military use.

I haven't experimented with other calibers in my AR-10's for several reasons. .308 works great. Lake City brass (and IMI) is designed for semi-autos and I have lots of it. Powders that work best in 6.5 Creedmoor (RE-17 and H4350) produce more port pressure and I don't want to monkey with an adjustable gas port or other work arounds.

I do have a 6.5 Creedmoor bolt action, but I don't consider it superior to any other caliber that I already own. My most accurate rifle is a 6mmBR Norma/6.5x47mm Lapua switch barrel rifle. Almost every rifle I own will shoot moa or better with match grade bullets. If you want better groups with your AR-10, feed it some 168 or 175 grain SMK's. You may discover you don't need a new caliber, just better ammo capable of bringing out the best from the rifle you already own.

While it makes sense to shoot a caliber that I already have ammo for. My Ammo for my .308 is a custom hand load for my rifle my dad uses different dies for his .308. Mine is built on a surgeon action while his is a factory 700 action. This one most liking mean a third set of dies LOL and as such why not shoot a different caliber?
42.3 grains of Varget

My custom .308 with  S&B optics and a 30" Bartlein Barrel on a Surgeon Action with a Jewell Trigger.

If you don't have a 6.5 Creed, do yourself a favor and build that. It's a sweet round and very easy to reload for.

I have both a 6.5 Creedmoor and a 6mm Creedmoor.  Both great rounds. 6mm a little less recoil but suppressed they are about the same.  Ballistic tables have the 6.5 going more distance.  

Can't go wrong with either.

Meh..yes and no.
Inside 500yds, yes this is true.  Beyond 500yds, and especially pushing 1000+yds, the cartridge makes a difference for accuracy when you consider the factors involved.  This is not because of the shooter, but because of the external ballistics reducing the margin of error.  6.5CM and other high-BC calibers have demonstrably less drop and as such, the margin for error on long range targets is substantially less.  This is also the case with wind deflection.  You have less adjustments necessary, and as such, one or two clicks off make less of a difference on the target.  While the 6.5CM might not be inherently more accurate than the .308, it is EASIER to be accurate with it at longer ranges.  That’s what ultimately counts and why in the PRS and other arenas, the higher-BC rounds are winning and .308s often are relegated to the Tactical division to provide a more equal field of competition.   Using factory load data, the 6.5CM with 147gr ELD has about 83” of drop between 900yds and 1000yds.  The .308 with 175gr FGMM SMK has 101” of drop between 900 and 1000yds.  While that translates into minor adjustment differences in the scope, that makes a major difference when it comes to your hold and when you pull the trigger.  An additional 18” of drop in that range means there’s more to go wrong, and it’s notable when you consider the the size of common targets engaged at that distance.  Having your own hold be 0.1 or 0.2mils off will make a much greater difference with the .308 than it will with the 6.5CM due to the fact that the .308 has a much faster rate of drop at that distance and an less than exact hold will yield a great effect.  With a higher-BC load, a less than completely perfect shot will have a higher chance of a hit due to less deviation of the bullets trajectory at that range.  In this example, a 0.2mil error in your shot hold will yield a greater deviation with a slower, lower-BC round since it’s dropping at a faster rate at that distance.  It’s ultimately all in context.  An expert marksman can make the round work within the round’s operating window (i.e. prior to transonic threshold), but it’s nice when your job is made easier because you have more room for error.  

I think that contextually, one can make a strong argument that that translates to better accuracy, because your hit rate tends to increase when there’s a lesser margin of error.

Great post, there is a lot of wisdom in his words….