Gathering all the bits for an AR10 in .243, and I’m looking for ideas on magazines.  The intention of the rifle is primarily target shooting anywhere from 300 to 1200 yards. Based on this intention, I know that I will be handloading and that I will need access to the largest internal overall length magazine available (depending on how load development goes). My question to you is I am aware of the Knights Armament and DPMS mags, are there any others I should look at? I had some of the MagPul 308 mags before, but didn’t like the internal limitations.

Thanks in advance.

Larue will have longest length that I am aware of, why not 6cm?

I already have the barrel.

What twist rate is your barrel?  What bullets are you going to/wanting to run?  Hunting or just banging steel?

Yeah, I was curious on the barrel twist also.  I have an Armalite AR10 .243 barrel, but it is their somewhat standard 1:10 twist.  Pushing past 100g bullets gets "iffy" depending on the particular bullet.  A lot of 105g-115g match bullets require at least a 1:8-1:9 twist, and frankly the 108g-115g match bullets usually need a 1:8.  Some of this depends on the exact bullet, as the bearing surface of the specific bullets varies.  The longer match bullets often have less bearing surface and therefore require a tighter twist.

It's hard to predict an "exact" formula here, but these are some generalizations for .243/6mm bullets at the heavy end of the scale.

It has more to do with the length of the bullet in question than with the bullets' bearing surface.  The is not an exact formula, but there are some very good models out there.  Berger bullets has a twist rate calculator on their site and
JBMballistics.com has a stability calculator.  JBM also has a list of bullet lengths if you want to play.  Plug in your numbers to their calculator, you'll know if your bullet will be unstable, marginally stable, or stable.

Some things that can make a difference ate environmental factors.  Air temp, humidity, elevation can all play to increase/decrease stability.

A 10" twist 243 can be a very accurate long range rifle.  I've been messing with one a lot over the last year.  Generally speaking, with a 10" twist 243, bullets with BCs up to .43 will be stabillized, between .43 and roughly .49 will be marginally stable, and higher than that will be unstable.  I use BC as a guide because there is a strong correlation between length and BC, higher the BC in a given caliber, likely the longer the bullet.  Also, marginally stabilized bullets can shoot very well.  In fact, they can make for phenomenal groups at 100 yards.  But, according to Bryan Litz, the BC of the bullet will not be optimized.  Also, you may have a load that shoots great in the summer, but key holes in the winter (ask me how I know).

Yeah, the combination of barrel twist, bullet length, bearing surface, BC, bullet center mass, bullet shape, velocity, etc., etc., almost seem like black magic, but we all know it's just a matter of actual physics.  Still, the debate goes on as to how much of a percentage each element plays to make the whole performance envelope.  Most would agree that all of them are important, and it's pretty much guaranteed that the percentage factor changes a bit amongst them as variables amongst those elements change.

This is probably why most everyone has to develop their personal loads for their rifle in steps to find the most ideal combination...nothing unusual in that.  That Berger formula is probably one of the best I've seen, but if it was exact/perfect, there would be no need of as much load development.  But then, that's what makes reloading a fun challenge.

I  recently experienced an interesting issue when shooting some 220g subs from a 16", 1:10 twist AR15 barrel.  I bought this barrel for shooting 110g supers which it does with excellent results.  Using the Berger formula, I need an 1:8.5 twist for true bullet stabilization.  I shot over a dozen shots at 100 yards with this 220g sub ammo.  MOA was between 3-5 MOA with a red dot and amazingly no keyholing.  I know that's not the do-all-end-all of a barrel twist vs. bullet type laboratory study, but it did show me how "shifty" some of these elements can be.

The Berger optimum twist rate recommendations aren't for stability at 100yds, they're for long range supersonic stability.

Looser twists really come apart at distance for long projectiles.  I've watched it with .308 12" twist after 800yds repeatedly.

Did the OP say what twist rate they're going with in the .243 Barrel?

Yeah, I know it's more relevant at distance, but I was still surprised to see that accuracy without keyholing at that low a velocity and barrel twist.  These are long match 220g BTHP's.

No, OP hasn't reported on the barrel twist.

Run your numbers through JBM or Berger's calculator.  I would bet you were marginally stable.  I've gotten .4 MOA groups with a set up that had about 1.05 SG.  That was last summer.  Grouped and shot awesome from 100 to 350 yards.  Shot the same load over the winter and had 12" groups that key holes.  Adjusted the temperature on the JBM calculator and my SG was below 1.0.  They were officially unstable and the proof was in my targets.

OP as far as mags, I agree with the MagPul recommendation.  I asked about twist rate to recommend some bullets that might be better with the mag length restriction.