Which Action to rule it all?

I assume by "AICS" and "mk13" with no Mod designation we're talking the old school thumb hole chassis (AICS 1.0,1.5 or 2.0?) and not the AT/AX aics chassis?

It has been several years since I thought about these so I might be wrong, but if memory serves me, there were 2 different long action legacy thumb hole AICS chassis systems (LA non-CIP and CIP). Might be worth measuring for piece of mind before you make the jump to 338Lapua or similar.

Also, If you have never handled the AICS, shoot it before you sink more money into it. Not as polarizing as the love it or hate AT/AX pistol grip, but they still aren't for everyone.
So if i dont have a cip magazine is just measuring the magwell and verifying that it is slightly bigger than the stated cip mag length good enough?
 
Get the gun as is, get some factory 300WM, get dies, and proceed to shoot the barrel out over a year or so. At that point, you're in a great position to know if you need more FPS (300 PRC), diameter or grain weight (338 LP), or could just slap another 300 WM barrel on it. Big calibers usually need big distances to really shine, and if you aren't set up for that, then you're just paying money for more muzzle flash and recoil.
 
So if i dont have a cip magazine is just measuring the magwell and verifying that it is slightly bigger than the stated cip mag length good enough?
I have an older Remington 700 MLR in 338 Lapua. I have that installed in a AI AICS chassis. It's one of the older models with the non-CIP length magazines. The outside dimension of the mag is 3.73" front to back. Those magazines will not accept a 3.680" CIP length cartridge. You can load a cartridge out to about 3.64" and have it run in those non-CIP mags. If your mags are around 3.73" front to back, you have a non-CIP chassis. No sense in attempting to build a 338 Lapua in that chassis. Keep it a 300 WM.
 
We were out at the range last week, several of the shooters next to us were running 6.5/6 Creedmoors.
All of them were complaining they couldn't spot many of their impacts at 1K and beyond- and one group of them had a dedicated spotter with NF glass.

Nor could we spot our impacts at one mile with .338 LM, and at 1200 with a 7-08 despite the above spotter helping us, AND a long-range Bullseye camera system that had a lousy refresh rate. No issues spotting .338 impacts at 1200.

The more energy you have at impact the easier it is to spot it, whether it be mirage or wet sand that barely kicks up a signature.

In my book, nothing is more frustrating than not being able to spot your misses when conditions are tough. Might as well pack up the drag bags and go home (and we have) rather than throwing a few dollar bills on the ground with every shot.

Me, if I'm building a dedicated ELR rifle- it's gonna have plenty of ass. I want every advantage not just in getting the bullet there- but being able to see it when arrives. For those that shoot small calibers at these ranges, and don't have issues related to spotting more power to ya. Location and composition of the berm probably makes a difference one way or the other as well.
6 Dasher was doing just fine at Nightforce ELR. Hits (or misses) were no problem.

Our local club match can go as far as 1600+ yards. Everyone shoots 6mm. No issues with wind, heat, rain, snow, etc.
 
I wont tell you not to buy a 338LM because if it's what you want then go for it. We all have those wants that might not make sense to others. I will say that a 300WM at 1200 will get easy pretty quick. They guys I shoot extended range with dont even shoot their 300WM rifles until they go past 1000 yards. They shoot 200-220 grain bullets and those rounds only take something like 17mils to get to a mile. And you can see the impacts on steel at a mile with a good spotter (Razor 20x mrad eyepiece).

Talking with some dudes I know that shoot 338LM have all said something along the lines of "If you arent shooting past 1500 yards you are wasting money."