I�m thinking about building a long range rifle on a Rem 700 .243 action, I want to go for the 6mm AI.
What do I have to know before I start?

for one, you have to know which bullet(s) you intend to use so you can select the appropriate twist rate.

George

You also need to state how long is long.....is it 500 yards.....800 yards.....1000 yards???

You can also opt to make your gun a single shot.....and that way shoot things like the 6.5-06 in a short action.

Being very specific in your goal will go a long way in achieving success.

Is this "long range" for deer?.....for prairie dogs?...for targets?

There are so many trade offs that make these topics interesting. Just some of them are the actual effect on the game at long range, richocets and recoil.

Your 6mm has a chance to shoot bullets fast and small enough to reduce the danger of richocets. A larger bore would have more effect on the game however at any range. When one is treading on over bore capacity rounds the next larger bore should always be considered otherwise.

Then there are the bullets. Some will twist it for VLD's or MK's and try then on game or pests. I would not but that's up to you.

So many choices.

I have shot the 243 long range at pests since 1957. It's really effective with just medium light spitzers and letting a good shot off from a reliable rifle.

Today the laser range finder has improved the sport so much.

For myself I would use the standard 6mm Rem with a 1-10 medium sporter barrel 26" long. But if you have not had an improved cartridge yet then your like I was in the 60's.

What do you intend to use the rifle for, and who do you plan to hire to build it?

In my opinion, wildcat cartridges are an unnecessary pain in the a$$ that I'd rather do without.........

AD

Allen asked the right question, what is it used for. There a lot of better choices depending on what you want to shoot and even what you consider long range.

Here on the "Ice" it�s mostly foxes and seals that we shoot and some paper targets.
The range is about 50 to 400 meters, on and off, and this is a windy place,and these things are small so we need accurate tools.
I�m now using a 22/250 and need a bit more killingpower.
The bugdet is minimal so I was thinking about buying a new Savage or Remington in 243 or 6mm and change it in to an 243AI or &mm AI.
A single shot rifle will do and the bullet weight is 70 to 90 grs.

A few people who have used the "short" Remington action for a 57mm base case round, then find that the short magazine compromises case capacity (because you have to back the bullet into the case to get it to fit into the magazine) and loading close to the lands. You are better off using a full-length action for a 6mm Rem or 6mm Rem AI.

As noted above, you have to use a fast-twist barrel with the longest bullets.