6mm or 243AI?


Active Member
Apr 5, 2004
Cody, Wyoming
I am considering this for a long range varmit cartridge. I always thought the 6mm was under rated and much better ballistically than the 243. After reading on this site about the 243AI and factory brass for it, what is the difference between the two? Is a 6mmAI that much better than a regular 6? Advantages...disadvantages?
.. Basically, tweedle-dee or tweedle-dum.. Some will claim huge differences but when realistically investigated the differences are mostly cosmetic and statistical.. The .243 has better brass available from Lapua closer to a ready state than the 7x57 Norma available for the 6AI..
... Both are used with great success and I've enjoyed my 6AI 14tw.. Pick the one ya like the looks of and have at it.. d:^) JiNC
I chose the 6mmAI and went with the Norma 7x57 brass. My Hart barrel had a 12" twist and shot very well with 55gr-80gr bullets. The 55gr B-Tips can be (safely) shot so fast that they are flatter, and resist wind better, than the heavier bullets all the way out to about 750yds. After that the heavies take over.

Fireforming can be a pain in the neck. But if you're comparing the 243AI to the 6mmAI, then you would have to fireform either way.

The 6mmAI does have a moderate velocity advantage over the 243AI, if that's important. In a custom rifle either cartridge is capable of superb accuracy.

I hope I haven't just made your decision a little harder.
Hey Varmint Hunter,

When you say the heavier bullets take over after 750 yards, what do you mean by heavier? Are you talking heavier BTs (70-80grs), 90gr Bergers, or 100+ grain VLDs, or all of these?

I am just getting started this season with my first 243AI, and am trying to settle on "the load." I am looking for it to cover out to about 750 yards or so, after that I'll pull out the 300WSM.

Thanks. Rick
Yes - by heavier I was refering to bullets from70-80gr. I have not looked at the ballistic charts for the real heavys (105-107gr) because my 12" twist will not stabilize them.

The 243AI will shoot a 55gr B-Tip bullet in excess of 4,000 ft/sec. If you do a few comparisons you will notice that this bullet will be flatter and more resistent to the effects of wind than most heavy but slower bullets out to about 750yds.

You would have to check on the VLD's but I doubt that they would beat the 55 grainers at their far superior velocity.

I have a 1-10" twist, so 90 grainers are about tops for me. I am getting ready to try out some 90 gr Bergers and Lapuas, but now I think I'll try some 55 BTs in the mix and see what looks best.

Thanks, Rick
I may have said it before, but I will say it again. The 6mm Remington is probably the most overlooked cartridge on the planet. I load 41.2 grains of "Varget" behind an 80 grain "Nosler" ballistic tip bullet. That charge sends it downrange, out of a 26 inch "SS Shilen" barrel (1:8" twist)at 3386 fps. It makes .176" groups at 100 yards and performs exceptionally well downrange. The 6mm AI may be popular, even trendy; and the .243 tried and proven, but so is the 6mm Remington. It is my favorite varmint cartridge.

[ 04-07-2004: Message edited by: W ]
Will a twist that is fast enough to stablize the bigger bullets such as 90+ do ok for the 55-60 grainers? From what I have read (I am still learning) the twists that handle the bigger bullets handle the lighter ones better than the other way around.
Varmint Hunter..
.. According to Point Blank Ballistic Calculator there's about a 16" disparity between the 55grBT and a 107grMK at 750 and about a 5" difference at 500 for windage.. Where are your calculations that show the lower BC'd bullet having better wind deflection resistance coming from.?? I was suprised to see how well the 55gr bullet maintained tragectory but I can fairly well predict trajectory and allow for it by clicking it in.. Wind, OTOH, is a different thing.. What's your priority.??
..I bet that little 55gr pill @ 4K+ is pure hell under 500.. Hehehe.. I run the 75gr VMAx's myself and have been very pleased with their performance.. Thanks.. d:^) JiNC

As I said in my previous post, by heavies I was refering to 70-80gr bullets and not to the 105-107 VLD bullets. My 12" twist was never intended to use them, as I preferred to use traditional "varmint" bullets. The VLD's always have an advantage with the wind due to their exceptionally high BC.

But to put some numbers on the comparison, here is what I got using moderately high velocity (but not max capable in the AI's)and calculated with a 10mph cross wind.

55gr B-Tip @4,000.....63" @750yds
80gr Sierra @ 3,300...67" @750yds
70gr B-Tip @ 3,600....61" @750yds

The little screamer 55gr B-Tip is very destructive on groundhogs and is as good or better than some others in the wind. It can also be shot quite a bit faster in many 6mmAI rifles but I tried to keep the comparison fair.

As for your Sierra 107's, I'm not sure what kind of terminal performance you get from these bullets but they sure do buck the wind. In my comparison the difference was not 16" @ 750yds, but 20".

Good shootin' - VH
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