Was going to post this in the .243 Ackley thread but the results were interesting enough that I thought it deserved a bit more prominence in its own thread.

I rebarreled a worn out .243 with a brand new Remington factory take-off barrel - a c-m barrel from a CDL, having it set back 1/2 turn and the chamber recut to re-establish headspace. The M700 action was trued up at the same time. This was part of an experiment to see if a factory barrel could be almost or just as accurate as a custom after market in a trued M700 action

It was.

I measured the distance to lands (DTL) of the new chamber, then 600 rounds later I measured again. It had receded by .070".

At that point I took this same barrel and had it chambered to .243 AI, again setting it back 1/2 turn. I measured DTL again for several bullets. This rechambered barrel now has exactly 632 rounds through it. Just measured DTL with three separate bullet types and the measurements show that the throat had receded .061" (give or take about .002" depending on my measuring accuracy).

The rifle has never been fired enough to get really hot, neither as a .243 or a .243 AI. 3 shots in a row (say in 1 1/2 to 2 minutes) are the norm with some 5 shot groups in about 4-5 minutes thrown in. No rapid fire 10 round strings or anything.

One of the reasons I wanted to AI the chamber was to mitigate the standard .243's voracious appetite for throats. It did, but I would not say appreciably. .061" is less than .070", but not enough to AI the .243 for that reason alone. There were other reasons for AI'ing the .243 which aren't important here but they have lived up to my expectations.

Anyway, I think the "experiment" was pretty well controlled so the results are pretty valid. Same barrel start to finish, same powders , mostly H4350, some IMR4831 or RE-15 loads, all from the same 8 pound jugs start to finish, same rate of fire, same measuring device, etc.

I have a stainless .243 caliber Pac-Nor super match barrel sitting in the closet waiting to go on this rifle when the current barrel is worn out. Going to have to go do some rapid fire and burn the hell out the throat now so I can justify putting that Pac-Nor on. Oughta be fun!

what you need to do is ream the barrel to 6mm or 6mm AI and do the same test and see what happens. but by now there is prolly quite a bit of alligator in the barrel.

Actually seems like a win-win. More speed with the AI, and less wear.

PS, how do you keep a barrel from getting hot?

I should add about the 6mm suggestion, there are some that say the longer neck of the 6mm case protects the throat alot more than the 243 case does.

That was one of the reasons I wanted to AI the .243. Apparently the neck length and 20 degree shoulder combine to focus the gases on the throat just ahead of the neck and make a real barrel blaster. Increasing the angle to 40 degrees keeps the hot gases from being "focused" right on the throat so much.

These results would seem to show there is some truth to that since I'm loading to the same pressure as a standard .243 and actually using about 4-5% more powder. But you're still pushing a lot of hot gas through a small hole so it's going to eat that throat sooner or later.

I like the 6mm Remington a lot but .243's are far more common, particular for left handers. Throat geometry and magazine length considerations favor the .243 as well. The 6mm at standard OAL is right up against a M700's maximum allowance. Even with less throat erosion, it will still erode the thraot and you don't have any room to chase it. With a .243 as the throat gets longer you can seat bullets further out and still have lots of room in the magazine.

The other reason to AI the .243 is to mitigate the wider pressure swings it shows. With the AI it behaves more politely so you can take it back up to where the factories originally loaded it. Basically a .243 AI equals a 6mm Remington safely.

Got two Winchester LH M70's and want to rebarrel them but still waffling on what to do with them. They are long actions but I want a short or medium chambering. The 6mm or 6mm AI is definitely on the list of candidates for those.

Love my 243, but always wanted a 6mm just for the cool factor. Nobody has those anymore.

Yeah, me too!

Jim Thanks for the write up. I love my 243AI and have had great luck with almost all loads developed for it. A 243AI throated for 105VLD on a M70 LH action would be awesome!!!

Jim, the first 600 rounds put a lot of taper on the height of the lands, the second test is fubar in my opinion.

I shot out a lot of 243 AI's on dog towns years ago using Hart stainless with zero freebore, 12 twist.

I would put 1000 rounds on each barrel per trip and measure the throat wear. We shot these barrels as hot and hard as you could imagine, dumping out .050 rounds and shooting as fast as you could load and acquire targets.

Typical throat wear was about .070 in the first 1000 rounds and after 2000 rounds we would set the barrel back. On the second chamber, the throat wear was usually about 0.100 or a tad less on a 1000 rounds.

We shot Win 760 exclusively with 70g bullets.

Stick powders ate barrels, especially 4064.

Jim, in your test, chrome moly was used on your barrel and it may indicate just how much faster chrome moly may wear and the powder you used was a factor also along with the bullet weight.

I thought ball powder was harder on barrels than stick powder?

Ackleyfan, I don't know where you came up with that notion. Military uses ball powder in their 5.56's and 7.62's....it's for a reason.

Really not sure but I must have had it backwards, are we talking a huge difference ?

I have a 243AI, 6mmRemington AI and a 6mm-284. All 1-8" twist. By far the longest barrel life was with the 6mmRemington AI.
It has a powder capacity between the other two but I always thought the long barrel life was because of the long throat.
One absolute I have found with tinkering with these 3 cartridges is that your barrel life is cut in half with a 3 groove barrel. Hope your Pac-Nor super match is not a 1-8"twist 3 groove you will be disappointed, if it is sell it and buy a 5groove.

5 groove, 1 in 9, Remington standard sporter contour at 24". I'm gearing toward the 85 grain range and it gives me the option of staying at 24 or cutting to 22" if so desired.

Still waffling about the next chambering (my middle name should be IHOP) and which rifle to rebarrel. Got a LH stainless M70 long action sitting in a McMillan BDL stock that is inletted for a Remington standard sporter contour. As noted in a post above, the 6mm or 6mm AI is a definite possibility for this one, this P-N barrel would drop right in and that magazine would give a 6mm round all the room in the world with lots of room to grow.

The current barrel on this .243 AI still has a lot of life left in it. The throat on the rifle's original barrel, which was built circa 1990, got so worn a 100 grain bullet touching the lands would not be in the case at all. But even with that it still shot great right up until it didn't.

I'll let this barrel tell me when it's time to retire it by how it shoots and not just by an arbitrary mesaurement. In the meantime that Pac-Nor is looking wistfully at me and whispering, "shoot me, shoot me...".

Just had another gunhead call me a couple of days ago about
a 243AI he just had built on a Remington M40X. Havent seen it
yet, hopefully this weekend.

828, hard to imagine a 3g vs 5g having just 1/2 the life, as I would not have thought groove count would affect life that much, let alone it's been said the wider lands in a 3g make a bbl last longer.

Too bad we don't see more good test results proving things one way or another and to what degree, but many industry insiders probably enjoy the confusion.

My personal .243 has 2100 rounds and still shoots very well. Of course I believe that a hot barrel is a happy barrel.

I suppose someday I will have to move the seating depth or actually screw a new barrel on but according to Mr Bore Scope that day ain�t today.

The idea that the AI rounds will �focus� the gases is not one to which I subscribe. If you want an AI then buy all means have at it but don�t expect an �increase� in barrel life if you are sending more powder down the bore.

Hard to imagine a shot I would take with an AI that I would pass on with the pleasant and easy to live with standard version.

I don't think this is a level playing field....with all your fancy Proprietary rifle parts

One minor correction, the standard version is the one that �focuses� (my word) the gas on the throat, the AI�s sharper shoulder angle theoretically stops that.

This whole thing really wasn�t planned as an experiment in barrel life from the beginning. It just worked out that I had a new unfired barrel chambered in .243 and after 600 rounds had it set back and rechambered to AI, then noticed that I had right about 600 rounds through it with the new chamber. I had the DTL written down when each chamber was brand new and also when they each had 600 rounds through them. I just took advantage of the situation to compare throat wear to see if there was a major difference.

My measurements indicate that in my chrome-moly Remington factory barrel the AI version shows about 13% less wear (12.857% for those that love digits to the right of the decimal). Whether someone wants to call that a significant difference is up to them.

Again, I like the AI version for a variety of reasons. If it gives me a few hundred more accurate rounds over the life of the barrel that�s okay with me.

I don't see how the shoulder angle is going to refocus the direction of hot gases/powder. It still goes through the same neck, into the same throat, against the same bullet, for the same outcome.

The various powders you used can drastically change the test one way or the other. Stick powders are a LOT harder on barrel throats than ball powders. You mixed and match both and probably not the same between both tests. It is interesting though, but far from conclusive in any way. It is fun to mess around with such notions Flinch

Aren't Ball powders all or mostly double base powders? And I believe they are so wouldn't they be harder on a throat than a single base stick powder?

I only know what I've read - generally - so this time decided to see for myself. The quotes below are some of what I've read. I've only shown those portions of the threads relevant to questions in this one, but lest I be accused of taking them out of context links to the original threads are included so those interested can go back and check for themselves.

All quotes are from Mule Deer and extracted from threads on the .243.

http://www.24hourcampfire.com/ubbthreads/ubbthreads.php/topics/1529522/1
"The .243 tends to erode throats faster than other rounds its size, probably because of the geometry of the case. The sloping shoulder and short neck tend to send hot powder gas right into the bore just ahead of the case."
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"6mm Remington barrels do tend to last longer than .243's, probably because of the longer neck and steeper shoulder.

The same problems that occur with the standard .243 are MUCH reduced with the .243 AI."
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"Montana Man,

Yeah, powder can affect barrel life. I generally run the cleanest burning powder I can, which cuts down on both powder and jacket fouling. Both have some effect on accurate barrel life.

Single-base powders have a lower burning temp than double-base. This supposedly does help in the long run, but the big factor is not frying the barrel with too many fast repeat shots."
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https://www.24hourcampfire.com/ubbth...e/Re_Mule_Deer_others_re_243#Post3677696

"The .243 is generally considered a throat burner. This is not just due to the bore/capacity ratio, but the fact that (as Ramblin' Razorback suggested) the rather sloping throat and tends to direct gas toward the throat just in front of the short neck. Thus it's partly a problem of gemoetry."
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"If the neck were quite long on all cartridges that would be true, but the gas is directed some by the shoulders before it gets into the neck--and if the neck is very short (as it is in the .243) then some the gas coming off the shoulder blasts right into the sides of the throat just in front of the case mouth.

Take a look at a .243 case, or a drawing of one, and you'll see what I mean. With other commercial rounds based on the .308 case (all of larger bore diameter than the .243) the neck is longer."
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I remember some other discussions about the properties of the .243 and how AI'ing mitigates or reduces them but hopefully the above will suffice to help explain some of my reasons for wanting to AI the barrel in the first place and perhaps to add some credence to the results of my little "experiment".

As a .243 the barrel saw rounds loaded with 100 grain, 85, 80 and 75 grain bullets using IMR4831 (call it 50% of those loads), about 45% H4350 and the rest a tad of IMR4350 and some RE-15.

As a .243 AI it has seen 100, 107, 87 and 85 grain bullets fueled about 90% by H4350 with some RE-15 and maybe 30 rounds using Hunter.

So the powder mixture was a bit different but I don't think there is a lot of chemical difference between IMR4831, IMR4350 or H4350 which accounted for the vast majority of rounds fired in both chambers.

Rate of fire was exactly equal - mostly 3 round strings in the space of 2 minutes or so. Several 4 and 5 round strings spaced out over 4 to 5 minutes.

This wasn't by design, that's the rate of fire I use for all my rifles. Bench testing gets 4 or 5 round strings, after that I'll get off the bench, load three in the mag and practice from kneeling or offhand, firing about as fast as I can work the bolt, breathe, steady down, aim and squeeze off a well aimed round.

Those three rounds get the barrel warm but not hot to the touch, even in summer. After three shots the rifle is set aside to cool while I shoot my other rifles.

Whether as a .243 or .243 AI, the barrel was never shot to the point where I could not comfortably lay my hand on it.