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Discussion in 'Gunsmithing' started by Booney, May 25, 2013.
What does it take to change a 243 over to a 6mm rem?
The 6mm Rem has more taper in the case and is longer but has about the same case capacity
as the 243 and there would not be any gain in velocity.
You would have to cut at least 1'' of the back of the barrel, re thread the tenon, cut a new bolt
face recess and re chamber. (A lot of work for no gain.
If you want more power/velocity you can re chamber to 243 AI and pick up 100 to 150 ft/sec
and not have to do anything to the action.
J E CUSTOM
you'll need a new barrel blank as the 6mm chamber won't clean up in a .243 barrel. But a 6mm Ackley (or RCBS rockchucker will). I suppose that you could set the barrel back about a 3/4 inch and maybe barely clean up. I'd just make it an Ackley and be done with it
a few years back I made CAD drawings of the two in layers to see what it took, and the Remington case looked like it needs about .010" on each side to clean up. But he 6mm Ackley as slightly less taper than the .243, and will clean up.
If the .243 has much more than 500 shots thru it, it probably is starting to errode anyway. So a setback will get you there. Performance wise the Remington design is better all the way around with a far greater barrel life. Better yet would be to ream the .243 chamber with a 6mm Ackley using a .243AI head space gauge. You'll still have to use 6mm brass or necked down 6.5 Sweed brass, but the barrel's gonna last a lot longer and the neck handles those 105 grain bullets a lot better.
Trickymisfit, I am interested in your comments as to barrel life. I am not critical, but would like to hear on what you base your opinion. It seems so hard to get a valid true comparison of barrel life, with ALL things being equal. By the way, I have been shooting a .243 Rockchucker since 1972.
Tricky I was going to go the AI roughy I should have specified that but I don't under stand what you mean when you say clean up.
first of all my comments were directed towards the .243 Winchester case design, and not the Ackley or Rockchucker. These case are fairly similar except for the shoulder angle (I actually like the rockchucker better than the .243 Ackly). My comments about reaming the .243 chamber with a 6mm Ackley reamer, but using a .243 Ackley headspace gauge are fairly simple. The chamber will clean up nicely, and without a lot of effort. You get the much desired .35" neck length, but like you, I'd have prefered the 30 or even 35 degree shoulder angle. Around my parts a .243 Rockchucker reamer is about as rare as it gets (I don't know that I've ever seen one), but the 6mm Ackley is fairly easy to find. That's why I said to ream the chamber with the 6mm reamer, but cut it short to use the .243 AI gauge.
Barrel life has always been an issue with the .243 Winchester if you shoot a lot of rounds thru it a year. Dosn't load very long and heavy bullets as well as it should due the the excessively short neck length (the bullet ends up being inside the shoulder area, and that ain't a good thing). I guess you could cut the throat excessively long to cure this issue, but then create some others. On the otherhand thse .243 case don't do very short bullets as well as it should, but in most case will get by. But most also shoot with a lot of bullet jump. A 6mm barrel will usually go twice the distance of a .243 Winchester, and the Ackley could be slightly longer (debatable). The problem with the Ackley is that the 6mm Remington case is really about the max capacity for a 24 caliber round without getting into overbore. The Ackley version is into overbore, but not a huge amount like something like a 6mm-06. If you CAD out a standard .243 Win case, you will see that the turbulance points is inside the barrel's throat. Not a good thing! The 6mm Remington's TP is about 20% inside the neck as measured from the lip. Better! The 6/284 actually is slightly better, but suppers from a great overbore condition. The Rockchucker case is about the same as the standard 6mm case (still good). Never quite figured out why Fred settled on that 28 degree shoulder instead of a more common 30 degrees! I'm sure he had his reasons.
I shoot the 6/250AI, and the barrel looks like it's toast in less than 1200 rounds, and maybe gone at the 1000 round mark. It has the Ackley 40 degree shoulder and also the vaunted doghnut issues. The neck comes a tad on the short side (I see necks in the .25" area using .243 Win cases as a parent). I think that had that chamber been cut with a 6mm Ackley reamer, I could have cleaned up .243 cases for an O/L length of 2.00". Giving me a neck length of about .35", and probably an honest 1800 shot barrel life. Plus the .243 Lapua cases are great cases to start out with. I like using the .243 brass as a parent case anyway, because there is a lot less brass displacement during fireforming. Of course the 6BG solves all these issues in spades, and has the desired 30 degree shoulder angle.
the Winchester case is actually fatter by design than the Remington's use of the 7x57 Mauser case. (or is the 6.5 Mauser? Don't remember). The Remington case is longer with a lot more taper than the Winchester. I thought it'd clean up like others, but was smart enough to draw them both up on the computer. They just will not clean up. The winchester taper is about .012" per inch, and the Remington's taper is .027" per inch. And Ackley almost always uses .010" per inch. All these cases start out with a .47" case head, but also use different amounts of taper in them