I build alot of big 6mm rifles for varmint hunters, big game hunters and longrange shooters. Everything from 6mm BR up to the 6mm-06 AI.
I can tell you without a doubt, if your going to be shooting lightweight bullets, by that I mean 75 grains or less, the 6-284 is **** near about the largest capacity you want to use. That class also includes the 6-06 and 240 Wby in the same capacity class.
I had a customer bring in a 6mm Mach IV which is the 264 WM necked down to 6mm and shoulder angle sharpened slightly. He said the barrel was shot out, it was not, just severally copper fouled, after three days of cleaning on the Foul Out electronic bore cleaner, the barrel was clean. The rifle was not overly accurate so the customer asked if I could try to work up a load for him, for yotes. I have long sence forgot the exact loads but I know for a fact that this rifle(28" barrel, 1-10 twist Shilen) did not break 4100 fps with the 70 gr Ballistic Tip and that was a top load and accuracy was only around 1 moa. Now I am not saying thats the fault of the chambering because the rifle was not accurized or anything special, just fitted with a new barrel.
The bad thing was extreme spreads were in the 100 fps range with the 70 gr bullet.
Best load I came up with accuracy wise was with the 95 gr Ballistic Tip and Rl-25 at that time. Again, I do not have the load data but the velocity was in the 3800 fps range with its best load which averaged right around 1/2 moa with spreads in the 30 fps range.
Now, that is faster then a 6-06 AI will drive the 95 gr BT for sure but the lighter weight bullets, you can push very close to 4000 fps with a 70 gr BT in a 6-284 with this length barrel. Maybe not breaking it but **** close so yes the 6mm-264 will add 100 fps or so to the larger 6mms on the 284, '06 or 240 Wby case but thats about it.
With heavier bullets, it will have a greater advantage but still, you get to the point of diminishing returns. You will still gain velocity to some point in capacity but it does not stay proportionate to the amount of capacity increase. Then once you reach a certain point, you will actually experience a loss in velocity when the capacity gets to large.
Personally, I have studied and worked on building a 6mm Allen Magnum and I have yet to see a combo that really offers that advantage over the 6-284, 06, 06AI or 240 to the degree that it would be worth while to invest on the new cases design.
Of the designes I like the most, I was going to use the SAUM case, set the shoulder back very slightly, and then trim the neck to be roughly .300" in length. Capacity would be slightly less then the SAUM case but still more then the standard big 6mm.
No it would not be as large in capacity as the 6-264 but it would be more practical and I would be amazed if it did not equal or exceed it in performance.
The main reasin I did not do this, the life of the SAUM is very uncertain at this point. If they were selling like hotcakes, there would be a 6mm AM based on this round. There still may be but I have other projects in the works that are more pressing.
In any big 6mm the size of the 284 and larger, your really fighting a loosing battle if you are using bullets lighter then 95 grains. +100 are much better and 105s and larger are even better. Working up a load for the heavy 115 gr Bergers or some of the really heavy Wildcats would work even better.
You hear all the time about these guys getting ultra high velocities, when they are pressed on it, generally, for accuracy, they either drop velocity dramatically with the light bullets or switch to a heavy bullet with these large capacity small bores.
By the way, powder bridging can be particularly serious with the 6-264 WM improved type rounds, ball powders are still the best. With the 284, 06 and 240, these are not needed as much, with the sharp shouldered '06 AI, it can also be a problem with some of the more aggressive stick powders.
Kirby Allen(50)
