243lbc vs 264lbc velocities don't jive to me

204 AR

204 AR

Well-known member
I've been researching quite a bit for some time now on these and the 6x6.8. I've got a bhw 264 on it's way I can't wait to get put together.

But here's my question, and what doesn't make sense to me is comparing speeds with same weight bullets. Every other case family, when the bore size goes up so does speed with the same weight bullet. But not this one. Are they loaded to drastically different pressures or what?

An example, I've read the 243 can push 95 gr bergers to 2950+. But I've yet to find a single load that shows a 95 vmax out of the 264 to anywhere close. If not a pressure disparity, then what?

And if the bolts can take the 243 at higher pressure then why not the 264?

Ritch and Greg, help me understand this?
 
Originally Posted By: 204 ARI've been researching quite a bit for some time now on these and the 6x6.8. I've got a bhw 264 on it's way I can't wait to get put together.

But here's my question, and what doesn't make sense to me is comparing speeds with same weight bullets. Every other case family, when the bore size goes up so does speed with the same weight bullet. But not this one. Are they loaded to drastically different pressures or what?

An example, I've read the 243 can push 95 gr bergers to 2950+. But I've yet to find a single load that shows a 95 vmax out of the 264 to anywhere close. If not a pressure disparity, then what?

And if the bolts can take the 243 at higher pressure then why not the 264?



Your statement about bore size and speed is partially correct. The best case of this can be seen in the 25x6.8 and the 6x6.8 With a 75 grain bullet in it and the 6mm the 6mm will out run it by over 100 FPS. This is using exactly the same powder charge and primer in both cases.

The reason for this is expansion ratios. With close to the same powder charge of the same powder in both bore sizes the larger bore will loose pressure faster than the smaller bore. In the case of the 264 LBC and the 243 LBC the same holds true. The case simply can't hold enough powder to keep the pressures increasing at the same rate in the larger bore size.

As a side note: There is published data out there that will push a 123 6.5 bullet close to 2750 from a Black Hole barrel and still meet the maximum pressure ratings. It has to be done from a 24 inch barrel, but the load you asked about in the 243 LBC is from a 24 inch barrel.

 
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I found that my 6.5 LBC shoots 85 grain .264 Sierra's way faster than my 243 LBC shot 85 grain .243 Sierra's.
 
With the same barrel length, same rifling pattern, same exact powder charge of the same powder, same primer? If the 264 is faster then it just rewrote the book of physics. The difference between the 243 and the 264 bore size in surface area of the heel of the bullet is huge.
 
Originally Posted By: MPFDI found that my 6.5 LBC shoots 85 grain .264 Sierra's way faster than my 243 LBC shot 85 grain .243 Sierra's.

I calculated the base ratios and the 264 is 18% (APPROX) large in area so the expansion ratio gets big fast with the same charges of powder. We found that in the A-30 you can't get enough powder in the case to run a rile length gas and have stuck with the carbine system because of that.It holds more than a 7.62X39 but it is just a huge hole for that little thumper.

I know I haven't even looked at anything under 123 in the 264 LBC as I want to use it for longer range target stuff but this winter we'll see what happens when I get down on the smaller pills. The 243 LBC gets a lot ore varied diet. A well balanced cartridge that is like it's cousin the 6X6.8. It has better big bullet potential but with a 95 in it it screams way out yonder.

I'll go to some faster powders and see what shakes out but I know that same same does make a difference. My shooting in other variants on the 6.8 on overlap bullet weights showed me that phenomena quickly. The classic example is the 223 vs. the 6X45. Lots of overlap but the speeds are definitely not the same.

Greg
 
Originally Posted By: ackleyman243 shoots the 95g at 3150 in some rifles, 24" barrels

I'm shooting 95 Sierras at a comfortable long brass life at 3000 or so in my 243 LBC's. With the nice BC they fly very well out yonder.

Greg
 
Originally Posted By: R Johnson With the same barrel length, same rifling pattern, same exact powder charge of the same powder, same primer? If the 264 is faster then it just rewrote the book of physics. The difference between the 243 and the 264 bore size in surface area of the heel of the bullet is huge.

Must have then. Both 20" BHW 1-9 twist P3 barrels, both with 28 grains of 8208 and primers from the same box. 85 grain .264 bullet was 100 fps faster...
 
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