"Overbore" discussion...continued

[Tommie]

The subject has been discussed here before but the recent dialy bulletin got me thinking about what the definition is or should be.

Many high capacity cartridges are routinely labeled as over-bore. So does "highly inefficient" mean the same thing as over-bore? Just my opinion but I would say no. I suggest that powder capacity is beyond bore capacity only when no velocity increase is possible when compared to a lesser capacity chambering, regardless of bullet or powder type.

In other words: when the law of diminishing returns reaches zero velocity increase, overbore has been achieved. Anything else is an argument defining relative levels of efficiency.

 

[CatShooter]

If there is an "overbore", does that mean that there is a "bore", and an "underbore". ??

There was a time when we didn't have the slow powders we have now. At that time, "overbore" meant that the case was so large that the slowest powder could not fill the case, so the case was "over sized" for the bore.

Now, it has become loose slang for "any big case".

But in my way of thinking, it is no longer a relevant term because of the powders we have now.

As to efficiency, when we make any cartridge larger for a bore diameter, the energy (in foot pounds) delivered per grain of powder goes down because the expansion ratio always goes down.

If we were to keep the expansion ratio fixed (i.e., 8:1), so as we made the case larger, we made the barrel longer to make use of the powder, I think the foot pounds per grain of powder would be pretty consistent.

 

[357Mag]

Howdy -

I believe Ackley addressed " ideal bore capacity " and " ideal case capacity " in his 2 volumne treatise.

With the newer-generation of heavier bullets per calibre, and heavy/ slippery VLDs; what might be
" overbore " case-capacity wise.... is mitigated somewhat.

Using one rifle/barrel as a "for instance":
- A high-intensity .224" calibre wildcat w/ a 28" 1-8 twist that shoots 55s successfully; even though
considered to be classically "overbore ".
- Same rifle/barrel is used to shoot .224" calibre 75s, and the resulting performance / loads no longer
meet the definition of "overbore" .

Not eloquent fer sher, but I hope I was able to pass along a useful notion about " overbore " capacity.


With regards,
357Mag

 

[Tommie]

I've read and own three of Ackley's books and I owe most of my cartridge interest and knowledge to him. When I pass through the pearl gates I plan on picking the man's brain.

That being said, his writings aren't beyond debate. For example: read his comparison of the 6.5-06 vs 6.5-06 Improved. When comparing equal charges of the same powder he finds that the unimproved '06 case provided a higher velocity than the improved case. He therefore declares the standard case maximum capacity. I don't know if Mr. Ackley was suffering from a brain fart but a larger capacity case will always result in lower velocity when compared to the same charge is loaded in a smaller case.

Maybe some day powder technology will reach the point that there is no such thing as over-bore. Until then we will have to live with the fact some cartridges are more efficient than others. And some (not many) are truly crazy.