Can someone give a simple answer as to why there are 6.5 bullets of these 2 diameters?

new one on me.....where or who is making the .263 dia bullets?

Speer has always listed their 6.5 bullets at .263, as have a number of European bullet makers.

I used the Speers extensively a number of years ago in a .264 WM. They shot well, but when I switched to .264" Nosler Partitions, I noticed no particular difference in loading, pressure, or velocity.

Oh, I almost forgot: The simple answer is that the 6.5 was conceived as a metric bore diamter. The bore diameter converts to approximately .2559" (depending on what conversion factor you use). The difference in the bore and the groove (normal bullet size) is typically .006" in .22's, .007 in 243's and .257's, and EITHER .007 or .008 in 6.5's and larger (although the .270 is typically .007"). So . . . some manufacturers use a .007" (.263") and some use a .008" (.264) difference in the bore vs. bullet diameter.

I believe Speer made their bullet .263" to use in the 264 Win mag.
The original Winchester cartridge had pressure problems with the short throat Winchester chambered in their rifles and full diameter bullets, so they used a two diameter bullet in it. The bullet was full diameter in front of the canalure and a hair smaller in front. Speer to keep pressure down with their bullet just reduced the diameter of their bullet for the whole lenght of the shank.
At least this is the reason I been told.

quote:

Originally posted by Ol` Joe:
The original Winchester cartridge had pressure problems with the short throat Winchester chambered in their rifles and full diameter bullets, so they used a two diameter bullet in it. The bullet was full diameter in front of the canalure and a hair smaller in front.

I think you meant that the bullets were full diameter BEHIND the cannelure.

the Winchester bullets were bore diameter infront of the cannelure and groove diameter on the shank behind it, so the parallel portion of the nose only rides on the lands.

FWIW I still have nearly 400 of those Winchester 140gr powerpoint softpoints.

AllanD

So barrel makers couldn't make the same conversion to thousandths, or just couldn't standardize on the OD of the bore. Bullet makers followed them by varing design and/or diameter just because.....
And the .001" probably means nothing in use.
Does that about summarize it?
Do some bullet makers furnish both diameters?

It has to do with the different CIP and SAAMI specs for the variety of 6.5s and 260/264s. There are four different permutations. See the specs here.
The 260 Rem and 264 Win Mag are the same bore/groove specs.
Edited to add:
We also do two different diameters as stock items (soft and solid) with the remaining two as special order items.

It is likely not a very useful or needed bit of info these days, what with the current standardization of American 6.5 barrels at about .264" groove diameter, but a fair number of European 6.5 bores were also .266" groove diameter.

My new (in 1979) Carl Gustaff sporter in 6.5x55, for example.....

It was also fairly common for some European cartridges to use bullets somewhat smaller than groove diameter, which leads us back to .263"/.264".

This is not uncommon. Some pistol calibers (like 44 and 45) appear in various diameters, thought very close. 9.3mm bullets are seen in both 0.366 and 0.365 diameters...