Re: 6.5 stw
Any true magnum cartridge, the 6.5 stw included can always use the barrel length. However, not living in a perfect world, we all know that they still perform without the extra long barrel.
Here are my thoughts, and this is the brutal truth. The 6.5 stw is a great cartridge. With a 26" barrel, you should get the velocuty ranges that I stated earlier since that was formulated on a 26" barrel. In fact, Layne Simpson's data was from a 26" barrel as well. I would not go any less than 26". If you do, say to 24", you would be better off with a 264 win. mag. At this point, you would only be a couple hundred fps behind the 264 WM, but burning a significant amount of more powder.....making it inefficent.
One more point....take this as food for thought: The 6.5 stw has more energy at 200 yards with the 140 grain bullet than a 300 win mag with 150 grain does at 200 yards! This is according to Layne Simpon's article on the 6.5 stw. We can thank the nice B.C. that the 6.5 has to offer.
I have seen several people taking the 270 WSM and necking it down to 6.5mm. The load data that I have seen on this is not promising. Other than having the ability to use a short action and eliminating the belt, the 6.5 WSM has no more to offer than the 264 WM. Some of the load data that I have seen on the 6.5 WSM was slower than the 6.5-284. Burning more powder to go slower is mot my idea of a great cartridge. Layne Simpson has compared the 6.5 stw to other cartridges in his article. These being the 7mm stw and the 25 stw. Today, one could argue that the 7mm RUM may be a good contender. I would agree, but it has always been hard to turn away from the 6.5's great B.C. and penetration power.