Just wondering why when I use AR Magnum Rifle Powder (AKA 3100) I get such differing results in burn rate in the 7mm-08 vs 308! Both rifles are Remington M-7's with short barrels!

The loads are as follows:
7mm-08 - CCI 250 primer - 42.0 to 45.0 grains of MRP - 139 grain Hornady SP!
308 - CCI 250 primer - 42.0 to 45.0 grains of MRP - 130 grain Hornady SP!

The 7mm-08 performs just fine, clean burning, etc!
The 308 always ends up with unburned powder left in the case and, the whole length of the barrel!

I realize that there is a bore diameter difference of .240, but to me it just seems wierd why this happens!

Chuck-

Just my guess, but because you are shooting 130's in the 308, which are light for caliber, and the 308 bore is larger than the 7mm (284) bore, you are getting less pressure, and therefore incomplete burning. Bump up the bullet weight in the 308 and see if it changes.

Actually I think MRP is on the slow side for both of those calibers. The slower burning powders work better in cases with large capacity relative to bore diameter. In other words, a powder that works great and gives optimum performance in a .243 win (like H4831 or MRP) won't give top velocities in a .308 win, and would do even worse in a .358 Win. You can't get enough of it into the case to allow it to develop the required pressure for peak performance. Clearly the optimal powders for the .308 are more in the medium burning range, like 4064, 748, or Reloder 15. If you have the opportunity to chronograph your loads, I will bet that your velocity will be much lower than you think, given your choice of powder and caliber. Adding in a short barrel will make it even worse. Your 7mm-08 might do pretty well with a slow powder if it had a 24" barrel. Even the .308 might be okay with a 24" barrel and heavier bullets. I really don't think either cartridge would be very efficient with powders slower than 4350.

If you're determined to use MRP, try crimping the bullet in the .308 -and the 7-08 for that matter. That should make it a bit harder for the bullet to "start" which should inhance your burn.

Chuck, the powder you speak of MRP is a very slow burn powder. IT is a mag type powder for large cases such as the 7-mag. cag. It is up there with AA3100, H4831 and RL-22 This powder is to slow for 308. with a 18 in. ballel. therefore you do not get all the burn. you need to go to a med. Powder like RL-15, 4320 4064 or H380. I have loaded the 7MM-08 for many years and use RL-15 at 44.5 grains with CCI 250 and a 140 sierra B.T. at 2920 fps and get great groups at 100 meters

I think the other guys answered this, but you need to match your powder to your bore size/bullet weight. The .308 is best w/ med. burners like IMR4064, RL15 & Varget. The 7-08 will also work fine w/ med. burners but you can often get better vel. w/ heavier bullets & slower powders (RL19, IMR4350, etc.) in the 7-08. Check the various loading manulas & you'll see the trend. If you want one powder for both I would select one of the med. burners.