Originally Posted By: cjclemensPardon my ignorance on this topic, but why the interest in a 16" 6x45 barrel? I was under the impression that the advantages of the 6x45 really started to shine in the 20-24" barrel range. Full disclosure: I have one of the Midway/AR-Stoner 20" 6x45 barrels. I haven't had a chance to put round #1 through it yet, so I couldn't make any comments about accuracy. I'm just curious about the rationale behind a 16 incher.
The 6x45 will gain you about 200fps over the 556 in the 16" with similar weight bullets. Or, the 6x45 will launch 70 grainers at or greater than the 556 can push a 55gr.
It's all about bore diameter. Same/similar weight bullets launched from the same case will always see an increase in velocity as the bore diameter is increased.
Here are some examples:
260 vs 7mm-08 with 140gr bullets: 7-08 wins
7-08 vs .308 with 150gr bullets: 308 wins
308 vs 338 Federal with 180gr bullets: 338 Fed wins
Much the same as .270 vs 30-06, both shooting 150gr bullets. The 30-06 will win.
Remember, these are for initial starting velocity from the muzzle. At times, the smaller bore cartridge with higher BC bullet may overtake the larger bore, even though it started slower.