Originally Posted By: Dark moon 631badshee, only problem with that load data is 29.5 is an 1/8th of an inch from spilling out. I'm good with 3310 with a 55 grain bullet. What I'm not getting is a 40 grain bullet with more powder is only 3260. I'm not expecting bolt action performance but was looking for 3500 or better. I'm sure a deep throat would cause this with a shorter bullet but a flaw is a short throat. Can't load magazine length without hitting lands. Ruled that out. Lol
I would imagine this has to do with a calculation regarding dwell time in the barrel, gas system length and barrel length. Similar to some pistol caliber carbines actually being slower than longer pistol legnth barrels. The 55 gr while heavier, is also longer allowing pressures to build and velocities to increase. Since the 40 gr is so light and has a shorter bearing surface, it just might not be building enough pressure before it leaves the barrel.
I ran into a similar scenario with Nosler's Combined Technologies Lubalox coated bullets. Book was claiming 3250 fps from a 24" barrel with a 150 gr Nosler ballistic tip and a given charge of IMR4350. Using the same barrel but the slicker Lubalox coated bullet, I was getting an average of 2997 fps with a SD of about 5. Throw a standard bullet in there and velocities increased. I was surprised having never really played with the Lubalox bullets, at least not with a chrono but after thinking about it, it made sense.
This seems to be a common theme with "overbore" cartridge that are shooting light for caliber bullets.