I'm new to the forum and relatively new to reloading so this may be a somewhat simple question. I have recently realized that the C.O.L. I've been using was determined using a Nosler Partition bullet as opposed to an Accubond which is what I'm loading with. COL with Partition is 3.374 and the COL with the Accubond is 3.428. I've had pretty good luck loading Accubonds with a COL of 3.350 and 69.0 grains of Retumbo @ 2950. What will happen to pressures, accuracy, and velocities if I load 69.0 of Retumbo into a case with an increased COL of 3.428 (minus ~0.015) as opposed to the original 3.350? Any responses would be great.

Shoot the OAL your rifle preferes for any bullet you care to use.. It's unlikely your rifle much cares what a book says.

What shell and barrel lenght?

Hey craig, welcome to AR

What you have to realize is that the distance to the lands is the important consideration when setting your OAL. Different bullets have different contours and dimensions from the point at which they are caliber size (.308" in 30 caliber, .284" in 7mm calibers, etc.) to the point of the bullets. You need to find how far out you can seat the bullets before your bullet hits the lands and then back off an appropriate amount (.02", .05" or whatever you decide).

The distance you can seat your bullets close to the lands often depends upon your magazine length. IOW, you may want to seat within .020" off the lands, but can not because the longer bullet will not fit in your mag.

But to answer your question, a long jump to the lands will decrease your velocity. It may or may not affect your accruracy.

What caliber and gun are you shooting?

measure to the ogive not the tip you can then use 50 different bullets and have the same jump to the grooves.

With bottle neck cartridges COL makes very litle difference in pressure as lond as you aren`t already on the ragged edge. Load to the lenght your rifle wants and work up as recommended and you`ll be fine.
Small straight walled cartridges such as the 9mm or 380 can show some pressure variations due to COL. It is recommened one go with the manuals lgt used to develope the data in them.

Nothing. Unlike deep seating handgun rounds, seating a rifles bullet a bit deeper may actually decrease pressures depnding on how far from the lands you are seating.