John Perry
John Perry
1 post
Joined December 2008

Reducing COL variations

While posting to the XL-750 board, I believe this info applies to all Dillon presses with interchangeable tool heads. I started with a used RL-450 many years ago. For those too young to know, the RL-450 has a fixed head for die mounting. Die holes and threads are applied directly to the frame of the press. During the years I was using that press, my COL variation was in the range of +/- 0.002". Dillon offered a good deal on an upgrade to the RL-550 with interchangeable tool head. I didn't notice until I was reloading some .308 Win for precision long range that the variation had jumped up to about +/- 0.009". I was so discouraged that I tore down the 550 frame and built back up the RL 450. My COL variation went back down to +/- 0.002".
About 3 years and many thousand rounds later, I couldn't bear not knowing why the RL 550 wasn't consistent. The +/- 0.009" variations returned but only with certain rounds.

I am not a subscriber to the notion that +/- 0.009" variations are not important. When you know exactly how much bullet jump yields the best group in a particular gun, that is a lot of variation ... including muzzle velocity. Precision shooting is all about reducing variations that affect precision.

Across the board when loading multiple cartridges I noticed more variation in the RL-550 but none more than the .308 where I single load. I've looked at topics in this forum and none of them mention what I am doing to solve this issue.

OK, so my .308s are loaded more like a single stage press.I don't have a case in stage 1 and don't want one there. I weigh each charge and dump powder into the case. I then insert the casing into the seating station (the only station with a die) and seat the bullet.

After looking at the possible variables, bullet ojive (Sierra match ... no problem there, seating stem (using the same dies, no problem there), ram head (no problem there) or tool head. I am aware that the pressure difference applied in stage 1 can cause height differences on the other side of the ram head. But wait, if that were the issue, I would have seen it in my RL 450.

That led me to conclude the rather loose fitting tool head might be the single largest factor. With a casing in the resizing die, the approximate force on the ram head is probably consistent enough to force the tool head upward and tight with each stroke. However, with nothing applying upward pressure in station 1, the seating station height varies based on neck tension. My .308 cases are resized, de-primed then trimmed using the Dillon trim die and motor then mouth chamferred. They are then annealed. The neck tension variations are not enough to be noticeable but I'm convinced they exist.

I measured the vertical travel variation on the tool head with a dial indicator at the seating die and found 0.0067" variation between the relaxed position and when pressure was applied upward at that stage.

To eliminate or minimized that motion, I created an aluminum shim that I glued to the bottom of the tool head rails. The tool head fits snugly but allows just a bit of motion with some minor friction to allow for tool head centering. I put the shim on the bottom of the tool head rails so that the major force in the ram's upward stroke occurs on the normal machined surfaces between tool head and frame.

While single loading my 308 Win cartridges, the COL variation has been reduced to +/- 0.003" and I don't need to have a fired case in station 1. That is very close to the performance of my old RL-450.

Sorry for being so long winded.