If you are trying to improve SD, several things may be done.
1. Make sure your powder burn rate is compatible with the bullet and its weight. A powder that is too slow for the bullet weight and case size is harder to ignite consistently and will have trouble burning as completely as it should.
2. Make sure your primers are seated hard enough. Failing to set the primer bridge
properly causes erratic ignition. In general, you want to see the small primers at least 0.003" below flush with the primer pocket.
A better approach is to measure the depth of your primer pockets and the height of your primers, including the anvil feet. Subtract the latter from the former, then add 0.002 inches. That's the minimum your primer should be below flush with the case head. Federal likes to stick with that number for a small rifle primer, and wants you to add 0.003 inches for a large rifle primer. Remington and Olin, in documents from the 80's, recommended to the military that 0.002" minimum and 0.006" maximum addition should be used. I find if you aim for the middle of that range, 0.004" addition, you generally do well, even with Federal primers. Note that these numbers are just depths that are in addition to where the anvil feet just kiss the bottom of the primer pocket.
3. Uniform the bullet pull force. Polished brass can stick to gilding metal in irregular degrees. Run a bore brush into each sized case neck and turn it a couple of times to remove carbon and lightly scuff the surface. Apply a dry lube, like graphite powder suspended in alcohol, inside the neck with a cotton swab to prevent brass-on-gilding metal bonding and to uniform the friction of the bullet release (note, this can affect your sweet spot load, so be prepared to adjust it).
4. Find each bullet's favorite seating depth. This Berger procedure
for their VLD's is something to try, though in the little 20 caliber you may want to reduce the adjustment steps to 0.020".
5. If you're worried about hard holding, read this
. Practice, practice, practice.
In general, a free recoiling (like floating in space) rifle's rearward velocity at the moment the bullet leaves the muzzle is just the muzzle velocity divided by the ratio of the rifle weight to the weight of the bullet plus about 40% of the powder charge weight (the portion that chases the bullet down the bore). This is due to Newton's second law of motion which says the momentum of the gun and all forward traveling ejecta must be equal and opposite at that moment the bullet base arrives at the muzzle. So if the bullet is going 3000 fps and the gun weighs 1000 times more than the bullet, then the free recoil velocity of the gun at the moment the bullet gets to the muzzle will be 3 feet per second. That's as much as it can affect muzzle velocity. Far more influential are the powder position in the cartridge, the barrel temperature, and whether or not your bullet goes through the chronograph screens perfectly parallel to the axis between the screens.
6. Chronograph setup. High readings, low readings, and erratic readings can all can all be caused by bad lighting conditions and, most of all, failure to set the instrument far enough back to avoid muzzle blast triggering. We had one board member with a .338 Lapua Magnum that had to set his chronograph back 18 feet before his readings settled. Based on the size of your cartridge, that much shouldn't be required, but you are asking for trouble if you use less than 10 feet with a rifle other than a .22 Rimfire. I use 15 feet normally, just because that's what the SAAMI standards screen midpoint is.
Your use of the Lee Collet Die is just fine
. I combine them with the use of a Redding Body Die
for shoulder setback and which leaves the neck alone. This makes sizing a two-step operation. If you can't stand the extra work, then a Redding S type FL sizing die with bushing or a similar device in another brand would combine the efforts into one. Just be aware that where the Lee Collet Die is immune to neck brass thickness variation the bushings are not, and you will want to segregate your brass by lot and history for the purpose of choosing the most appropriate bushing size for the job. The bushing die with no bushing and no expander in place can serve as a body die. It just costs more to get into the bushing dies.