Factory ammo typically runs + or - 2% of average velocity. My serious bullseye loads are under 10 fps max velocity spread (+ or - 5 fps) ... plinkers are + or - 20 fps. You won't see much difference on a 25 yd target until spreads hit + or - 50 fps.
Thanks, I recently picked up a chronograph and some of the factory ammo I have checked is a little worse than 2%. Kinda surprised me but then maybe it's me learning how to use the crono. One of my hand loads sucks with a spread of around 100, but they shoot OK for what I use them for. At least now I have something to work off of.
Reb, If you had a 38 Special load with a +or-2% spread ...lets say 900 fps, that would be +or- 18 fps or a total spread of 36 fps. This would be quite normal for quality ammo. The white box stuff could be twice that. Rifle ammo is usually +or- 1% for quality factory loads.
When you get wide max velocity spreads, it's usually because of poor powder selection, light weight bullets, or weak cases. The 38 Special is an odd ball in that the case is quite large and the pressures are quite low. That means a "normal" powder charge will only partially fill the case to get the desired chamber pressure and velocity. Herein lies the problem with a 38 ... too much room for the powder to "splash" around between shots thus a different ignition rate for each shot. This is also called "position sensitivity" because changing the position of the gun will also change the ignition rate.
Powder granules like to all be touching and located directly in front of the primer flash hole to get uniform ignition. When powder lays in the bottom of the case (different for each shot) you may get good ignition or you may not but for sure, ignition will be different from shot to shot so velocity will follow.
The faster the burn rate of the powder, the more uniform it will ignite. That means Bullseye (fast burner) will get way more uniform velocities than Unique (medium burn rate). You don't use slow burning powder such as W-296 in 38s because they ignite very poorly unless the case is nearly full ... which would be way too hot for a 38 Special.
Next is the case. Most reloaders keep reloading their cases until a split shows up or the primer pocket will no longer hold a primer. For plinker loads ... no big deal but for more serious accuracy, you really need good cases. Each time a case is fired, it fire forms to the chamber (expands) then gets resized back to nominal diameter. This process work hardens the brass so after 3 or 4 times fired, cases no longer have the uniform neck tension to hold bullets. Proper neck tension is what helps the powder get a good burn. Light neck tension causes the bullet to start moving too soon .... pressure will drop and the powder won't get good ignition. The next round fired may have tighter or looser neck tension so it too will cause the powder to burn at a different rate. Crimp plays a role too. Although a normal crimp only accounts for 10% of the neck tension, this could be the difference between good and bad velocity spreads.
Last is the bullet. Heavier bullets always generate more uniform chamber pressure (thus velocities) than light bullets. With 38s, I like 140-158 grain bullets the best. If I load on the hot side, 125 gr bullets will work but never anything lighter than 125 gr. Additionally, bullet seating depth is critical for uniform chamber pressure. The deeper a bullet is seated, the higher the chamber pressure will be. That means your OAL should be nice and uniform.
So there are a few solutions. The best is to use a proper charge of fast burning powder. If you insist on using slower burn rate powder, use polyester fiber filler to hold the powder together and position it in front of the primer. I use the polyester filler made for stuffed toys. A .5 grain pinch works just fine when tamped down over a powder charge just before seating a bullet. Yes, it all burns up ... no issues except maybe a little more soot. Velocities are way more uniform and accuracy is better.
Next is to sort your cases by "times fired" and by headstamp. Each different manufacturer's cases internal volume will vary in slightly so using mixed brass will generate different chamber pressure. After my cases have been fired 4 times, they go in the "plinker" bucket. I still use them but for "match grade" loading, all my cases will be the same headstamp and fired the same number of times (3 or less).
When I load for anal accuracy, I weigh each powder charge and sort bullets by weight. If you do the math, a powder charge or bullet weight that is off by X% will also drive the velocity off by a like amount. Worst case you could get a light bullet with a heavy powder charge and get a way higher velocity than a light powder charge and a heavier bullet.
I do keep my cases sorted by head stamp and I trim them to a uniform length before there first reloading. I think you hit on the problem with this particular load, low case fill. It is the first time I have used universal and I like the way it burns but in a 38 special load the case is about half full or a little less. Although I am very careful it also concerns me a little that I could double charge in a 38 case and still have room to seat a bullet. I shot some of them yesterday tipping the gun up before each shot and it helped. This also confirmed my suspicion that case fill is part of the problem with this load.