More specifically in 5.56 for an ar15.

I'm getting 3 or 4 very consistent rounds (es 10, sd 4.5) then 1 or 2 rounds that are +/- 20fps or more, then 3 or 4 rounds that are consistent like the first few.

I didn't notice this phenomenon until I switched to a new lot of powder (8208 XBR) that is running roughly 40fps slower than the first, and different brass (FC 223)

I'm using a Sierra 77gr SMK, rem 7-1/2 primers, FC 223 brass, and 8208 XBR now.

I was using LC brass, and a different lot of 8208 XBR that got me 11fps ES and 5 SD. But those rounds were 40fps faster with the same powder charge.

I'm thinking the neck tension on the brass isn't consistent, but I'm not sure.

Any ideas on what's causing this? And what do you do to ensure consistent velocities?

There are many variables that play thier role in part, in whole and/or jointly exponentially.

Best explanation of why and what causes high ES and SD numbers and how to reduce them is David Tubbs.

It's old and long, but still valid.

Two parts.

https://youtu.be/NUnrYp0NH38

Make sure your scale is accurate.

Use an accurate beam scale to double check your electronic scale if you are using one.

Use check weights to verify your beam scales accuracy.

Stick with LC Brass and weight sort them so they are within 1 grain of each other.

Use a Hornady comparator to make sure your bullets are seated to the same depth, keeping the base to ogive dimension as close as possible.

Anneal the brass to get consistent neck tension.

overall case prep also:
-neck tension (as stated above)
-uniform case head space
-uniform primer pockets
-clean and uniform flash holes
-uniform case volumes (after sizing)

Aside from all the case prep and sorting, more powder gives me better numbers.

Make sure your chronograph is accurate.

Actually, just the opposite.  The best thing you can do to reduce your ES and SD numbers is to use a chronograph with a slow clock speed.  An older very expensive chronograph may be accurate, but because of the slow clock speed it may not have the resolution to "split hairs", so to speak.  In other words, it lacks the resolution to detect slight differences in MV.  That leads to small ES and SD numbers.

A modern chronograph, even it it's a cheap one, may have a very high clock speed  with comparatively good resolution.  The accuracy may not be perfect because of improper calibration.  That means it might measure all your loads 50fps slow for example.  But if it can resolve tiny differences in MV, then it will record ES and SD numbers which will appear worse than that measured by the older chronograph under identical circumstances..

"More powder" may be the most effective way to reduce ES and SD.  But not because of increased velocity, but rather because of higher loading density, also known as fill ratio.
It's the ratio between the amount of powder in the case and the case capacity.  All other things being equal, the closer you can get to 100% full, the more consistent your MV's will be and the lower your ES and SD numbers.

I think that's why so many have such good success with Varget, particularly with the .223.  Varget is a good powder, but it's also a low density powder.  If you fill the case to near the neck/shoulder junction you will, in many cases, have a good stout load with very little air space.  That tends to produce small groups.  Of course I'm not advocating anyone go out and buy Varget, fill their cases, and go shoot the resulting ammo.  You just might blow your face off.  The point is that a nearly full case of low density powder helps give you good ES and SD number.  ALL hand loaders MUST work up their loads starting from reliable published data.

Consistency. Same brass.  Powder charge as absolutely close to each other as it can be, and consistent neck tension.  Once I started annealing every 2-3 firings I noticed my numbers come down.   I only am that particular about my precision rifle stuff.

Trying different primers can have a huge effect. I find lower ES/SD numbers usually when approaching maximum loads.

Ball powders tend to give lower ES/SD numbers but not better accuracy.

Unless you are actually competing at 600+ yards it will have very little effect on target. ES of 50 fps is very good for general purpose ammo. Anytime I'm under 15 SD for ammo intended inside 300 yards that shoots great groups I don't worry about it.

So many things can be in play it's almost impossible to narrow it down. Try bumping your charge up .3 grains and your velocity will be virtually the same as the Lake City brass. If the load is shooting just as well as before I wouldn't worry about it.

I think expecting all the rounds to fall under 10 ES and 5 SD is unreasonable, and probably impossible. The only ammo that shoots that well over a chronograph is custom tuned handloads using premium sorted components (Lapua brass) and hand weighed charges in long range rifles.

8208 XBR is just below the neck on my SMK reloads.

Is this also true for compressed loads? The top end of my TMK work up loads are slightly compressed.

What primers have you tried? I started with CCI 400s but they seem a little soft for upper end loads.

223 is a beast when it comes to reducing ES/SD. Small case capacity makes small variations have disproportionate impact. Case capacity, powder weights, primers, neck tension, seating, and a bunch of things I am omitting can impact results.

CCI-BR4's and CCI-450's both work well for me. The CCI-#41's should too. All of these handle pressure better than Winchester or CCI-400's IMO.

Russian 5,56-kvbm primers are tough, the m stands for magnum. They are milder flamed and should improve ES/SD because they don't introduce as much pressure into the mix when they ignite.

CCI-BR4's are known to work well with Varget and I have used close to 5000 teamed with that powder and SMK's.