OK, there was a conceptual fork in the road and I took a left turn. Was looking specifically at the word "inertia" in the sense of physics, not in the sense of "effective increase" thereof. On the point of reduced velocity SD, it makes sense that it would and I think it one of reasons factories crimp their ammo. A crimp is an easier method of regulating neck tension that most any technique one might dream up. Not the best, but it provides statistical envelopes that are practical. The bullet engaged in the lands would do the same, but not to the same degree. Contrary to conventional wisdom, doing this does not raise peak pressure as much as the use of different primers in cartridge guns, and that includes jacketed bullets. In context of muzzle loaders and round balls, I think what you bring up has validity, but the variables in regulating it are vast. So we wipe....
I have a very high regard for Lyman and generally recommend their load manuals to neophytes first and foremost. That said, if they report air pressure or sonic shock waves the cause of deformation in the photo you mention, they are wrong. Now I gotta go find the danged thing and find out...... 
Note this was written while you were posting your finding of the photo.
But will post anyway.
A friend of mine laid "load inertia" on me sometime back but its a perfect description so I use it too. Saves a lot of words.
Pg 167 of the 1975 copyright edition of their "Blackpowder Handbook"
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"Fig. 9 Shadowgraph of a .445 RB at 2220 fps Velocity. Note the flattening of the ball by the pressure behind the shockwave."
Looking at it I think its distortion of the shadowgraph by the shockwave or ?. I have not looked at this photo in years and took it at face value years ago.
The problem with crimp is that it more often than not hurts accuracy. Lead bullets especially soft ones don't like being pushed though a crimp. But sometimes it helps. BPCRs can be horribly individualistic especially when different reamers are used even for the same cartridge.
BP is far different that smokeless in cartridges. A gap much larger than .010 between case length and chamber length will often result in lead or paper rings at the case mouth as the powder bumps the bullet before it starts to move.
The BPCR shooters work load inertia in various ways, some use no neck tension at all and generally use some bandaid approach like very heavy compression but its not a good answer IMO.
Little of this is applicable to MLs. But ML arms do benefit from drop tubes but they are only practical as range accessories at matches.
Some shooters report heavy compression in MLs helping with accuracy but there is no way to get much compression on powder by hand, some mechanical advantage is needed to produce more than 1/8" of compression on drop tubed powder charges. Yes, I have actually tested this with a arbor press with a scale to measure handle pressure and a cartridge case with a false bottom in a chambered chunk of barrel. Crude perhaps its still informative.
While most ML charges are not properly drop tubed, the powder is generally poured too fast and the inside of the barrel is not smooth which could also effect compaction as well. Properly drop tubed powder, especially Swiss is pretty hard to compress. The PSI at the punch face gets pretty high once past about 5/16-3/8". Compression is WAY past "pushing on a rod" pressure at about 1/8 to 3/16. So I do not consider compressing the powder practical in a ML.
Properly drop tubed (cartridge loading) is poured at a consistent rate for each cartridge taking perhaps 5 seconds for the typical charge of powder. Pouring too fast greatly reduces compaction. Some shooters vibrate the cases.
Uniform compaction via drop tube or vibration, allows a uniform compression of the charge for a uniform seating depth. It also produces more uniform ballistics, so uniform that the SD gets down to the accuracy error of chronographs. It also greatly reduced fouling in the bore.
Why? *Probably* due to the way the flame propagates in the charge.
So careful drop tubing is surely a good idea in MLs and this is confirmed by its use by ML slug gun and picket rifle shooters.
It really helps (from reliable "rifle cranks") though I have not tried it personally. The tube must end about 1/2" above the highest level of the powder in the bore to be effective.
Note than many straight case BPCR shoot best with NO compression at all. But without compression transporting the ammo will cause vertical dispersion problems so about 1/16" or a little less is recommended. I used to use 1/8 for BN cases with GOEX.
In some testing, limited, I found the lubing the bore after cleaning seems to help my experimental picket rifle's accuracy. But this is not set in stone.
I suspect it will shoot as well or better with 1.5 Swiss as FFG. So far best groups have been 80 grs of FFG Swiss in a 40 with a 132-135 grain swaged FP, FB picket. But its still nothing to right home about, probably 2-3" at 100. 70 grain group is much bigger.
Why the picket takes so much powder I have no idea but it seems to be fairly typical.
This is a 48" twist and perhaps the twist needs more powder.
But then Chapman tells of using 2" of powder in the bore for a 38 picket rifle with a gain twist and if IIRC it ends in the 30s twist range. Would have to re-read to be sure.
He indicates that hot powders do not work as well with the picket.
Sorry about all the BRCR content but there has been a lot more serious shooting done with BPCRs over the past 20 years than with PRBs. Lots of shooters shoot a LOT with sophisticated sights at ranges to 1000 yards and even beyond. There is a measured mile match in Northern Wyoming and some people are VERY serious about this. Then we have the Schuetzen shooters who try to shoot 250s at 200 yards and the 25 ring is not all that big.... The level of development is pretty darned impressive. But they are still having problems beating some of the old records.
We have people doing a lot of very good work with MLs to but the numbers are not as high. Few ML shooters chrono all their target work as well as shooting for group. It is amazing what can be learned with one of these speed meters. Some pretty heavy BPCR shooters are buying ML picket and slug guns and shooting these as well. Some of these were ML shooters who, like me, drifted to BPCRs. It was a living and shooting 200-1000 yards is a lot of fun. But BPCR is also a lot of work and I shudder at detailed load development these days. To much stress
Hope this makes sense.
Dan
