For the past two years i have been experimenting with shooting reduced loads out of my Swiss K31 7.5x55. I have been using 150gr plated xtreme and 185 cast gas checked bullets with Red dot, Unique and 2400. I typically shoot ~4moa groups but would occasionally see a random flyer, sometimes a complete miss with one shot in the string. I then got a chronograph and started chroning my loads. Below are some of the my recorded data. The slow shot seems to be the first shot when the rifle is cool.

The 150gr xtremes seems to shoot just fine up too `~1800 fps. This is the highest I have taken them with 2400.

150gr plated over 18gr 2400
String 1
1673
1743
1734
1724
1762
String 2 after checking target.
1653
1732
1720
1741
1746

150gr plated over 13gr red dot
1494
1644
1600
1614
1608

150gr plated over 7.5gr unique (I shot this 6/24/15. tilted to rifle up and down to position the powder. Up to this point I assumed my issue was powder positioning.)
String 1
919    down
1068  down
1071  down
1084  up
1066  up
1064  up

String 2
918    down
1089  down
1072  down
1107  up
1082  up
1072  up

These are only a few examples. My question is what could cause the first shot to be so slow. Without the slow flyer, my SD and ES are really good. I don't clean my barrels. I just checked and no leading, just minimal powder soot.
If anyone is interested in more data, I would be happy to load it, shoot it, chrono, and post. I have Red dot, unique, 2400, titegroup, and trail boss. I have not tried trail boss yet. Reply with suggestions.

Bullet set back when loading from the magazine. My guess.  Try working up some loads with cast/gas checked and  IMR or H  4895

Never shoot for group or velocity with a clean barrel. Always shoot at least two fouling rounds through the barrel first.
Why? Barrel condition changes.

I rarely clean the bore. These flyers occur after the bore has been fouled with my new powder combinations.

Im going to try Trail boss next.

Looks like those reduced pistol powder loads may just be extremely temp sensitive.  Barrel transfers

heat to brass pretty quickly once it warms up after the first shot.  Or could be technique related. Powder

forward, or powder against primer makes a significant difference.  You load the first shot with muzzle up

for instance, but subsequent shots the powder is forward in the case because of recoil. Try an experiment

of intentionally tipping the rifle either front or back each time after clambering a round.

Pretty sure he specifically stated that's what he was doing.  His velocities are listed with which way the powder was positioned.

2400 definitely seems to be temperature sensitive (test here), but I do not know for sure about Unique or Red Dot.

Try loading the rounds one at a time, as identically as possible, and giving the rifle time to cool between shots. I'd say give it 5 minutes between shots to be safe, action open.

I shoot 16gr of Unique in a 7.62x54R with a cast bullet. Also 6gr of Bullseye with the same bullet without gas check.

I get consistent velocity from both loads.

All fouling has some sort of residue in it. It is not all dry carbon. I suspect that the first shot is warming and loosening the hardened fouling in the bore then the fallowing shots have less drag.

Unlike full power loads, reduced loads using fast powder reach pressure peek very quickly. If there is resistance in the bore it will slow the bullet down.

Motor

So today, 6/29/2015, I shot my K31 with trail boss loads.
150gr plated flat nose with 11.7gr trailboss using lee 2.5cc dipper
6 shots at 50rds, iron sights, shooting of range bag on shooting bench. Shot my usual 3-4 moa groups.
String 1
1082
1299
1305
1260
1284
1268
Extreme group spread: 1.5" 50yds

String 2
1178
1261
1250
1269
1292
1287
1234
Extreme group spread: 1.6" 50yds

String 3
1248
1292
1292
1324
1287
1234

Extreme group spread: 1.9" 50 yds

Conclusion, I experienced the same first round cold bore lower velocity shot as with other powders. At 50yds however, the velocity difference was not enough to ruin my group sizes. I am guessing at 100 yards, the difference would be much more noticeable. I guess i will just have to accept and work around my cold bore flyer. For years, i assumed that it was something i was doing wrong.

I also had the sad experience of 2 case mouths splitting on my batch of 20 Prvi brass. I got 13 firings and full length sizing out of them. I think for now on, i will fire my brass 10x, then put them aside to load with a full power load and put away for shtf.

Today you say you were shooting plated bullets. Before you said cast bullets and that you don't clean the bore.

Looking at your results from today the velocity difference between the first round and the rest go less and less with each string.

Your 3rd string looks normal with the last round about the same as the first.

I think shooting the plated bullets has cleaned out some of the bullet lube left by the cast bullets and maybe some other fouling.

Motor

I have thought about it, I have just never tried it and Im not sure of a consistent way to do it. I have a propane torch. How do you use the temp indicating liquid? Once I buy another set of 20, i would love to try annealing and seeing if i can get more life.

I have tried both but the last 5-6 times out ~(150rds) have been with the 150gr plated. The groups where shot one after the other. There was some time (5-10min) between groups 1 and 2 and very little time (few minutes) between 2 and 3.
I am concluding that the flyer is due to literally a cold bore/chamber as one poster suggested.