The first rifle used was the No4Mk1/2 scoped with a 8.5-32×50 Hammers. The ten shot group measures 2.53"×2.67"

The second rifle used was the Faux L8A5T scoped with a BSA 24×44 target scope. The ten shot group measures 2.15"×2.01"

The last rifle used is a No4Mk1 Long Branch with a donor 2 groove barrel and scoped with a Lucid 6-24×50. This one totally surprised me because what looks like only seven shots is actually seven shots in one large hole and three to the right. That ten shot group measures in at 1.33"×2.51"

Not a bad day today I must confess...

The first two targets were hit with Sierra Matchking's, 174 and 168grn respectively using IMR4064. 

The Man in Black used Hornady BTHP .312 174grn bullets with Norma 202 powder. 

I've only ever seen it once.

I was given some .30 cal 170 gr bullets for the 30-30, they were specifically marked that way, we reloaded them into .308 cases using 308 180 gr load data.

Our assumption is we simply over-drove them to .308 velocities & they broke down in flight somehow.

 We retrieved a couple of them & there was a jacket, but almost no lead, we assume it melted somehow. Either from the hotter charge weight or the increased speed.

The first time I noticed this I had just sat down to sight in the rifle with the 2A1 Ishapore barrel installed.  I was using Hornady 168grn HPBT with N202 powder. I was letting my daughter do the actual shooting and scope adjustments. Through the spotting scope it originally looked like the bullet was going sideways into the paper. It wasn't until we walked out to get a better look that we noticed the tiny vortex marks. My daughter thinks it is unburned powder residue...

The Sierra projectiles using IMR4064 left no marks...

In the photo you can see the difference between the Sierra on the top and the Hornady on the bottom. I marked the area in question. 

Another explanation is the bullets are being driven at excessive speeds and getting damaged, but I really doubt that is the case with your loads.  These bullets are typically good for speeds higher than .303 British loads. 

Not every bullet hole showed the pattern, so it might be an occasional bullet with an uneven jacket thickness.  There only appears to be one small area on the bullet that gets the jacket scrapped off.  The amount of lead that is leaking out I would have thought those bullets would go wide from imbalance. 

This group came from my All Black Long Branch with the 2 groove barrel. The bullets in question came from a batch that I loaded a few years back using the .312" GRAF BTHP's and the N202 powder. As I stated earlier in this thread, I checked the chamber where the rifling starts and checked the grooves and lands forward to the muzzle and found no trace of anything unusual. Actual cleaning of the barrel just brought out the normal fouling you normally have. 

You mentioned that those pistol bullets might have been damaged in the bore and then you also mentioned thin walled rifle bullets, specifically the Sierra brand.

 I pulled a bullet from that batch and saw something that would possibly contribute to the swirl marks. It is also my assumption based off of what I saw that the bullet was damaged prior to entering the bore.

I used to crimp the case as the bullets were seated. After pulling the bullet, I can see a definitive deformation in the ogive that can and would strike the rifling hard enough to remove enough outer material to allow the core to melt away from the heat build-up from the excessive friction caused by that deformation.

I have ten of the crimped cases left. Then next time out I am going to fire ten rounds of uncrimped cases first and then the remaining ten rounds of the crimped cases and I bet I see swirl marks from those bullets...

Ah HA!

you may have hit the nail on the head.

I never crimp unless there's a specific crimping groove in the projectile.

What I do instead is to very slightly reduce the expander ball diameter, usually by spinning it in a power drill & polishing firmly to a mirror finish with a very fine abrasive.

This increases  neck tension without distorting the projectile at all.

My issue now is digging out six boxes of loaded Norma brass that I originally crimped years ago. I know I'm going to find the same thing. I'm going to have to find an extremely clean container to catch all that N202 powder in...

I’m amazed that those bullets even hit the target at 200 yards let alone grouped at under 2 inches.

Years ago I used a light crimp with the Lee Factory Crimp die on the 174 SMK.  The thin jackets deform easily.  I stopped doing this and did what Shamu did by sanding down the expander ball in the FL sizing die and also bought a -0.001 mandrel for my Lee Collet sizing die to increase neck tension.