Unknown UK/Kynoch M8 API

I asked on the BOCN forum, but got no info, so I thought I would ask here.

I was given two British/Kynoch M8 API silver tipped .50 BMG (12.7x99) caliber rounds and asked for identification. (I’ve already checked Labett, but nothing this new.)

They both have the exact same headstamp, primer, case, primer sealant, etc. All that is different between the two rounds is that one has a single knurled ID cannelure on the projectile, the other has TWO knurled ID cannelures on the bullet.

What is the difference?

K 75 50 headstamp, brass primer, purple sealant, ring primer crimp.

I pulled the projos and will attach the photo. Single cannelure: 660 gr, longer, with a longer boattail and narrower bullet base but what also looks like a white glass base cap. Double cannelure: 687 gr, shorter with shorter boattail and fatter bullet base and a green plastic base cap. Neither are tracer (the standard US M8 API is lead base plug, 650 gr). Xrays only reveal what we see from the outside, though the core tips appear to be slightly different inside.

Xray photo, the twin cannelure is left, single cannelure is center, US M8 API on right.

Since both rounds have the same mfgr and date, why the two bullets?

  1. One experimental, the other standard UK issue?
  2. One is an early design and the other is the later, a production change mid-year?
  3. One is standard issue UK product, the other built to the specs of some unknown customer?
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