We haven't been too busy since the last round of testing loads last summer. I have however not been completely idle. My brain is functioning even if my body feels the passage of time more harshly.
I have been looking at even heavier projectiles in 7mm and I have enough 158 grain Grom projectiles from Privi Partisan to make it worth continuing.
This is for this summer along with the continuing 150 grain testing.
In the mid 1960's the advent of the 5.56 round and the AR 15 adoption by UK forces for special in theatre use along with Nato testing of helmet penetration testing showed that a high velocity projectile of 4 to 5 mm driven at sufficient achievable velocity was far superior to existing 7.62 x 51 ammunition.
As part of this testing an EM2 and a rifle No4 were rebarrelled to a 6mm projectile using a cartridge which was made using existing .280/30 cases necked down to accept a 6.25mm projectile.
Research requests have turned up information courtesy of one of Peter Labbett's publications that the bullets used were in fact what we would know to be .243:
One form of rifling considered for the 6.25 round had a minimum land diameter of 6.06mm and a groove diameter tolerance of 6.24-6.27mm - but I do not know if this was the form used for the barrels of the 2 EM2 auto rifles converted to 6.25mm
6.25 mm with several weight variations. Only ball, dummies and proof known. Case headstamp was RG date 6.25mm
Regarding your enquiry. Please see attached pdf – scanned page taken from ‘Assault Rifle Ammunition 5.6mm to 11mm Calibre’ by Peter Labbett, published March 2000, where he states that ‘various bullet designs existed with this cartridge with weights varying between 5.9 grams and 7 grams (91 grains and 109 grains).’
The ECRA database on this cartridge gives Bullet diameter data on 7 specimens as: 6.30, 6.23, 6.25, 6.23, 6.22, 6.26, 6.15.
To look at the developing of this round it means that I need to get some dies, a reamer, headspace gauge and a barrel for testing. I am on the case but in the meantime I have ordered in some existing dies to carry n the work.
I have recently received another set of .280/30 dies as mine are now on long term loan and a set of 25 Souper to work on a .25-08 using a .243 case as a parent. Why? Because I need something to keep me busy whilst waiting for other things to be done. That and the original .25-06 is a good round and I'm looking to see if that can be matched or improved using a shorter case. I know its another rabbit hole.
I am not as yet for some strange reason interested in recreating the 4.85 for reasons I have yet to elucidate but its somewhere in the future. I am however interested in the alternative theory that the .280/30 case could have been considered as a parent case for a sub 6mm projectile. This would in effect have made a 43mm cased version of the .22-250. Whether or not I lose my marbles on the way remains to be seen but this means that I will hopefully be using a single receiver (a Sako L691) and three barrels one in .25, one in .243 and one in .22 to get these loads looking like something of possible use.
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