Right, managed to get them weighed, big one is 8.4Kg, small one is 5.3Kg, give or take a bit for scales.... Still think the larger is US tank? that's what we'd suspected at the start, it just looks just like a Cockerill round...
1) Both the length and mass of the NR-188 (in the middle) are consistent with this projectile being a HE-TP-T for the Cockerill 90mm Mk1/2/3.
2) The mass of the NR-246 seems to high for this projectile to be a HEAT-TP-T for the Cockerill 90mm Mk1/2/3. From there, I can only think of a couple of alternatives :
a) In the early 1980s (1983-84 IIRC), Cockerill developed a couple of 90mm which were more powerful than the Mk1/2/3 : the Mk4A3 and the Mk7. The Mk4A3 was intended as an upgrade for the M41 light tank and the Mk7 eventually morphed into the Mk8 (after it was somehow merged with the MECAR Kenerga 90/46). According to Jane's 1990-91, the Mk4A3 and Mk7 offered the same performances. The HEAT projectile was said to have a mass of 5.8 kg and a MV of 950 m/s, meaning a muzzle energy of about 2.6 MJ. So it may be assumed that a 8.4 kg HEAT projectile could have been fired at a MV of about 800 m/s from the Mk7, which could make sense. From there, one may imagine that this was an early HEAT design for the Mk7, later replaced by a lighter (and faster) projectile. Problem with this is that your projectile is marked A1, meaning it's the second iteration of the NR-246, and it doesn't seem to make much sense given given the timeframe.
b) The Belgian Army was an M47 user (until the mid-1980s), and it's possible that PRB may have developed a HEAT projectile for the US 90mm M36/M41 guns. The M431 HEAT round was 5.8kg with a MV of 1,220 m/s (i.e. muzzle energy of about 4.3 MJ), so it may be assumed that a 8.4kg projectile could achieve a MV of about 1,000 m/s, which sounds quite reasonable for a HEAT projectile. The NR-246 doesn't look similar to the US 90mm HEAT projectiles I'm familar with (i.e. M348 & M431 series), but I seem to recall that other HEAT projectiles were developed in the US for this gun (don't remember their designation though).
This second option (US M36/M41 guns) sounds more plausible than the first one (Cockerill Mk4/7), so you might want to research the various 90mm HEAT projectiles that were developed in the US and will hopefully end up with something that matches your NR-246.
That's unfortunately as much as I can help, since I've reached the limits of my very limited knowledge.
I nevertheless hope it might be useful and that someone will eventually jump in with the correct answer.
Kind regards.
Matt.
PS : you could also try to contact the PR department of NEXTER (formerly GIAT) in France and ask them if they can help you with the Nr-188 and NR-246 designations. Since they took over from PRB when the Belgian company went bankrupt in 1993, they might be able to help you.
