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I have a load of once fired 7.62 GGG brass that i'm using to reload, and having read that Military (NATO) brass is thicker, therefore with reduced case capacities and increased presures, you should drop the powder by 1.5-2 grains.

Using the Sierra loading data for their 155gn HPBT Match, for H335 powder, 39.5 gn should give around 2500 fps, 40.9gn = 2600fps and 42.2 gn= 2700fps. Max load of 44.9gn =2900

I went to the range today with my old Chrony and tried 5 batches with loads between 40.0 and 42.0gn.

None of them went higher than 2550fps...

If GGG cases have reduced capacities, i was effectively using 41.5 up to 44gn of powder, but getting nowhere near the velocities.

So.... either the GGG cases have commercial brass capacities, and I can increase the loads by a grain or 2, or the short barrelled Mossberg MVP (18.5'') isn't generating the same velocities as the Sierra test gun (26'')

What is the opinion of the learned gentlemen..?

You can be losing about 25fps per lack of inch of barrel.
That would account for about 180fps.
If you have other cases try weighing them for a comparison.
You could compare the internal capacity with water if you have a suitable measure.

Have you compared the wet and dry weights to known military and commercial brass for fired brass?

Ok, So it looks like it is just barrel length thats the issue...

My 42 gn loads averaged 2523fps, so adding 180fps that makes 2703fps, which is close enough to Sierra's 42.2gn @2700fps

That also works with my 40.5gn load average 2430fps +180 =2610fps, close to their 40.9gn @2600fps

I do still have some unfired GGG, I should have taken that with me to chrono, but didn't think until afterwards.

I was aiming to get them up to around 2700fps, but it looks like that will be tricky and close to max pressure with this rifle, so i'll stick with the 2500-2600 range, and just add an extra few inches at longer ranges.

Thanks for the advice.