Hi Mike,
SAAMI standards usually deliver a 9mm para round that's actually too short, so I doubt that the length of the Winchester round could be the cause. Did it produce feeding issues?
The guys in the US seem to swear by the White box Winchester stuff, with normal MFJ round nose bullets.
Over here in the old world, we tend to lean towards the Sellier & Bellot 124 grain, although even their 140gr subsonics perform well.
My only objection to GeCo is that it seems to be a bit dirtier than the S&B. Need more cleaning time afterwards
The absolute best 9mm round is the one loaded to your personal (and your gun's) specifications, I guess