The AR-10 was never designed around the 6mm with OTM meplats. What happens with the narrower neck is exactly what you’re experiencing.
The neck and projectile are able to present lower than the feed cone cut into the breech.
A very talented machinist, gunsmith, and engineer I know tackled this problem by cutting an extra set of extended feed ramps from the feed cone, without cutting into the feed cone depth (because this will jeopardize case web support under pressure during firing).
It’s very hard to get in there though without damaging the barrel extension or screwing up your breech.
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What I would try before doing any mods is get some Lancer 7.62x51 mags and seeing how they feed the 109s.
That will probably solve your problem since they are thinner lips, allowing higher cartridge presentation.
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You can also reach out to Seekins and ask if they have any recommendations.
Another thing to try would be to make a tool with a .308 case wrapped in 400 grit sandpaper. Install the tool in a hand drill.
Take your PMAG and spin the tool inside the feed lips to change the inside lip angle a bit higher, so the cartridge biases more upwards upon stripping from the magazine.
You can also add material to the back of your PMAG spines right as low as possible in the mag well location where there is contact, so it biases your magazine angle nose-up.
This is what Geissele did with their 6mm ARC, with the trigger guard nose. It protrudes into the mag well to bias the magazine so they don’t experience the same thing you are, as it is a problem with 6mm and certain projectiles, especially secant ogives with OTM meplats.
Let us know and we’ll help you along the way to get it sorted.