So if you load 180gr VLDs to max length in your magazine, you're still ~.250" short of touching the lands? And, the rifle is grouping 3.5-4" at 100yds?
If so, then I think there's nothing to say that it can't group well with that much jump provided the rifle setup is good and you do a proper load workup.
Here are a few things to consider...
- check the twist rate for your barrel, 1 in 12" would be on the slow end for 180 VLDs and may not stabilize them
- has the rifle ever grouped well with any bullets/ammo
- try a lighter flat based bullet
- good scope, well mounted, without stress
- action screws are properly torqued
- barrel is free-floated and nothing touching the barrel when you take a rest on bags or bipod
- crown is clean and burr free
- barrel is clean and not copper fouled
- if the barrel has a high round count, consider having it borescoped to check the throat
If everything appears to be in order...
Try a load workup as a single shot without placing rounds in the magazine. If you're now able to load to within .010-.140" from the lands and achieve suitable accuracy, then you can consider either using it as a single shot or a magazine modification.
Wyatts makes extended magzine kits for various rifle. I'm not familiar with fng. Do you mean FNH? ...as in Mod 70 Winchester style? Or, is it the old Mauser 98 action?