I have had two M50s- with only two tapped holes in the bridge and they had the wider .860 spacing. I think the factory barrel is fairly slim so I would be a bit concerned about balance if shortening much…and I like shorter barrels. Like someone else wrote, I like the factory FN safety on the left side of the bolt sleeve. The 51 has a higher comb as I recall. The factory wood on my current 50 has a low comb better suited to irons so I am re-stocking with a Hi Tech.
